It's an early 70's piston ported aircooled 350. The engine looks very much like the later Rd range of aircooleds. It had morphed into the Rd350 by about 74 I think. Any bike that has stood for a time will suffer from the crap petrol that we have nowadays. I would be stripping the carbs down and making sure they are clear of varnish and crap. Points ignition on these, so check that too. Electrical system may be iffy if it's worst once hot. This bike could be as old as 50 now, so electrics could be throwing a retirement party. You say loads of work has been done. What, and by who ? Not all rebuilds are good ones, and not all parts are reliable in use ! Does it start easily, run and idle as it should, rev freely before the issue arises, get better once cooled down ?
My mate has had a YR5 since 2010. Often referred to as an R5. It is very similar to an early RD350B in many ways. Internally, many of the engine components are similar to the RD range: the Lc clutch basket tool fits, as does the oil pump. Its basically a YDS7 but 350cc rather than 250cc. We have never seen another on the road – despite many trips to the classic TT. When he bought it, it had an RD250E engine in it – he bought a basket case YR from D&K and used the engine and loom from that. (sold the rest and the RD250 motor and was in profit!) Its not particularly high revving but has a nice spread of torque. He has been after a set of exhausts since he got it but spares rarely come up, so its still running RD350 aircooled standard pipes. The brakes are drum front and rear and are surprisingly good when properly set up. As dusty says, check the carbs are clean – worth an ultrasonic bath. The points are easy to set up but I seem to remember him playing with the timing to make sure they were bang on. I would try the coils too, from memory there are 2 separate ones, but I would bet an LC one could be adapted to trial it out?
Thanks all .. Mate got the bike and it turned out to be a sheep in wolf's clothes! ... looked great but was cr☆p, engine wise. Barrels re-lined and ported back to standard.Crank rebuilt by Grampian and re-fitted with new seals and and an electronic ign fitted. Believe bike had pod filters and the carbs were rebuilt and original air box re-instituted and re-jetted back to standard. Upon return from the " shop " it starts ok but takes awhile to get going ... the bike is remote from owner and isn't started regularly ( lock down ). Plugs are sooty as hell and it's described as drinking fuel.Dont wish to suspect the mechanic who did (Lots) of the work ...but none of use are immune to errors/mistakes. All the great info appreciated and welcome .... please keep it coming 😁
Sounds like someone has spent a lot of time and money trying to get it back to stock. There is a big bill at the end of that lot. It may well be carb settings. I wonder if it has been set up too rich to help preserve the engine for running in, maybe ? There is nothing wrong with relined cylinders, Grampian crank or retro fitting an airbox, as long as the parts and process are good of course, and it sounds like a previous owner has tried to get it back to how Yamaha intended it to be. Carb settings are crucial on 2 strokes, and you will really notice poorly set up or functioning carbs. Here is a quick guide (not mine);
Spanky's jetting 101:
A correctly jetted carb makes a tremendous difference in the torque, midrange pull, top-end pull, and over-rev of your engine. If you have never jetted your bike correctly, you will almost certainly gain some performance at some point in the bike's powerband.
A cleanly jetted pilot circuit can be the difference between having to clutch the bike out of a turn or not. Hard starting when hot or cold, poor response when opening the throttle, reluctance to idle, all of these are symptoms of an improperly sized pilot jet or incorrectly adjusted air screw.
The needle can make all the difference in the world for the power of the machine in most situations, as it controls the throttle range that most riders spend most of their time using.
A correctly sized main jet could mean the difference between being able to rev out high enough to not have to shift one more time at the end of the straight, or the power falling flat on top and requiring you to make that extra shift.
Are you fouling plugs? Many people will tell you all sorts of band-aid fixes, from running less oil, to running a hotter plug. Both are incorrect fixes for plug fouling. It's all in the jetting. An engine that is jetted too rich will have combustion temperatures that are too low to burn the fuel and oil effectively, leading to deposits and wet fouling of the plugs.
Do you have spooge? There are the rare instances where a mechanical issue, such as a leaking wet-side crank seal, can cause spooge. But, by and large, this isn’t the case. In most instances, spooge is caused by rich jetting. It has nothing to do with how much oil you mix in the gas, or how hard you ride. An engine that is jetted too rich will have combustion temperatures that are too low to burn the fuel and oil effectively, resulting in deposits, plug fouling, and spooge. Spooge is nothing more than unburned fuel and oil entering the exhaust.
The only way to know what jetting changes you will need is by trial-and-error. No one can give you jetting specs, because every bike is different, every rider has a different style, and jetting is totally weather dependent. Unless the person telling you what jets to use is riding an identical bike, on the exact same track, at the same time, his recommendations are meaningless. Someone with a good understanding of jetting can get you in the ball park, but you need to do the testing to determine the correct jetting yourself if you want it right.
Jetting is fairly simple, and is a useful skill to learn if you ride a two-stroke and want it to perform at it's best.
It's very important that you start with the pilot circuit, because the pilot circuit affects the entire throttle range. When you are at full throttle, the main jet is the primary fuel metering device, but the pilot is still delivering fuel as well, adding to the total amount of fuel that your engine is receiving.
Before you start to re-jet your bike, you need a clean air filter, a fresh plug (actually you need several plugs to do plug-chop tests for the main jet), and fresh fuel.
One important detail: Make sure the engine is in good mechanical condition. If your engine has a worn top-end , fix it first. Trying to jet a worn out engine is a waste of time. The same goes for reeds that don't seal properly, and a silencer that needs re-packing. Worn reeds will mimic rich jetting, and worn rings will mimic lean jetting.
Before you start the jet testing, Install a fresh plug. Set the float level to the proper specs, an incorrect float height will affect yourjetting all across the throttle range.
All jet testing must be done with the engine at full operating temperature.
As already stated, start with the pilot circuit. Turn the air screw all the way in, then turn it out 1.5 turns to start. Start the engine, and turn the idle screw in until you get a slightly fast idle, or hold the throttle just barely cracked, to keep the engine idling. Turn the air screw slowly in, and then out, until you find the point where the idle is fastest. Stop there. Do not open the screw any farther, or your throttle response will be flat and mushy, and the bike may even bog. This is only the starting point, we will still have to tune the air screw for the best response.
Now is the time to determine if you have the correct pilot installed in your carb. The air screw position determines this for you, making it very simple. If your air screw is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet. If your engine doesn’t respond to air screw changes, then you either have a dirty carb, or the pilot jet is way too rich. When the pilot jet is way too rich, you are forced to use the idle screw to open the slide so far in order to keep the engine running that the pilot circuit is partially bypassed, and the engine is actually starting to draw fuel through the needle jet.
Once you have determined (and installed it if it's necessary to change it) the correct pilot jet size, and tuned the air screw for the fastest idle, it's time to tune the air screw for the best throttle response. Again, make sure the bike is at full operating temperature. Set the idle back down (the bike should still idle, despite what you read in the Moto Tabloids), and ride the bike, using closed-to-1/4 throttle transitions. Turn the air screw slightly in either direction until you find the point that gives you the best response when cracking the throttle open. Most bikes are sensitive to changes as small as 1/8 of a turn.
The air screw is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the air screw to compensate for changing outdoor temps and humidity. An air screw setting that is perfect in the cool morning air will likely be too rich in the heat of the mid-day.
Now, it's time to work on the needle. Mark the throttle grip at 1/4 and 3/4 openings. Ride the bike between these two marks. If the bike bogs for a second before responding to throttle, lower the clip (raising the needle) a notch at a time until the engine picks up smoothly. If the bike sputters or sounds rough when giving it throttle, raise the clip (lowering the needle) until it runs cleanly. There isn't really any way to test the needle other than by feel, but it's usually quite obvious when it's right or wrong. A too-rich needle can often be felt simply when revving the bike on the stand. The bike will sound rough and raspy when blipping the throttle on the stand. A correctly jetted bike should rev cleanly and crisply.
Last is the main jet. The main jet affects from 1/2 to full throttle. The easiest way to test it is to do a throttle-chop test. With the bike fully warmed up, find a long straight, and install a fresh plug. Start the engine, and do a full-throttle run down the straight, through all gears. As soon as the bike tops out, pull the clutch in, and kill the engine, coasting to a stop. Remove the plug, and look deep down inside the threads, at the base of the insulator. If it is white or gray, the main is too lean. If it is dark brown or black, the main is too rich. The correct color is a medium-dark mocha brown or tan.
Please note that, when reading plugs, the tip of the insulator, threads, etc. are meaningless for jetting purposes. They can tell you a lot of things, but jetting isn’t one of them. Only the mixture ring at the very base of the insulator, inside the threads, can tell you anything about the jetting.
The slide is also a tuning variable for jetting, affecting the throttle range from 1/8 throttle to approximately 1/3 throttle. If you can’t clean up the small-throttle jetting on your bike no matter how lean you go with the pilot or the needle, the slide is the next step. But few bikes need leaner slides.
Keep in mind, even though this article is intended primarily for two-strokes, four-strokes also need proper jetting to perform right, although they are not quite as fussy as their oil-burning cousins. The only real difference in the two is with the pilot circuit.
Two-strokes have an air screw that you screw in to make the jetting richer, and screw out to make the jetting leaner. Four-strokes, on the other hand, have a fuel adjustment screw that you screw in to make the jetting leaner, and out to make it richer. Their purpose is the same, they just do it in different ways
Well if it now 100% stock then the jetting just needs to be 100% standard. Maybe the jetting is still set for the full tune ? Either way, sounds like the shop either don’t know what to do or weren’t given the time to do it. All the indicators point to the carbs unless the sparks are poor. You really just need to swap on another set of ‘lender’ carbs and see what happens. Obviously in the current lockdown circumstances it’s not easy to do that. I have a set of 278s if needed. Got a bike actually but in bits at the mo ....as with most things around here !
Bike starts and Idles well ... just shows problem upon riding.
Hope to start going thru the basic tomoz ... ie checking jet sizes - needle position - float heights - ign timing. Would anybody suggest the oil pump set being part of the problem ? or would "too much "oil just encourage it to smoke more.
When carbs are off what indicates which slide is which ( any photos would be great ! ) and where specifically does the Tank Cap vent / breath ?
Hi all Finally got to checking all possible variables with the carbs etc Set float heights checked jets and so on. Needle position was away from book spec. Bike ran more cr***y if put right. Had half an idea the carbs were blocked from day one ! So left alone to contemplate ..... one of our TT mate's came back to us and suggested as the plugs were reading rich we were thinking " wrong " and it wasn't too much petrol but not enough air. Out came air cleaner and bike ran best ever. So this ( in my mind ) confirmed " not enough air " Ultrasonic'd carbs ..... it then SEIZED ... ARG ! The bike is now due to go back to the " shop " and to the mechanics credit , after hearing the tale said he'd sort it F O C . Hopefully more soon ....
Thanks for replies. Battery is new. Think what has happened is that after ultra - sonic cleaning the carbs any " crud " became dislodged ( or dissolved ) This allowed the carbs to breath substantially altering the mixture. We didn't revert the mixture screw or needle clip position to " factory set " and ..... well the rest is history 😣