Here are some engine performance numbers I got with the little NGH 9cc gasoline RC engine the other day, when I was test running it.
I was using some Master Airscrew plastic fiber filled propellers and a APC propeller too.
MA 11x6 11,700 RPMS
MA 11x7 11,500
MA 11x8 11,000
APC 11x8 10,700
MA 12x6 9,800
Engine low speed idle was pretty good down to around 2400 rpms, but going lower usually results in a slow cycling surge like effect occuring where the RPMs increase and decrease, up and down.
Using a Master Airscrew 11x6 prop.
Master Airscrew 11x7 prop
Master ASirscrew 11x8 prop
Using a APC 11x8 prop
and a Master Airscrew 12x6 prop
engine idle speed with a MA 11x7 prop
Using a MA 11x7 prop shows the engine running pretty decent on temperatures too. Ambient temperature was about 82 degrees F at the time. Not like when it is 110 degrees F ambient temperatures during the summer. This measurement is at the base of the spark plug on the head right there. Off the back of the head of the engine the temps are around 225 degrees F.