Paul,

After making whatever adjustments per all the tips guys have given you,do the following when flying:

From safe height put the model in a fullthrottle dive (shallow 20-30 degree or so) to unload the engine. If the pipe length is correct, your engine will not pick up any rpm or lose any for the prop load you are turning.

If it goes fat, (it will likely burble some).....pipe is long for the prop you are using. Either increase prop load and/or shorten the pipe 1/4" and try again.

If it picks up speed,pipe's too short.....either lengthen 1/2" and/or decrease load.

This technique hasworked well on all sizes of glow type 2 strokes (including the big block 160's-180's), that depend on needle and pipe settings for ignition timing. It assumes you are using a good all around plugsuch as KB1L