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Post by Tobyjugs on Jul 3, 2020 0:25:12 GMT 1
Last weekend i was invited to go to Assen to watch some 250's hoon around the circuit. The agreement was to spend Friday evening at the circuit and Saturday was the day for the demo. Around midday on Friday Kees ( a lad in my village who i keep an eye on) asked me if his bike was ok to ride to Assen on Saturday. I checked it over and found he had not replaced the split pod filter on his bike. I replaced the pods with much larger filters from the same maker. I did think about the jetting and then thought "ah i cant be much different" then i went to Assen. Kees turned up on the Saturday and we both had a great day and loved watching little two strokes flying into the corners when the rest were braking to slow down. After the demo's we all packed up and went home. Kees was the first to leave, after about an hour i got a message which said his bike had seized. I think the jetting was really on the edge and when the bike went onto reserve it siezed. Here is the spark plug which came out the cylinder. 
Here is the rescue service who felt very guilty for being a bit lazy and not checking the carburation.  The above text is a bit of a story to lead to the thread headline. I changed Kees's engine for my old 400cc engine. Every thing was kept the same except the motor and reeds. When i test drove it it felt lazy and started misfiring at 9000rpm. I really thought it was ignition. It turned out to be exhaust and carb settings. The bike was using a 4LO engine, 28mm PWK's with 160 main jets, stuffed YPVS reeds and a set of TSA pipes and it flew. Now with the 400cc YPVS engine, valves pinned open, VForce 3 reeds and TSA pipes i had to drop the main jets down to 148 and it's still a bit rich. I dont have any smaller jets to try. I was very surprised to have to jet down for a larger engine using the same carbs. What do you chaps think about this? |
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Post by headcoats on Jul 3, 2020 7:49:15 GMT 1
That plug looks alright to me š±
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Post by midlifecrisisrd on Jul 3, 2020 8:45:00 GMT 1
All depends on how the motor is sucking and how effective it makes the venturi effect of the carbs
My 385 is similar on the pilots
Everyone with a ypvs, pwk's, tsa's and using an airbox has a 38 pilot
I'm on a 36 and it still machine guns on neutral throttle between 3 and 4k
I've got 34's to go in it but never got round to it
This horrified people as they can't believe it needs to go so lean
Steve
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Post by beardy on Jul 3, 2020 8:54:21 GMT 1
40 odd years ago I read an article by a 2 stroke tuner (canāt remember who). Basically he said that a main jet in a particular carb on an engine can only give an amount of fuel depending on the airflow. It wonāt be giving its potential maximum.
He then went on to say that if the airflow is increased then the main jet may need to be reduced in size?
Iād forgotten this article until I read your post.
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Post by beardy on Jul 3, 2020 8:56:15 GMT 1
Venturi effect. Thatās a word I should have used šš»
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Post by reedpete on Jul 3, 2020 9:03:18 GMT 1
Two things to summarise that might help.
1. The reason we need to change jet at all is because even the best designed carbs are not linear Airf/fuel mixing devices , meaning that the amount of fuel picked up does not give the same AF ratio for all flow rates. (Let alone that fact that different AF ratios are required in different operating modes.
2. The āsuckingā as Steve called it....Ie the shape of induction pulse , both time and magnitude, essential determines the opportunity the jet has to deliver the nescessary fuel. so a smaller jet that is flowing for a longer time can actually deliver more total fuel than a big jet that flows for a short time.
Thatās as brief as I can make it but hope that answers the seemingly conflicting reality of an engine thatās using more fuel having a smaller jet.
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Post by Tobyjugs on Jul 3, 2020 9:08:10 GMT 1
That plug looks alright to me š± Too lean for constant open throttle on a motorway. |
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Post by headcoats on Jul 3, 2020 9:09:13 GMT 1
Plus it's the old debate of can you seize going onto reserve !
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Post by Tobyjugs on Jul 3, 2020 10:35:17 GMT 1
Plus it's the old debate of can you seize going onto reserve ! Both plugs the same colour and it was the carb with the shortest fuel line. The only thing i have done so far is a leak test and endoscope inspection. No leaks found  |
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Post by hoist1 on Jul 3, 2020 10:51:21 GMT 1
Plus it's the old debate of can you seize going onto reserve ! Probably, if things are borderline and the bike is subject to the right (or wrong) conditions. |
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Post by headcoats on Jul 3, 2020 11:19:51 GMT 1
Silly question I guess but would putting "equal" length fuel pipes on the tank to the carbs potentially stop the one cylinder wanting to run on while the other side was empty , while going onto reserve ?
Or would it just seize both sides !
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Post by Tobyjugs on Jul 3, 2020 11:21:51 GMT 1
Plus it's the old debate of can you seize going onto reserve ! Probably, if things are borderline and the bike is subject to the right (or wrong) conditions. Totally agree as i've leaned it off much more for sprints and this bike has been ragged stupid on this setting but it is always on off with lots of throttle movement in between. Now with a head wind and constant throttle in one area i think it was on the limit and that fuel starvation with an open throttle slide was "the straw that broke the camels back". |
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Post by hoist1 on Jul 3, 2020 12:53:40 GMT 1
It can be a bit of a balancing act, when they go on reserve they seem to not give much time to reach for the tap. On another thread someone always tops up before reserve, something Iām going to do, keep an eye on the mileage and perhaps even switch over early as a loss of power can be more than inconvenient. Like while overtaking.
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Post by marrcel on Jul 3, 2020 20:23:02 GMT 1
There is no electrode left on the plug. Signs of overheating.
Replacing with larger pod filters you need to up the mainjet at least 2 sizes. Especially on track with lots of wot.
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Post by hoist1 on Jul 3, 2020 21:19:23 GMT 1
There is no electrode left on the plug. Signs of overheating. Replacing with larger pod filters you need to up the mainjet at least 2 sizes. Especially on track with lots of wot. I wasnāt sure about the pic, itās like those pictures when you can see 2 things. One of cogs the other day, it made my eyes go funny to distinguish the slot from the tooth. Perhaps a side on pic. |
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Post by Tobyjugs on Jul 3, 2020 22:21:52 GMT 1
There is no electrode left on the plug. Signs of overheating. Replacing with larger pod filters you need to up the mainjet at least 2 sizes. Especially on track with lots of wot. Hi Marrcel, the electrode is fine. Its an EG type plug. What Kees experienced reminded me of your experience last year. Normally i jet much richer, this is a case of me being lazy and saying nah it will be ok. |
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Post by marrcel on Jul 4, 2020 20:08:56 GMT 1
Thats a lesson for me.
I remember that Piet said: up the mj before you go on track. If i am lazy than probably have to pay a price. š¤
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Post by Tobyjugs on Jul 5, 2020 9:33:11 GMT 1
Thats a lesson for me. I remember that Piet said: up the mj before you go on track. If i am lazy than probably have to pay a price. š¤ I would still go one jet up on your set up if your going to go long distances. It's that middle section that can also be dangerously lean on the 4LO set up. |
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