94 Yz250

[dirtynogood]

i recently bought this bike from my cousin,a 94 YZ250,he has ran it alot,but has also taken good care of it. i was resently out riding when in the middle of the ride it wouldn't hit the powerband anymore. it runs fine and revs up fine in nuetral.when i try to crack it ,all it does is bog a little and through a little black smoke.it is almost like the choke is stuck.i have changed the reeds, cleanded the power vaulve.my cousin just changed the piston,rings,and all the inside so iknow those are in good shape.i do have to change the head gasket. but other than that i am stumped.lif anyone has any thought on the matter, any info would be helpfull,thanks

 

[Garryese]

Black smoke is indicating a carb problem. Check the choke cable free play and check that vent lines are open. If these check out then remove the carb and clean it; check/set the float height when re-assembling.

Why does the head gasket need changing?

How many hours since the new topend? What kind of piston and rings?

How many hours since crank seals were changed.

What color is the spark plug? Can you do a cutoff test with a new plug?

Do you have the ability to do a compression test and a crankcase vaccuum test?

 

[dirtynogood]

it is leaking out the shifter side,about a half inch break in the gasket. the top end is new.i had to use some crappy oil to mix with the gas i hope that wouldnt be all it is? but i will take the carb apart thanks for the input.

 

[dirtynogood]

the sparkplug was a little bunged up not to bad but i chaged it anyway

 

[dirtynogood]

im not sure what a cuttoff test is and i dont have a compressin gauge

 

[Garryese]

Cutoff test:

Warm up engine with an old spark plug. Install new plug. Get machine into high gear quickly and then hold throttle wide open for 1/2 to one mile. While still hold throttle wide open use kill engine with kill switch. Remove spark plug and check color on insulator. Also check for forgien deposits.

A one half inch hole in the head gasket will cause your problem. Actually, I am surprised it runs with that big of a hole.

Top end being new is relative. OEM piston and rings might last 15-20hrs; Significant losses at 8-10 hrs. Competition piston and rings will last 5 - 8 hrs max, with significant losses at 2-3 hrs.

 

[dirtynogood]

thanks for the info garryese.i figured it out when i took the carb apart. the jet on the bottom had unsrewed,well at least i hope thats what it was,im sure it was.i just cant check it till i get the head back on.