Jeff,

A tuned pipe cut too short, will require, after advancing the throttle to WOT, that you close the main needle, to get the RPM high enough, for the engine to get on-the-pipe. Then, immediately you will have to open the needle back to prevent a lean-run.

It would seem your shortened pipe is close to this point.

Pull the pipe back, even beyond the 1/2" that you cut off.
The prop you are using is larger than the 10x6 size intended for this engine/pipe combo.

You did not say what fuel you are using.
It must contain some Castor oil and 5-10% nitro. The pipe supercharges the engine and will make 15%, or more nitro, too much.


Too short a pipe will kill your engine, as will too much nitro with a pipe, on a standard (non-racing) engine.