1996 Yz250 "Rev limiter" Problem

My First Post

NEED HELP

I have a 1996 yz250 2stroke dirt bike.

 

I bought the bike about a month ago, it has a new top end and fmf gold pipe on it. The guy told me the bike had to be rejected for the pipe. so i rejeted the bike to spec.... i have been having issues with 3/4 4/4 throttle range in id say 1st 2nd 3rd gear. Know i figured it needed a bigger jet so i went up a jet size. nothing . ran a little fatter but still had problems in that 3/4 4/4 throttle range. It tends to feel like a hesatation or some what like a rev limter. I figured the bike was starving for fuel. i changed the main jet about 5 times along with various clip postions and no change in 3/4 4/4 throttle range. So i decided to rule out the carb.(cleaned the carb at least 10 times) I also checked to make sure the power valve was in working condition and it is . I also repacked the exhaust and put in a new air filter. Compression is good and reeds are in good shape. I’ve also tried a couple spark plugs but no change in performance .Spark looked good last i checked.

 

Now I’m starting to feel it’s something electrical . Possibly a bad stator? CDI? or Ignition coil? Can something go wrong where it doesn’t let the bike rev past a certain rpm? (in my case the last rpms of each gear) I also checked the wire harness that runs to the case and cdi box, everything checks out and is in good condition.

 

I’m starting to think the stator is going bad or possibly a wire is cracked... i don’t know too much about that part of the bike, but could that be the cause of the rev limiter feeling I’m getting?

 

any comments will help :)

in my experience those components like CDI or the coil wont have a repeatable range like you are saying for flaking out.

whats your float height set at?  if its too low you could be starving for fuel.  No air leaks in the rubber carb boot between the carb and reeds? or reed gasket?

how new is the top end? have you checked the compression?

less than an hour on the topend and I will check the float height and adjust accordingly .

 

thanks

in my experience those components like CDI or the coil wont have a repeatable range like you are saying for flaking out.

whats your float height set at?  if its too low you could be starving for fuel.  No air leaks in the rubber carb boot between the carb and reeds? or reed gasket?

hey I changed the float height in hopes it would be the issue but had no luck. after ruling out the carb and the electrical (cdi stator etc.) I'm starting to think its possibly a mechanical issue. The guy I bought the bike from did a new top end rebuild and is it possible he put the piston in backwards or possibly an upside down piston ring? I'm thinking its pretty possible this guy could of done something like that incorrect. I don't know how the bike would run if the piston wasn't set right. what's your opinion on that? thanks for you help shift1313

You can rule out the piston being backwards. It would have snagged a ring on the intake port. I would run a compression test and really make sure the power valve is completely working, if its not fully opening and closing that can be your issue. You should also check ignition timing, if its completely advanced (maxed out on stator plate) your top end power will really suffer. And I'm not sure on that year bike but if it has a TPS or the fuel cut solenoid unplug them see what happens. Well that all the ideas I got for now. Good luck!

+2 on the power valve

Did you ever try to go the other way in jetting(leaner)? I have seen rich jetting on the top end keep the bikes loaded up so bad they would not clear out the extra fuel. Is it blowing alot of oil out the exhaust? I had a 98 that never ran right until I replaced the PJ carb with a PWK from a KX bike. The stock needle was too rich.

You can rule out the piston being backwards. It would have snagged a ring on the intake port. I would run a compression test and really make sure the power valve is completely working, if its not fully opening and closing that can be your issue. You should also check ignition timing, if its completely advanced (maxed out on stator plate) your top end power will really suffer. And I'm not sure on that year bike but if it has a TPS or the fuel cut solenoid unplug them see what happens. Well that all the ideas I got for now. Good luck!

compression is 250 psi . and the power valve doesn't stick, I pulled the cover off and worked it up and down with my finger and it works well.

 

how new is the top end? have you checked the compression?

the top end is brand new. But I measured the compression and it is 250psi. that seems very high to me. I could be wrong. the engine is bone stock

compression is 250 psi . and the power valve doesn't stick, I pulled the cover off and worked it up and down with my finger and it works well.

the top end is brand new. But I measured the compression and it is 250psi. that seems very high to me. I could be wrong. the engine is bone stock

You should check the squish clearance. That compression is very high, that will cause top end power loss. You might need to run a thicker head or base gasket... also check the PV governor

Did you ever try to go the other way in jetting(leaner)? I have seen rich jetting on the top end keep the bikes loaded up so bad they would not clear out the extra fuel. Is it blowing alot of oil out the exhaust? I had a 98 that never ran right until I replaced the PJ carb with a PWK from a KX bike. The stock needle was too rich.

I cant recall if I tried a leaner route. When I bought the bike the seller said it needed to be jetted for the fmf pipe he just installed. so ever since than I went up a jet size. ill buy some leaner jets and see what happens. I know there's a different needle than stock in the carb. I cant remember if its leaner or richer but i will check. Maybe try changing needles?

DID YOU EVER FIX THIS?   IM HAVING A SIMILAR PROBLEM WITH MY KX250 AND IM SUSPECTING IT IS THE CDI...     BUT I DONT KNOW HOW TO PROVE IT,    PETALS ARE GOOD, CRANK SEALS ARE GOOD, COMPRESSION IS GOOD  MOVED JETTING UP AND DOWN  ENOUGH AND STILL NOTHING!

I just had this problem on my 2000. The PV linkage was worn and not opening the PV fully. Fixed the linkage, fixed the problem.

Linkage could have been set wrong when he did top end.

I just had this problem on my 2000. The PV linkage was worn and not opening the PV fully. Fixed the linkage, fixed the problem.

What bike man? Thanks for the advice

yz250

Just a possiblity, but with a bike that age sometimes the needle jet gets worn out

(the fixed orifice on the bottom of the carburator's venturi where the needle slides into)

 

that is equivalent to having a smaller diameter / narrower taper needle which changes the metering at about 1/2-3/4 throttle

(clip postion mostly affect initial needle movement at about 1/4 throttle)

 

The 'stuttering' feeling (also feels like hitting a rev limiter) is indication of overly rich fuel metering.

 

Have a closer look inside your carb, not always visible to the eye but if that needle jet orifice shows

signs of wear / is oval shaped, that could just be the issue with your 3/4 throttle hesitations.

At that age, if original even the needle itself could be worn to a ever so slighly smaller (richer) diameter.

 

EDIT...just noticed the original post dates from 2013....hoping the OP got it fixed by now!

Edited by mlatour

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