ORIGINAL: 2slow2matter
OK, I checked and indeed I have a 13 X 6 prop on mine. That is definitely the ticket as it swings it with authority, so fix your carb problems, and go up to a 13 X 6 prop and you will love this engine.
Or, a 12 X 8, but I would more prefer the 13 X 6 if it were me....
I agree that 13x6 is the correct load for a 70 four stroke. 12x8 will work fine too. With a 13x6 prop and 15% nitro fuel you can expect about 10k rpm. If you get about 10k your engine is running correctly.