What is it like when you are over revving?

[rablack21]

Here is what I am trying to figure out. I have a 88 motor with stock jug with spark mx pipe, 38 A/S, and rad valve. This is what it is doing. I can open the throttle anywhere from off idle to Wot and it revs nicely and pulls hard. But once the rpms get to the top, the power just kind of drops off and it almost makes a spuddering sound. I can then shift gears and open the throttle all the way and it will pull hard again till in reaches high rpms. My question is this, am I just over revving it? Have I reached the top of the powerband and am going past the point of power?

[wilkin250r]

Yes, and there are a variety of reasons it acts like that. Your particular case, one reason will have more influence than another, but I can't tell you which one without being there in person.

Your bike does drop off power at high RPMs. As the engine revs higher and higher, it takes more and more power just to keep it running. Eventually, you get to a point where it takes more power to run it than the engine actually produces at that speed.

Your CDI is programmed with a rev limit, for protection. Extremely high revs can create too much stress on the engine, so for protection, the CDI will cut the spark before it gets to that point. Once the revs drop back to a safe level, it starts sparking again. But it happens so fast, it doesn't feel like a surge or anything, it can stop and start the spark all in a matter of a few revolutions, the end result sounds like a sputtering. This is very likely what you are experiencing.

These carburetors aren't perfect. They actually get richer as the RPMs increase. So going from 4000 to 8000 rpms is a 100% increase in airflow, but ends up about 120% increase in fuel. It might just be sputtering and low on power because it's just incredibly over-rich at that RPM.

[rablack21]

Thanks for the help. You are I are on the same page. That is exactly what I am describing. I did double check the jetting, it is pretty much dead on without getting lean. I figured I might be running into the cdi limiter. Is porting still the best way to broaden the powerband?

[rooster300ex]

The more u ride it u will be able to tell when the engine isn't making anymore power its just revvin higher. shift when the engine stops pullin hard.