There seems to be kind of problems with bigger props. 14x6 or 15x6 are a bit too heavy. If you stay with 13x8 or 13x7 they look lot more 'acceptaple' by this engine. It is not low RPM engine as many conider it. Also, over 10% nitro is not making any good, just heats too uc the engine. OS8 plug seems optimum. Cheers, Nick.