just got a YZ 250. it wont crank

[Tommygunn]

hey everybody..im new here. i just picked up a 95 YZ250. the guy i got the top and bottom end rebuild back in march of last year. he said he got it back from the dealership and fired it up and it ran for about 15min and when he would give it over 1/2 throttle it would cut out like it was getting too much fuel.

well i got it anyway because of the price and it looks brand new. we could not get it to crank, my foot is killing me from kicking that thing so much. i ended up having a buddy pull me down the road on it and it fired up doing that. seemed to run good but once you would give it over half throttle it would bog down. also will not idle. i have checked the main jet and its clean. the entire carb is really clean. gas is new and mixed right, spark is good. reeds look pretty good, and the key on the flywheel looks good too. sometimes when you kick it over it will make a backfire noise. i just pulled the cover off for the power valve on the left side (if your sitting on bike). behind the cover everything was black from the oil but im sure thats common. i pulled the other piece out thats behind the cover and looked at that. it was alittle dirty but i think that is just a stop. i took an allen wrench and stuck it on there and turned, the power valve seems to not be stuck. it moves back and forth pretty easy. should i take it all apart and look even more coming in from the other side?

this is just got me so confused.. feel like i have looked at everything but something isnt right. the engine has great compression and everything is new. i really want to get this going so i can ride it but i dont really have the extra money to take it somewhere to have fixed.

thanks in advance,
Tommy

 

[sick 96 250]

For easier starting I would focus more on the reeds, take them out completely and look and see if they are sitting flat on the cages with no gaps. It could also just be a pilot jet issue for starting as well. Have you tried to turn the air screw in and see if it helps? If its not getting past half throttle it does sound like a main jet issue, could also be the powervalve is adjusted wrong or not even connected properly, esp if he put the whole motor back together and had the same issue then. I would check that real good and clean the carb yourself, not just pull a jet and say its clean, completely remove the carb, disassemble and clean the crap out of every hole in there.

How is your clutch adjusted as well? I never had it happen to me but I have seen people over adjust there clutch way to much and when it would try to get into the higher RPM's just slip like crazy.

 

[rmc_olderthandirt]

I am a little confused about the sequence of events. The previous owner had work done at the dealer and when he got it back it wouldn't run past 1/2 throttle? Why didn't he take it back to the dealer and have them fix it?

I am a little suspicious of the power valve. It is possible that the power valve itself is OK but it has become disconnected from the governor or the governor is all messed up. If you don't do it right you can really mess up the linkage when you take it apart to rebuild the top end.

If the bike did run OK when the previous owner got it back from the dealer and then all this started happening after it sat for months then I would suspect the carburetor, probably the pilot jet.

When you were trying to start it did you check the plug? Was it wet or dry?

Normally when you are unsuccessful at starting the plug will be wet, if the carburetor is working even halfway right. If the plug is dry then you have a fuel delivery issue. It could be that there is a clog at the tank that simply isn't letting enough gas into the carburetor, although I would expect it to start and idle OK.

When you got it started did you run it long enough to get it warmed up? Another possibility is that there is raw gas down in the lower end. If this is the case it will run very poorly until it is all gone.

 

[Chris888]

you should change the reeds even tho they look good. I had the same problem with my yz 250, i messed around with the carby and pulled it apart about 20 times in 1 week, but still no difference. then i put the reeds out of my yz 125 in it and it went perfectly fine, put the 250 reeds back in and it worked again but not as good as it use to.
my reeds looked fine but turns out they werent.

 

[FNG]

The needle/seat could be worn out which could have caused a lot of fuel to pour in to the motor/pipe. If the pipe is full of fuel it will run on the bottom but cause troubles as the RPM's rise.

I had a similiar problem on my 95' when I rebuilt the top end last fall. I put 1 piece in the governor linkage on backwards and it caused the powervalve to not open.

http://www.yamaha-motor.com/sport/parts/home.aspx

It is the part right above the #7. It has to be installed in the direction shown. It will "fit" both ways but will not work.

Make sure the pilot is clean as this will cause the bike to start hard and also not idle.

As for reeds, I just picked up a RAD valve so I have a set of Boyesen Superstock reeds with 3 rides on them. When I replaced them I found a rip in my intake boot. I tried to seal it using some stuff my buddy uses in the aircraft industry but it could not hold up to the vibration.