That is strange, another member of our club just purchased a 91-cd and it runs great on a 14x6 and on a 12x9, personally I am getting about 9400 - 9600 RPM on a 14x6 APC, idle is reliable at 2100-2200, with glow it idles nicley at 1700-1800 RPM, How much run time do you have on that engine. Oh by the way the fuel I was using was Powermaster 15% nitro with 18% synthetic oil mix 6% of which is castor blended