AT 8000 rpm's

It's acts like it hits the rev. limiter, that's at 9000. Any ideas? I back off the throttle a tad, and it smooths out!!!!

Carb or EFI? How's the fuel pressure? Tried a different battery (I don't care how hot you think that one is, try another). :)

Also, try a smaller prop...that will allow you to rev further for the same speed. If it will turn past that RPM with the smaller prop, you may just be out of HP.
What ECU do you have on it? Digital or Analog? A??

It's a carb. 2.5, I back off the gas a bit, and it smooths out and climbs to 8500... Then when I give it full throttle, back to that bumping( so to speak)

Look at your butterflies, on my carb'd 2.4 they could be set, so that at w.o.t they would actually start to close on the wide open end. Keith's right about the fuel pressure. 6-8 lbs no more no less, any more and you'll blow the needle of the seat at idle any less and you'll starve the motor. Do you run pyrometers? If so what temp is the exhaust when this "bumping" is starting? Also have you adjusted the timing? You don't want it to start detonating. Trust me it's not pretty when they do. burningm Later, Gary.

Check your fuel filter, check your fuel pressure.
To much air and not enough fuel = a lean condition. When you back off the throttle it smooths out because you now have the less air.
Think of it this way, If you have to little fuel flow the motor will only run good until you put to much air to it.
That's just one thing to check, there are factors.
You Te