YZ250 Poor top end

[johnnyl]

I bought a 2001 Yz250. Top end power seems poor. Bike pulls well down low, then runs out of steam feeling lean. It has new crank bearings and seals, barrell re-plate and piston kit, plug, air filter, new jets stock apart from a leaner 48 pilot. Ignition timing and float level ok. The power valve appears to be working ok but is it possible to check if the valve is timed correctly and opening fully? Any other ideas?

 

[Spine_Thrust]

Do you have a spark arrester type silencer? If so, take out the screen and see what happens. You might also check the reeds.

 

[johnnyl]

Thanks for the reply. The silencer is stock although needs re-packing, will do that today. The reeds also looked good last time I checked. I will test the bike later after the re-pack and may be able to give more information.

 

[Spine_Thrust]

No problem mate...did you use the dowel pin to help remove the power valve push rod assembly? If you don't use the pin you can bend the rod assembly and that would make the power valve function improperly.

 

[johnnyl]

I stripped and re-built the powervalve today, all seems ok. I don't have the proper dowel tool but used a screwdriver of the same diameter which worked well. Also re-packed the silencer.
Bike sounded good on startup, powervalve mech seems to work well. It is definitely opening fully.
Didn't get time to test so will do tomorrow, I will let you know how it goes.

Thanks John.

 

[marcusgunby]

Some 250s does go flat if you ride them like a 125, have you rode other 250s that felt like more top end?

 

[johnnyl]

This is my first motocross bike, I have not ridden any other 250 only smaller bikes. It feels quite fast but not nearly the white knuckle ride I expected. Doesn't lift the front like I have read it should. I am certain that the power dies off far too soon around about where I feel it should pull really hard.

 

[johnnyl]

Does anyone know how to test the solenoid and throttle position sensor on the carb?

 

[johnnyl]

Thanks for the replies guys, the problem is sorted.
The powervalve was not opening fully. :)

 

[trev]

I had the same problem. How did you set your power valve any help would be greatly appriciated

 

[johnnyl]

The power valve settings appear fixed.My problem was that the centre valve had some marks on it suggesting it was sticking at some point. I just cleaned and lubricated it. That worked for me. Have you torn down your powervalve? The reason I ask is that the ramps for the sub exhaust ports can be fitted reversed, this will give a flat top end.