Tom, something just isn't right. Timing, fuel mixture, something. My Kioritz 23.6cc turns an APC 16 x 8 prop 7900+ rpm's here at 4500 ft. and 90 degrees, with a measured thrust of over 10.5 lbs. It will turn the same prop around 8400 or more at sea level. I don't remember if you mentioned opening the exhaust up, but take out any/all baffles, and install two 7/16" or larger outlet tubes. My older 23.6cc is the best of the small Kioritz models, and I have modified it so don't expect to see quite the numbers I got, but you should still get very decent power out of yours. I think a 15 x 8 APC prop will probably end up being the optimum for you, and you should DEFINITELY see 7000ish if not more with that prop. I once cobbled together a 21cc Kioritz made out of a 16cc crankcase and a larger cylinder I had lying around. With nothing more than a larger carb and an opened up muffler I got 7000 plus on the 15 x 8 APC prop, here at my high density altitude. Keep after it, and good luck.

AV8TOR