A forum for owners of Yamaha TTR250 trail and enduro bikes!

BennVenn · Member

Engine 'misses' on rapid release of throttle anywhere from 1/2 to WOT...

ttboof · Super Guru

yep I had this prob and it is annoying , there will be a few responses to this no doubt but im my case when I went to the stain tune I went up a size in the pilot and main jet only to have the symptoms you describe . once I went back to the standard main all was good as it turned out I believe it was a bit rich and with the increased vacuum at the instant the throttle was closed it hesitated for a second until it cleared .

to back this up the main jet is in play above 1/4 to full throttle taking the pilot out of the picture .

depending on the bike altitude ext you may be able to cure it with a needle clip change but I went for the quick fix and went back to the jet id taken out (no point in being slack unless your good at it )

also opening up the air box is part of the stain tune and jetting mod if not done may richen things up as well .

as said that's my experience and may help but give it a few hours and im sure more advice is on the way .

im in ozz as well

BennVenn · Member

Thanks for the info ttboof,

I was trying to think of it from the engines perspective and what you say makes sense. Over rich at WOT and backing off still leaves more fuel in the manifold than necessary. Explains why it clears up right away, and why it happens when only the Main jet is active. I'll ride it to work tomorrow with the airbox cover off to see if it helps at all. Maybe all it needs is a bigger airbox opening (though doesn't exactly makes sense why I never had this problem until now). Might as well order a stock jet too (150?)

Ben

ttboof · Super Guru

the type of riding might be the deference as its annoying in the dirt but a real pain on sealed roads .

as for the jet im not sure on the original size just reverted to the my original I took out , but im sure confirmation isn't far away

TTRfan · Super Guru

A quick check to see if it is running rich would be to do back to back runs with and without the airbox cover on. If it runs better with the airbox cover off then it indicates the mixture is too rich.

After standing for that length of time, is the air filter flexible and not clagged in dried oil?

Brian

BennVenn · Member

Hi Brian,

The filter was a bit old, wasn't hard or clogged. Gave it a wash and re-oil too. I've got another arriving tomorrow at my local Yamaha dealer so I'll fit that and see if it helps. I took it for a ride with just the filter cage fitted and it's hard to say if it gave any improvement. It certainly revved a lot higher and pulled harder at the top end (I had never hit the rev-limiter before). I just fixed that leak in the exhaust so I'm keen to see if that helped. I ride to work around 4.50am in the morning and it was less noticeable in the cool morning air than peak hour on the way home. I suppose that indicates its rich on its own.

I've got a set of jet drill bits for jetting my VW beetle, So I'll probably order 3 or 4 stock pilots and mains and see what performs best. I'll try what ttboof suggested with an oversize Pilot and stock Main to begin and go from there.

Ben

BennVenn · Member

Just an update,

Couldn't remove the airbox cover because of the amount of rain this morning. The exhaust fix didn't make any different to the symptoms. Release of the throttle just after throttling on exaggerates the symptoms whereas after a second or two of WOT and releasing it isn't as dramatic. Which confirms the over rich theory with the TP adding even more fuel to the already rich mixture.

I've ordered a 52 pilot to fix the hunting and a 140 main. It may still be too big but at least I'll know what is in there and can go up or down from there.

It's odd that I'm only experiencing these problems now. Perhaps its less noticeable on the trails, or the perished intake manifold was leaning the mix enough to mask the problem. Either way, I'm hoping the new jets will fix the problem - Or I'll find a stock exhaust to put back on the bike (I'm sure the neighbours will appreciate that)

I'll update when the jets arrive ( for those interested, found the compatible Mikuni jets at www.motorcyclespareswarehouse.com.au . A little more expensive than those available OS. I would have liked to buy a range of Mains)

Out of curiosity, how hard does your TTR pull towards the higher RPMs? Is it strong all the way to the limiter? Or does it kind of drop off? I've never hit the limiter until yesterday which is making me wonder has my bike always been this disgustingly rich?

Ben

locky · Guru

I have the Mikuni jets ready for if i decide to mod the airbox and fit a FMF that i have . From all the info on the net they recommend (Mikuni) 52.5 pilot and 140 main jet for the airbox and exhaust mods.
The 140 Mikuni jet is equal to a stock 150 jet .
Stock jet sizes are equal to Dynojets.

vmaxoutlaw.com/tech/dyno-mikuni.htm

www.4strokes.com/tech/carbjetxover.asp

-- Edited by locky on Friday 27th of February 2015 12:00:36 PM

ttboof · Super Guru

mine runs well ridden as a trail.bike .and i ride in grass track days riding it up in. the revs and it loves it .never hit the illusive limiter though

-- Edited by ttboof on Friday 27th of February 2015 10:19:00 AM

BennVenn · Member

Thanks for the links Locky, I've stumbled across those a few times in my searches.

ttboof, maybe it isn't a CDI rev-limiter, could be anything from valve float, overlean etc... Certainly never reached it before removing the air filter though.

BennVenn · Member

OK, so new jets are in. The hunting that was just above idle is almost gone but can be found if you search for it. So I'm more or less happy with that. I feel it is idling very rich as the revs drop after 10-20 seconds of idling. I'll have to pull the plug to be sure. The mixture screw has a lot more effect than it did with the 50 pilot in.

The main must be larger than what was in there, the missing on backing off the throttle is noticeably worse. I'll swap mains over tomorrow and see what effect that has. I'm yet to ride it with the air box cover off, that should make some kind of difference and give an indication of which way I should go on the Main. The bike sounds much more 'crisp' if that makes sense.

I was really hoping the new jets would fix the problems. Ahh well. I'll keep an eye out for a stock exhaust, that with the original jets should give me a nice smooth bike for city riding. Or just buy a DRZ and keep the TTR for the trails...

I'll keep this thread posted with any developments.

There is one more thing, I swear there was a spark arrestor when I first got the bike. There definitely isn't one now. Do you think the added back pressure could make all the difference?

Ben

ttboof · Super Guru

the missing spark arrester shouldn't make any noticeable difference . from what you describing I get the feeling something's just not quiet right . im thinking about it and its good you are keeping us up to date .

I have the stain tune exhaust ext and running spot on

BennVenn · Member

It hasn't always been this way so I could put it down to something wearing or degrading. The carb is (with the exception for the new jets) as new from the factory. There is no wear and all passages are clear. It's probably time to start looking elsewhere.

I'll pull the rocker cover tomorrow and check clearances and if the shims/lifters might be sticking. Flat spots on the lobes could also cause this kind of thing (excessive overlap, exhaust flowing back through the intake or the more obvious exhaust gas scavenging)

I cant get my compression tester in there because of the unusually small plug diameter. I'll see if I can borrow a mates and do a wet/dry compression test.

My girlfriends KLX cracked a ring landing and behaved all kids of crazy, poor idle, backfiring, missing but was OK at freeway speeds. Got us around tassie before stripping it down to find a cracked piston. So it could even be compression related.

Might as well photo-document the whole process and add it to a thread. Someone might find it useful