ORIGINAL: gpar
My spe 26 turns a Menz 17x6 at 8700 rpm. I had to change the original carburator for other with a bigger venturi.
The original carburator wt-500b is 9.53 mm venturi. You have to replace it for other ranging from 11.1 mm to 13.4 mm.
gpar, I found BCMA sells a 11.1 venturi Walbro carb and want to swap carbs. Did you enlarge the insulator block opening to match the new larger carb opening?
When I tested the engine with the original carb, I was getting 7500 rpm with a APC 16x8.