Was easier for me to replace the head on glow plug version. For comparision I run the engine first on original head and then on the shorter one.

Enhine: Poulan 25cc short shaft (weed whacker)
Exhaust: Original gutted out and drilled the muffler case with two holes 14mm dia. This is more like no muffler at all.
Carb: Original Zama C1U with 8mm venturi.

Fuel: 47.6% Methanol, 46.2% 91 oct gasoline, 6.2% oil (Motul/Castrol mix). I made the fuel : 40cc methanol, 40 cc gas/Motul mix (1:33), 4cc castrol oil. First I mix Castrol oil with methanol, and then I add gas/Motul mix. Shake it well and it looks OK.

Propeller: APC 17x6

Now the performance:

Standard cylinder.
Engine can easy start by hand. Peek 7020rpm , reliable idle 1760rpm

Shorten cylinder.
Engine can easy start by hand. Peek 7160rpm , reliable idle 1820rpm

Low end in both test was uneven. Engine was slowing down to quoted rpm but from time to time jump to 2000. However was idling like that for about minute and when I opened throttle full it immiedietly reved up without hesitation.

BTW, the glow plug needs to be powered only at start up. For running glow starter was unplugged.

Next before I mess up with ports I will try bigger carb. I need to find some cheapo walbro that will have 11mm venturi and will fit into the place.

RysiuM