Post by Norbo on Jun 13, 2010 22:31:55 GMT 1
What is Jetting
Carburetor jetting can be easily understood if we understand the basic principles of carburetor and engine operation. A carburetor mixes fuel with air before it goes into the engine. When the mixture is correct the engine runs well. The bottom line is a carburetor must be adjusted to deliver fuel and air to the engine at a precise ratio. This precise ratio can be affected by a number of outside and inside influences. If you are aware of these influences you can re-jet your carburetor to compensate for the changes. I'm going to show you some examples of how you can change your jetting for better performance and in some cases increase engine life. As with any engine work be sure you have good tools the correct parts and a good manual before you get your hands dirty!
Altitude Compensation
For our first example let's say we find a new riding area WAY up in the mountains. Our jetting is dialed in for our usual riding area which ranges from sea level to 1500 feet. Our NEW riding area starts at 4000 feet and goes up from there. Going to a higher elevation will require will require a jetting change but which way? Like our fuel density, air density can also change. Higher elevations have less air density then lower ones. At high elevations our engines are getting less air, so they need less fuel to maintain the proper air/fuel ratio. Generally you would go down one main jet size for every 1750 to 2000 feet of elevation you go up (info for Mikuni carbs). If you normally run a 160 main jet at sea level you would drop down to a 140 at 4000 feet. Something else goes down as you go up in elevation is horsepower. You can figure on losing about 3% or your power for every 1000 feet you go up. At 4000 feet your power will be down about 12%-even though you rejetted! For our second example, let's say we are still at our new 4000-feet elevation riding area and a storm comes in. We head back to camp and ride it out overnight. The next day there's a foot of snow on the ground the skies are clear and it's COLD! Aside from getting the campfire going and making some coffee you should be thinking about jetting again! Cold air is dense air and dense air requires bigger jets. If the 140 jet ran good the day before you will need a bigger jet to run properly today. If the temperature is 50 degrees colder than it was the day before you can actually go back to your sea level jetting, a 160 main jet! If you don't rejet you can kiss your assets goodbye when you rebuild the seized engine. Air temperature makes that much difference!
Our final example will deal with something often overlooked. We are still up in the hills enjoying our NEW riding area when we notice the old fuel supply getting shorter. No biggie; there's a little store/gas station just down the road. A short trip a few bucks change hands and we are ready to go again. Out on the trail the bikes are running funny, sometimes "pinging" and running HOT. What happened?! When we changed jets to compensate for altitude and temperature we were still using SEA LEVEL gasoline. Gasoline sold at higher elevations have a different blend of additives to compensate for the altitude. Generally high elevation gasoline is less dense to compensate for less available air going into the engine and to aid starting. The lighter specific gravity of the high elevation fuel actually "leaned out" our mixture! One to two sizes bigger main jet will get us back into the hunt. If you ride in vastly different areas try to bring enough or your normal fuel along to last the entire ride. It will save you hassles and gray hair in the long run!
Pilots, Needles & Mains
So far we have only talked about main jet changes to compensate for altitude, temperature and fuel density. As most of you know there is a pile of jets in a carburetor. While main jets are the most critical for ensuring full power operation and engine longevity, the other jets are equally as important for a good running engine. Let's run through them quickly.
Pilot Jets: Pilot jets control the low-speed and idle mixtures. Many times an adjustable jet is used in conjunction with the pilot jet. The adjustable jet allows a precise setting of the idle mixture. If the adjustable jet is located to the rear of the carburetor and usually on one side it is a AIR adjustment. It controls the amount of air that mixes with the fuel coming from the pilot jet. If the adjustable jet is to the front of the carburetor, on the side or bottom, it controls the amount of air/fuel mixture going into the engine. In either case if adjusting the mixture screw won't improve the low-end running speed it's time for a different pilot jet.
Slide: Throttle valves (the slide) control the off idle, to one-quarter open, mixture. Some aftermarket carbs have replacement slides available with different "cutaways". Changing the cutaway changes the mixture. More cutaway is lean, less cutaway is rich. Some carbs do not have different slides available, so you have to compensate by changing the mixture on the idle circuit or needle circuit. Partial throttle hesitation or rough running can be caused by the slide cutaway.
Needle Jets: Needle jets control the amount of fuel going by the needle and into the engine at low to mid throttle. There are 2 types of needle jets used in a carburetor. One is a primary type that has a very precise hole hole drilled through the middle of it, along it's length. The size of the hole relative to the size of the needle determines how much fuel goes into the engine. The other type of needle jet is constructed essentially the same except for a bunch of holes drilled into the side of the jet. These holes allow air to mix with the fuel before it's metered into the engine. Either type of needle jet works well in most cases but there is power to be gained on high performance four-strokes by going to the needle with the holes in the side. These are called "bleed" type needle jets and produce more midrange power in a four-stroke. In any engine going to a leaner (smaller) needle jet is the easiest way to rejet the midrange running when going to higher elevations. Changing the needle jet leans out the mixture evenly at all the midrange throttle settings moving the needle clip doesn't.
Needle: Jet needles more commonly know as the "needle" control the fuel mixture throughout the midrange. The shape or taper of the needle dictates how much fuel goes into the engine at a given throttle opening. The needle must work in conjunction with the fueling requirements of the engine relative to slide position. If you have an engine with a strong hit in the midrange the needle will probable have a noticeable reduction in size the the slide is half open. Remember it takes fuel to make power and when the engine makes power it needs fuel NOW! If it doesn't get the right amount of fuel it pings or misses. You many have cleared up a little midrange pinging by moving the needle up a notch but at the same time you may have over richened some other areas. If the problem isn't too bad you won't even notice the rich condition. If the machine stutters before it comes on the power that part of the needle's taper is too small and the only way to cure it is to get a needle with a different taper. Finding the right needle can be difficult so hopefully moving the clip will do the job.
Main Jet: Finally the good old main jet comes into play at three-quarters open to full throttle conditions. Most of you already know a bigger main jet has a bigger hole so it lets more gas into the engine! Pretty simple! As simple as it is the minuet is absolutely CRITICAL to high-speed engine operation. Not only does it meter the gas into the engine, it can aid in cooling the engine as well. A properly sized main jet will let the engine make good power for a long time. One size smaller main jet may make greater power for awhile. A slightly rich mixture burns cooler than a lean one so be sure the main jet is big enough!
One final note on jets. All of them and the carburetion functions then perform tend to overlap into some other jet's territory. If you mess with one jet, you may have to mess with a few of the others. My best advice is to not change more than one jet at a time. Slowly work out the correct jetting and keep notes on what you are doing. If you get totally fouled up at least you can go back to where you started.
Sign, Symptoms & Causes
How would you know if there was something wrong with your jetting? If you listen, your engine will tell you! All you need is an interpreter. Since I speak and understand several different engine dialects, I will give you a hand. Let's start with lean conditions because they can cause the most damage. In a lean condition the engine will surge and sometimes ping under acceleration. The engine will also be "cold-blooded" (hard to start and keep running) but will run better when hot. The spark plug will look bone white or burned in extreme cases. The engine may spit back or sneeze through the carburetor once in awhile too.. If the engine is running rich the throttle response will be fuzzy and not too quick. The engine will burble, miss and blow black smoke. It will start easy but will run funny when fully warmed up. The plug will be dark, wet or fouled (possible all three!).
Ok so what do you do first to cure the problem? The very first thing is to check and adjust the float level. If it's off one way or another it can throw the jetting off too. Set the float to the specs and retest the running. The next item is to determine a rich or lean condition. Let's say the engine gets hot and doesn't pull well. This is a lean condition so the engine wants more fuel. Stick in at least a two size bigger main jet and try it again. If it's better but still not right go even bigger on the jet. and try it again. Bear in mind that drastic or sudden changes in jetting usually mean an air leak has developed somewhere in the engine. Find it and FIX IT! When the engine burbles on the top end come down one jet size at a time until it winds all the way down. Don't drop and more sizes! If the engine seems sluggish and lumpy or want to load up on the bottom end the mixture is TOO RICH. Adjusting the low speed mixture screw helps a little but doesn't cure the problem completely. What you need now is a new pilot jet. Go one size smaller and try the adjustment again. When the engine runs smooth with the adjustment screw about one and a half turns out from the seat you have it!
Balanceing carbs:
Once you have the new carbs in place, you need to balance them. This is easy, they have a balance screw to make this task a simple one. LC's and Valves have an individual cable into the top of each carb and a idle adjust screw for each slide. To balance these carbs, you will be adjusting the cable adjusters at the top of each carb where the cable enters. These should have a locking nut to keep them from moving once set(you probably saw this in the Haynes manual ). There is a method described on the forum called the "lolly stick" method. I would need 2 pieces of 1/16"wood dowel about 6" long. With the bike level up on the center stand, slide a dowel into each carb inlet under the slides alongside the needle. With the slide closed, the dowel should be held sticking straight out of the carb mouth. As you twist open the throttle, the slides rise. This lets the dowel ends sticking out from the carbs start to point down from horizontal, at an angle relative to the amount the slide is opening. If you get down alongside the bike and look across the carb inlets, you can see if the dowels are at the same angle. If not, adjust the cable adjusters(slide height) so the angle of the sticks are the same. Be sure when they are the same that there is a little cable slack(throttle freeplay) with the sticks removed and the slides fully closed.
Screw the idle adjust screws in till they just touch the slides. To adjust idle RPM, adjust these screws in or out the same amount on each carb to keep the idle slide position balanced.
Float height:
Although this adjustment is done with the carbs off the bike, the best way to confirm that it is correct is to attach a clear piece of tubing to the drain/overflow fitting on the bottom of the carb and use it as a sight glass. Again with the bike level on the center stand, connect the tube and hold it up alongside the carb body. With the fuel supply on(prime position?) and the float bowls full, open the float bowl drain screw. The fuel in the bowl will seek it's own level in the clear tube and you can see where it is in the bowl. The fuel supply will make p the ammount sent out into the tube for an accurate level reading in the bowel. It should be within 2MM of where the top of the bowl meets the carb body. This is important as the different circuits draw fuel from different levels in the bowl. If not correct then you must re-bend the tabs on the floats to get it there(float height adjustment in manual).
At this point, if everything is mechanically balanced, you should be able to re-assemble the air intake and any other plumbing and start the bike. The idle will probably be off and require adjustment with the setscrews(remember, both screws at the same time). If you set up the carbs the same as the ones you took off, It should be similar to how it was but it will still require some adjustment and checks before you can declare it safe for the road.
I have covered a lot here, so lets call this phase one. Why don't you crunch on this info and decide if you want to tackle the carb swap yourself or get some local help to do it. It is not especially difficult and if you don't dis-assemble the old carbs, you always have those to fall back to if you run into difficulty.
Phase 2, Oil Pump Check and Adjustment:
You should probably check the pump output as this will have a large effect on the life of the engine and on the mixture adjustments on the carb in the final stage. For the first part, you will need to drain the fuel tank and mix up a gallon of 2 stroke 200ml to a gallon . this is ricjh on oil but its what they race with you can do less if you like but id syay to at least 175ml . Remembering that for permenent pri mis you need to go up a jet to componsate for oil displacement. Mix fuel/oil and put that in the fuel tank. Next is to remove both oil lines where they connect to the carbs and route them somewhere where they can both be put into a container and the pump output measured. Plug the oil inlet ports on the carbs, another piece of tubing(vacuum tubing from an auto parts store or model fuel tube works well for this) connecting them together works well.
As I am sure you have noticed, the gear driven pump strokes a plunger, the length of which is controlled by a pully and cable connected to the throttle. Adjust the oil pump cable to the manufacturers specs keeping in mind that the closed throttle position should be with the throttle free play taken up on the throttle to the point where the carb slides are just about to open. Once this is set, with the engine idling on pre-mix, pull the oil cable to its maximum position and make sure the oil lines are completely full of oil. Then let the pump cable return to the minimum position(point where throttle cable slack is taken up and the carb slides would just start to open) and measure the combined oil output from both hoses for 200 strokes of the pump(this takes a while and is a very small amount of oil). The volume should be 0.12CC - 0.19CC.
Next, with the engine SHUT OFF, twist the throttle to its wide open/full throttle position. Note the position of the oil pump pulley, I put a small paint mark on it with bright red model paint and a toothpic near a fixed structure in the oil pump compartment. Once you have marked the full throttle position, Return the throttle to the idle position.
Re-start the engine and with it idling, pull the oil pump cable until the oil pump pulley is at the full throttle position as noted by the mark you made. Measure the pump output for 200 pump strokes. Again, this takes a while to count 200 strokes and the output volume should be between 2.58CC - 2.85CC. As you can see, this is not much oil. I use a small measuring syringe for dispensing children's medicine. I once counted 1000 pump strokes at the pump idle position to get a larger more easily measurable volume.(I set up a fan in front of the radiator to keep the bike cool for this extended run).
If these measurements are good, re-confirm the oil cable adjustment with the throttle free-play taken up. THIS IS IMPORTANT! and WILL affect the mixture adjustments in the next phase. It took me quite a while of scratching my head over inconsistent test results after de-installing/re-installing the carbs several times before I got fanatical about checking this adjustment ANY time I did anything associated with the throttle cable/carb linkages.
Drain the pre-mix fuel and re-connect the oil lines to the carb ports. I always like to confirm that I can blow through these ports into the carb before I re-connect the oil lines. Add some regular fuel to the tank and you are about ready for a test ride to check plug color again
Carb Theory and Adjustment:
For practical purposes, there are 3 basic carb circuits and engine RPM is basically unimportant. What is important is throttle position as this determines which carb section is providing the air/fuel to the engine.
The pilot section: This is from just off idle to around ¼ throttle. It is the most used section in the carb as the throttle spends 99% of its time in this region(unless you are racing). Because of this, it is provided with an infinitely adjustable airscrew and jet system for fine-tuning. The size of the pilot jet and the setting of the pilot airscrew determine the pilot airscrew mixture below ¼ throttle.
Needle jet/jet needle section: This is from the ¼ position to around the ¾ position. This range has adjustment capability by jet and needle selection and the needle clip position(needle height). The throttle does not spend much time here but has these adjustments to make for a smooth transition from pilot to main section when accelerating.
Main Jet Section: This section covers from ¾ to full throttle and is only really adjustable through main jet change out with emphasis on the widest throttle position.
It is VERY important that EVERYTHING else is properly set up(carb balance, throttle free play and oil pump adjustment/delivery volumes) before proceeding with any carb adjustments.
Since all these sections are additive and the amount of fuel delivered down at low throttle will contribute to the full throttle mix, the best place to start is the pilot section. One adjustment procedure talks about adjusting the idle speed, then adjusting the pilot airscrew for highest idle speed then re-adjusting the idle back down and trying again for a peak idle speed using the airscrew until the highest peak is found. I have used this method on several single cylinder engines but have NEVER been able to get it to work properly on a 2 cylinder RZ/RD. I think the second carb and crossover tube have something to do with it. The pilot circuit doesn't really come into play until the engine is placed under load. The idle circuit is fixed with it's only variable being the height of the slide as adjusted by the idle setscrew. On the RZ, I have been able to adjust the airscrew so far out that the engine dies as soon as I try to open the throttle and pull away, but the bike will still start and idle perfectly on the first kick with the throttle closed.
The method I found that works the best is to start the engine and warm it up to normal temp. With the pilot airscrews set 1 ½ turns out on both carbs, try and pull away in 1st gear from a stop on a level roadway or parking lot. Ideally the engine should pull you away nice and smooth. Make a small adjustment to both airscrews, say around 1/8 turn, either open or closed and try and start off again. Was it better or worse than the first try? If better, move the airscrews another 1/8 turn in the same direction and try again. If worse, return the screws to the original position(1 ½ turns out) and go 1/8 turn in the opposite direction. Keep doing this trial and error method past the point where the best pull-away is achieved and a decrease in performance is noted. Don't let the engine get too warm, as this will affect results.
Remember, the pilot airscrew controls the air being mixed with the fuel from the pilot jet. If you have to go below 1 full turn (airscrew in = richer) from the fully closed position to get best pull-away performance, your pilot jet is too small and you need to upjet. If you have to go above 2 full turns out(airscrew out =leaner) for best performance your pilot jet is too large(passing too much fuel) and you need to go to a smaller pilot jet. Below 1 turn or above 2 turns the pilot airscrew is no longer linear and adjustments are unpredictable. Above 3 turns there is no effect at all. Ideally you want a pilot jet that gives the best pull-away performance with the airscrew near 1 ½ turns out. Once you have it in this area with 1/8 turn adjustments, try and see if you can get it any more refined with smaller adjustments. Then try pulling away up a slight incline(greater load) and see if you can refine it any more
Once you have the best pull-away performance, go for a ride with new plugs and see what kind of color you get on them from a cruising "plug chop"(like the tests you posted pictures of, level road, up to highway speed using as low a throttle as possible). Here is where you can make small adjustments to the airscrew settings to get the desired color on a cruising chop. If your carbs are mechanically in good balance, small changes to one carb or the other can get the same color on both plugs. I have yet to see a factory multi cylinder carb pair with the exact same setting on all carbs. My own RZ factory settings were different nearly 1/8 turn from one carb to the other.
Once you have the pilot section ironed out, the next step up is the needle jet/jet needle. If the flow thru the engine has not been radically altered(porting, squish ect) then you will probably be fine with the stock needle and jet combo and only need to adjust needle height. Your needle may have 5 different clip positions (the US RZ only has a single position). What you are looking for in this section is a smooth progression from the pilot section into the midrange section. If the needle is too low(lean), as you reach the top of the pilot section(around ¼ throttle) the engine will start to bog(lack horsepower or hesitate) and perhaps even knock or ping if the case is extreme enough. Conversely, a too high needle position(rich) will also result in an engine bog but with a dull/flat engine sound.
The center needle position will probably work OK for your bike but you can try moving the needle up/down to see how it performs. If it seems better in a particular direction, you can also use small/thin washers as shims under the clip to refine the height of the needle at less than full clip positions. I use washers to adjust the needle on my RZ. Any time you do anything that involves the throttle cable, like removing the tops and slides to change the needle height, re-confirm the mechanical balance and carp/throttle/oil pump adjustment. Any changes in these will effect your results and make you wonder why you didn't get the results you expected(plug indication is richer from increased oil maladjustment when you expected leaner because you just put in smaller jets/leaned out airscrew or needle).
Once you have a smooth transition to mid throttle, You can check this mix by doing ½ throttle plug chops, accelerating through the gears on a new set of plugs. Warm up the bike by riding to your test road(long, straight, level or slightly uphill) Stop the bike and put in new plugs. Get ready for the test run, put the bike in first gear. Start the engine and pull away and start accelerating immediately, avoiding idle as much as possible so it doesn't skew your results. Get the throttle open to ½ as quickly as possible and keep it there while you accelerate, closing it only briefly to shift(Close throttle, shift and right back to ½ throttle). The goal here is to keep the throttle at the position being evaluated for as much of the run as possible. Accelerate like this for as far as your gears, nerve or the law will allow then hit the kill switch and pull in the clutch to get the engine stopped as quickly as possible and coast the bike to a stop. Remove the plugs and check your results.
You should have some indication of color on the plugs(may take a second or third run to add up). You should NOT hear any knocking or pinging under these ½ throttle load tests. If you do, you may want to re-evaluate the needle position and raise it some(richen) so you can get a smooth test in and see some color on the plugs. A little darker here is not necessarily a bad thing. This is an area that is usually passed through and not stayed in so a little on the rich side will help keep your engine healthy. Remember, too rich only really fouls plugs. Too Lean and you eat pistons and or seize the engine. (In my youth, I miss-timed a R5B too far on the advanced side causing slight pre-ignition and detonation same as a lean burn condition and melted a thumb-sized hole in the top of each piston while cruising down the highway).
Once you have the pilot and mid section sorted, it is on to the Main section. The main jet section is mainly done with the plug chop method I described above except that you are accelerating with the throttle wide open as much as possible. If the jetting is really off either way, you will feel it as you try and accelerate. The area above ¼ just wont feel any more powerful or it may even bog down. While you are doing your midrange or pilot setups, you can completely remove the main jets and try to run up above ¾ throttle(you won't notice they are gone until then) and see what a way too rich main jet would feel like.
The full throttle plug chops can be a little tricky sometimes as if everything is balanced, the transition through pilot and midrange are smooth and you are making good power, the front wheel may want to come off the ground in the lower gears. You can back off the throttle just slightly to keep the wheel on the ground till 3rd gear and above. As before, any knocking/pinging noted under wide open throttle acceleration is VERY bad. Again, the result should be some visible color on the plug(again may take a run or two to show). A little darker up here is not really going to affect your useable power but will affect the life of your engine. Remember, the throttle doesn't spend a lot of time up here, but the times it does are high stress. A little extra fuel for cooling is not a bad thing.
When refitting your YPVS carbs dont forget to refit the vacume pipe from your fuel tap to the carb. Shown here on your left. If you are fiting thiscarb to an lc just atatch a bit of pipe to this and foled it over and put a tye rap around it.
That is everything you'll ever need to know .
Carburetor jetting can be easily understood if we understand the basic principles of carburetor and engine operation. A carburetor mixes fuel with air before it goes into the engine. When the mixture is correct the engine runs well. The bottom line is a carburetor must be adjusted to deliver fuel and air to the engine at a precise ratio. This precise ratio can be affected by a number of outside and inside influences. If you are aware of these influences you can re-jet your carburetor to compensate for the changes. I'm going to show you some examples of how you can change your jetting for better performance and in some cases increase engine life. As with any engine work be sure you have good tools the correct parts and a good manual before you get your hands dirty!
Altitude Compensation
For our first example let's say we find a new riding area WAY up in the mountains. Our jetting is dialed in for our usual riding area which ranges from sea level to 1500 feet. Our NEW riding area starts at 4000 feet and goes up from there. Going to a higher elevation will require will require a jetting change but which way? Like our fuel density, air density can also change. Higher elevations have less air density then lower ones. At high elevations our engines are getting less air, so they need less fuel to maintain the proper air/fuel ratio. Generally you would go down one main jet size for every 1750 to 2000 feet of elevation you go up (info for Mikuni carbs). If you normally run a 160 main jet at sea level you would drop down to a 140 at 4000 feet. Something else goes down as you go up in elevation is horsepower. You can figure on losing about 3% or your power for every 1000 feet you go up. At 4000 feet your power will be down about 12%-even though you rejetted! For our second example, let's say we are still at our new 4000-feet elevation riding area and a storm comes in. We head back to camp and ride it out overnight. The next day there's a foot of snow on the ground the skies are clear and it's COLD! Aside from getting the campfire going and making some coffee you should be thinking about jetting again! Cold air is dense air and dense air requires bigger jets. If the 140 jet ran good the day before you will need a bigger jet to run properly today. If the temperature is 50 degrees colder than it was the day before you can actually go back to your sea level jetting, a 160 main jet! If you don't rejet you can kiss your assets goodbye when you rebuild the seized engine. Air temperature makes that much difference!
Our final example will deal with something often overlooked. We are still up in the hills enjoying our NEW riding area when we notice the old fuel supply getting shorter. No biggie; there's a little store/gas station just down the road. A short trip a few bucks change hands and we are ready to go again. Out on the trail the bikes are running funny, sometimes "pinging" and running HOT. What happened?! When we changed jets to compensate for altitude and temperature we were still using SEA LEVEL gasoline. Gasoline sold at higher elevations have a different blend of additives to compensate for the altitude. Generally high elevation gasoline is less dense to compensate for less available air going into the engine and to aid starting. The lighter specific gravity of the high elevation fuel actually "leaned out" our mixture! One to two sizes bigger main jet will get us back into the hunt. If you ride in vastly different areas try to bring enough or your normal fuel along to last the entire ride. It will save you hassles and gray hair in the long run!
Pilots, Needles & Mains
So far we have only talked about main jet changes to compensate for altitude, temperature and fuel density. As most of you know there is a pile of jets in a carburetor. While main jets are the most critical for ensuring full power operation and engine longevity, the other jets are equally as important for a good running engine. Let's run through them quickly.
Pilot Jets: Pilot jets control the low-speed and idle mixtures. Many times an adjustable jet is used in conjunction with the pilot jet. The adjustable jet allows a precise setting of the idle mixture. If the adjustable jet is located to the rear of the carburetor and usually on one side it is a AIR adjustment. It controls the amount of air that mixes with the fuel coming from the pilot jet. If the adjustable jet is to the front of the carburetor, on the side or bottom, it controls the amount of air/fuel mixture going into the engine. In either case if adjusting the mixture screw won't improve the low-end running speed it's time for a different pilot jet.
Slide: Throttle valves (the slide) control the off idle, to one-quarter open, mixture. Some aftermarket carbs have replacement slides available with different "cutaways". Changing the cutaway changes the mixture. More cutaway is lean, less cutaway is rich. Some carbs do not have different slides available, so you have to compensate by changing the mixture on the idle circuit or needle circuit. Partial throttle hesitation or rough running can be caused by the slide cutaway.
Needle Jets: Needle jets control the amount of fuel going by the needle and into the engine at low to mid throttle. There are 2 types of needle jets used in a carburetor. One is a primary type that has a very precise hole hole drilled through the middle of it, along it's length. The size of the hole relative to the size of the needle determines how much fuel goes into the engine. The other type of needle jet is constructed essentially the same except for a bunch of holes drilled into the side of the jet. These holes allow air to mix with the fuel before it's metered into the engine. Either type of needle jet works well in most cases but there is power to be gained on high performance four-strokes by going to the needle with the holes in the side. These are called "bleed" type needle jets and produce more midrange power in a four-stroke. In any engine going to a leaner (smaller) needle jet is the easiest way to rejet the midrange running when going to higher elevations. Changing the needle jet leans out the mixture evenly at all the midrange throttle settings moving the needle clip doesn't.
Needle: Jet needles more commonly know as the "needle" control the fuel mixture throughout the midrange. The shape or taper of the needle dictates how much fuel goes into the engine at a given throttle opening. The needle must work in conjunction with the fueling requirements of the engine relative to slide position. If you have an engine with a strong hit in the midrange the needle will probable have a noticeable reduction in size the the slide is half open. Remember it takes fuel to make power and when the engine makes power it needs fuel NOW! If it doesn't get the right amount of fuel it pings or misses. You many have cleared up a little midrange pinging by moving the needle up a notch but at the same time you may have over richened some other areas. If the problem isn't too bad you won't even notice the rich condition. If the machine stutters before it comes on the power that part of the needle's taper is too small and the only way to cure it is to get a needle with a different taper. Finding the right needle can be difficult so hopefully moving the clip will do the job.
Main Jet: Finally the good old main jet comes into play at three-quarters open to full throttle conditions. Most of you already know a bigger main jet has a bigger hole so it lets more gas into the engine! Pretty simple! As simple as it is the minuet is absolutely CRITICAL to high-speed engine operation. Not only does it meter the gas into the engine, it can aid in cooling the engine as well. A properly sized main jet will let the engine make good power for a long time. One size smaller main jet may make greater power for awhile. A slightly rich mixture burns cooler than a lean one so be sure the main jet is big enough!
One final note on jets. All of them and the carburetion functions then perform tend to overlap into some other jet's territory. If you mess with one jet, you may have to mess with a few of the others. My best advice is to not change more than one jet at a time. Slowly work out the correct jetting and keep notes on what you are doing. If you get totally fouled up at least you can go back to where you started.
Sign, Symptoms & Causes
How would you know if there was something wrong with your jetting? If you listen, your engine will tell you! All you need is an interpreter. Since I speak and understand several different engine dialects, I will give you a hand. Let's start with lean conditions because they can cause the most damage. In a lean condition the engine will surge and sometimes ping under acceleration. The engine will also be "cold-blooded" (hard to start and keep running) but will run better when hot. The spark plug will look bone white or burned in extreme cases. The engine may spit back or sneeze through the carburetor once in awhile too.. If the engine is running rich the throttle response will be fuzzy and not too quick. The engine will burble, miss and blow black smoke. It will start easy but will run funny when fully warmed up. The plug will be dark, wet or fouled (possible all three!).
Ok so what do you do first to cure the problem? The very first thing is to check and adjust the float level. If it's off one way or another it can throw the jetting off too. Set the float to the specs and retest the running. The next item is to determine a rich or lean condition. Let's say the engine gets hot and doesn't pull well. This is a lean condition so the engine wants more fuel. Stick in at least a two size bigger main jet and try it again. If it's better but still not right go even bigger on the jet. and try it again. Bear in mind that drastic or sudden changes in jetting usually mean an air leak has developed somewhere in the engine. Find it and FIX IT! When the engine burbles on the top end come down one jet size at a time until it winds all the way down. Don't drop and more sizes! If the engine seems sluggish and lumpy or want to load up on the bottom end the mixture is TOO RICH. Adjusting the low speed mixture screw helps a little but doesn't cure the problem completely. What you need now is a new pilot jet. Go one size smaller and try the adjustment again. When the engine runs smooth with the adjustment screw about one and a half turns out from the seat you have it!
Balanceing carbs:
Once you have the new carbs in place, you need to balance them. This is easy, they have a balance screw to make this task a simple one. LC's and Valves have an individual cable into the top of each carb and a idle adjust screw for each slide. To balance these carbs, you will be adjusting the cable adjusters at the top of each carb where the cable enters. These should have a locking nut to keep them from moving once set(you probably saw this in the Haynes manual ). There is a method described on the forum called the "lolly stick" method. I would need 2 pieces of 1/16"wood dowel about 6" long. With the bike level up on the center stand, slide a dowel into each carb inlet under the slides alongside the needle. With the slide closed, the dowel should be held sticking straight out of the carb mouth. As you twist open the throttle, the slides rise. This lets the dowel ends sticking out from the carbs start to point down from horizontal, at an angle relative to the amount the slide is opening. If you get down alongside the bike and look across the carb inlets, you can see if the dowels are at the same angle. If not, adjust the cable adjusters(slide height) so the angle of the sticks are the same. Be sure when they are the same that there is a little cable slack(throttle freeplay) with the sticks removed and the slides fully closed.
Screw the idle adjust screws in till they just touch the slides. To adjust idle RPM, adjust these screws in or out the same amount on each carb to keep the idle slide position balanced.
Float height:
Although this adjustment is done with the carbs off the bike, the best way to confirm that it is correct is to attach a clear piece of tubing to the drain/overflow fitting on the bottom of the carb and use it as a sight glass. Again with the bike level on the center stand, connect the tube and hold it up alongside the carb body. With the fuel supply on(prime position?) and the float bowls full, open the float bowl drain screw. The fuel in the bowl will seek it's own level in the clear tube and you can see where it is in the bowl. The fuel supply will make p the ammount sent out into the tube for an accurate level reading in the bowel. It should be within 2MM of where the top of the bowl meets the carb body. This is important as the different circuits draw fuel from different levels in the bowl. If not correct then you must re-bend the tabs on the floats to get it there(float height adjustment in manual).
At this point, if everything is mechanically balanced, you should be able to re-assemble the air intake and any other plumbing and start the bike. The idle will probably be off and require adjustment with the setscrews(remember, both screws at the same time). If you set up the carbs the same as the ones you took off, It should be similar to how it was but it will still require some adjustment and checks before you can declare it safe for the road.
I have covered a lot here, so lets call this phase one. Why don't you crunch on this info and decide if you want to tackle the carb swap yourself or get some local help to do it. It is not especially difficult and if you don't dis-assemble the old carbs, you always have those to fall back to if you run into difficulty.
Phase 2, Oil Pump Check and Adjustment:
You should probably check the pump output as this will have a large effect on the life of the engine and on the mixture adjustments on the carb in the final stage. For the first part, you will need to drain the fuel tank and mix up a gallon of 2 stroke 200ml to a gallon . this is ricjh on oil but its what they race with you can do less if you like but id syay to at least 175ml . Remembering that for permenent pri mis you need to go up a jet to componsate for oil displacement. Mix fuel/oil and put that in the fuel tank. Next is to remove both oil lines where they connect to the carbs and route them somewhere where they can both be put into a container and the pump output measured. Plug the oil inlet ports on the carbs, another piece of tubing(vacuum tubing from an auto parts store or model fuel tube works well for this) connecting them together works well.
As I am sure you have noticed, the gear driven pump strokes a plunger, the length of which is controlled by a pully and cable connected to the throttle. Adjust the oil pump cable to the manufacturers specs keeping in mind that the closed throttle position should be with the throttle free play taken up on the throttle to the point where the carb slides are just about to open. Once this is set, with the engine idling on pre-mix, pull the oil cable to its maximum position and make sure the oil lines are completely full of oil. Then let the pump cable return to the minimum position(point where throttle cable slack is taken up and the carb slides would just start to open) and measure the combined oil output from both hoses for 200 strokes of the pump(this takes a while and is a very small amount of oil). The volume should be 0.12CC - 0.19CC.
Next, with the engine SHUT OFF, twist the throttle to its wide open/full throttle position. Note the position of the oil pump pulley, I put a small paint mark on it with bright red model paint and a toothpic near a fixed structure in the oil pump compartment. Once you have marked the full throttle position, Return the throttle to the idle position.
Re-start the engine and with it idling, pull the oil pump cable until the oil pump pulley is at the full throttle position as noted by the mark you made. Measure the pump output for 200 pump strokes. Again, this takes a while to count 200 strokes and the output volume should be between 2.58CC - 2.85CC. As you can see, this is not much oil. I use a small measuring syringe for dispensing children's medicine. I once counted 1000 pump strokes at the pump idle position to get a larger more easily measurable volume.(I set up a fan in front of the radiator to keep the bike cool for this extended run).
If these measurements are good, re-confirm the oil cable adjustment with the throttle free-play taken up. THIS IS IMPORTANT! and WILL affect the mixture adjustments in the next phase. It took me quite a while of scratching my head over inconsistent test results after de-installing/re-installing the carbs several times before I got fanatical about checking this adjustment ANY time I did anything associated with the throttle cable/carb linkages.
Drain the pre-mix fuel and re-connect the oil lines to the carb ports. I always like to confirm that I can blow through these ports into the carb before I re-connect the oil lines. Add some regular fuel to the tank and you are about ready for a test ride to check plug color again
Carb Theory and Adjustment:
For practical purposes, there are 3 basic carb circuits and engine RPM is basically unimportant. What is important is throttle position as this determines which carb section is providing the air/fuel to the engine.
The pilot section: This is from just off idle to around ¼ throttle. It is the most used section in the carb as the throttle spends 99% of its time in this region(unless you are racing). Because of this, it is provided with an infinitely adjustable airscrew and jet system for fine-tuning. The size of the pilot jet and the setting of the pilot airscrew determine the pilot airscrew mixture below ¼ throttle.
Needle jet/jet needle section: This is from the ¼ position to around the ¾ position. This range has adjustment capability by jet and needle selection and the needle clip position(needle height). The throttle does not spend much time here but has these adjustments to make for a smooth transition from pilot to main section when accelerating.
Main Jet Section: This section covers from ¾ to full throttle and is only really adjustable through main jet change out with emphasis on the widest throttle position.
It is VERY important that EVERYTHING else is properly set up(carb balance, throttle free play and oil pump adjustment/delivery volumes) before proceeding with any carb adjustments.
Since all these sections are additive and the amount of fuel delivered down at low throttle will contribute to the full throttle mix, the best place to start is the pilot section. One adjustment procedure talks about adjusting the idle speed, then adjusting the pilot airscrew for highest idle speed then re-adjusting the idle back down and trying again for a peak idle speed using the airscrew until the highest peak is found. I have used this method on several single cylinder engines but have NEVER been able to get it to work properly on a 2 cylinder RZ/RD. I think the second carb and crossover tube have something to do with it. The pilot circuit doesn't really come into play until the engine is placed under load. The idle circuit is fixed with it's only variable being the height of the slide as adjusted by the idle setscrew. On the RZ, I have been able to adjust the airscrew so far out that the engine dies as soon as I try to open the throttle and pull away, but the bike will still start and idle perfectly on the first kick with the throttle closed.
The method I found that works the best is to start the engine and warm it up to normal temp. With the pilot airscrews set 1 ½ turns out on both carbs, try and pull away in 1st gear from a stop on a level roadway or parking lot. Ideally the engine should pull you away nice and smooth. Make a small adjustment to both airscrews, say around 1/8 turn, either open or closed and try and start off again. Was it better or worse than the first try? If better, move the airscrews another 1/8 turn in the same direction and try again. If worse, return the screws to the original position(1 ½ turns out) and go 1/8 turn in the opposite direction. Keep doing this trial and error method past the point where the best pull-away is achieved and a decrease in performance is noted. Don't let the engine get too warm, as this will affect results.
Remember, the pilot airscrew controls the air being mixed with the fuel from the pilot jet. If you have to go below 1 full turn (airscrew in = richer) from the fully closed position to get best pull-away performance, your pilot jet is too small and you need to upjet. If you have to go above 2 full turns out(airscrew out =leaner) for best performance your pilot jet is too large(passing too much fuel) and you need to go to a smaller pilot jet. Below 1 turn or above 2 turns the pilot airscrew is no longer linear and adjustments are unpredictable. Above 3 turns there is no effect at all. Ideally you want a pilot jet that gives the best pull-away performance with the airscrew near 1 ½ turns out. Once you have it in this area with 1/8 turn adjustments, try and see if you can get it any more refined with smaller adjustments. Then try pulling away up a slight incline(greater load) and see if you can refine it any more
Once you have the best pull-away performance, go for a ride with new plugs and see what kind of color you get on them from a cruising "plug chop"(like the tests you posted pictures of, level road, up to highway speed using as low a throttle as possible). Here is where you can make small adjustments to the airscrew settings to get the desired color on a cruising chop. If your carbs are mechanically in good balance, small changes to one carb or the other can get the same color on both plugs. I have yet to see a factory multi cylinder carb pair with the exact same setting on all carbs. My own RZ factory settings were different nearly 1/8 turn from one carb to the other.
Once you have the pilot section ironed out, the next step up is the needle jet/jet needle. If the flow thru the engine has not been radically altered(porting, squish ect) then you will probably be fine with the stock needle and jet combo and only need to adjust needle height. Your needle may have 5 different clip positions (the US RZ only has a single position). What you are looking for in this section is a smooth progression from the pilot section into the midrange section. If the needle is too low(lean), as you reach the top of the pilot section(around ¼ throttle) the engine will start to bog(lack horsepower or hesitate) and perhaps even knock or ping if the case is extreme enough. Conversely, a too high needle position(rich) will also result in an engine bog but with a dull/flat engine sound.
The center needle position will probably work OK for your bike but you can try moving the needle up/down to see how it performs. If it seems better in a particular direction, you can also use small/thin washers as shims under the clip to refine the height of the needle at less than full clip positions. I use washers to adjust the needle on my RZ. Any time you do anything that involves the throttle cable, like removing the tops and slides to change the needle height, re-confirm the mechanical balance and carp/throttle/oil pump adjustment. Any changes in these will effect your results and make you wonder why you didn't get the results you expected(plug indication is richer from increased oil maladjustment when you expected leaner because you just put in smaller jets/leaned out airscrew or needle).
Once you have a smooth transition to mid throttle, You can check this mix by doing ½ throttle plug chops, accelerating through the gears on a new set of plugs. Warm up the bike by riding to your test road(long, straight, level or slightly uphill) Stop the bike and put in new plugs. Get ready for the test run, put the bike in first gear. Start the engine and pull away and start accelerating immediately, avoiding idle as much as possible so it doesn't skew your results. Get the throttle open to ½ as quickly as possible and keep it there while you accelerate, closing it only briefly to shift(Close throttle, shift and right back to ½ throttle). The goal here is to keep the throttle at the position being evaluated for as much of the run as possible. Accelerate like this for as far as your gears, nerve or the law will allow then hit the kill switch and pull in the clutch to get the engine stopped as quickly as possible and coast the bike to a stop. Remove the plugs and check your results.
You should have some indication of color on the plugs(may take a second or third run to add up). You should NOT hear any knocking or pinging under these ½ throttle load tests. If you do, you may want to re-evaluate the needle position and raise it some(richen) so you can get a smooth test in and see some color on the plugs. A little darker here is not necessarily a bad thing. This is an area that is usually passed through and not stayed in so a little on the rich side will help keep your engine healthy. Remember, too rich only really fouls plugs. Too Lean and you eat pistons and or seize the engine. (In my youth, I miss-timed a R5B too far on the advanced side causing slight pre-ignition and detonation same as a lean burn condition and melted a thumb-sized hole in the top of each piston while cruising down the highway).
Once you have the pilot and mid section sorted, it is on to the Main section. The main jet section is mainly done with the plug chop method I described above except that you are accelerating with the throttle wide open as much as possible. If the jetting is really off either way, you will feel it as you try and accelerate. The area above ¼ just wont feel any more powerful or it may even bog down. While you are doing your midrange or pilot setups, you can completely remove the main jets and try to run up above ¾ throttle(you won't notice they are gone until then) and see what a way too rich main jet would feel like.
The full throttle plug chops can be a little tricky sometimes as if everything is balanced, the transition through pilot and midrange are smooth and you are making good power, the front wheel may want to come off the ground in the lower gears. You can back off the throttle just slightly to keep the wheel on the ground till 3rd gear and above. As before, any knocking/pinging noted under wide open throttle acceleration is VERY bad. Again, the result should be some visible color on the plug(again may take a run or two to show). A little darker up here is not really going to affect your useable power but will affect the life of your engine. Remember, the throttle doesn't spend a lot of time up here, but the times it does are high stress. A little extra fuel for cooling is not a bad thing.
When refitting your YPVS carbs dont forget to refit the vacume pipe from your fuel tap to the carb. Shown here on your left. If you are fiting thiscarb to an lc just atatch a bit of pipe to this and foled it over and put a tye rap around it.
That is everything you'll ever need to know .