I am at sea level. I have a few of these (conversions but the same engine) and mine fly best with a 21/8 prop. About 7200 RPM. Tried 22/8 Menz props but could get maybe 6700 or so and being hot and humid here for 8 months a year the RPM will sag. As for priming, check for cracks in the neoprene tube between the case and the carb.