yz 250 unpredictable power delivery at less than 100% throttle

[scottiedawg66]

I am tuning my yz 250 that I am racing at pikes peak this year at the supermoto track. In the sweeping corners when I need 20% throttle the power delivery is not smooth, and makes it really hard to hold a consistent line and or not spin the tire, (full slicks).

i am betting its a jetting issue, but I would not be surprised if it was a throttle position sensor or power valve issue.

the bike goes like hell at WFO and is smooth. Its just at partial throttle that I have a problem.

what yall got?

 

[dirt bike dave]

Sounds to me like the powervalve wants to open.

Not sure about the YZ, but some bikes have springs that will let you adjust the powervalve opening. IMO, you want it to open at a higher rpm than it is now.

If the PV is not adjustable, you could try richening the pilot jet (or turning in the airscrew) and see if it helps - throttle response on some bikes is very sensitive to this adjustment.

Seems like for your application you would trade a little low end snap for a smoother power delivery. Maybe a flywheel weight would help?

 

[scottiedawg66]

thanks for the advice dave. you are correct. I do not need that low end grunt. I need top end power, and smooth delivery.

the PV is adjustable on my bike, I will look in to that.

are flywheels measured for weight complete, or are they taken apart?

 

[FNG]

Flywheel weights go directly onto the existing flywheel. My experience with a flywheel weight on a '95 YZ250 was it gave me more bottom end, smoothed out the hit and actually gave the bike a little more over-rev. Flywheel weights spread out the power and soften the hit would be a good description of what they do.

Also X2 on what dirt bike dave said.

Check this site out:
http://www.steahlyoffroad.com/flywheel-weight.html

 

[BSWIFT]

I would check the timing. Your ignition advance & pv opening points do not match. Got a broken leg with that problem once. You described it to a tee.