Mine has run great with the Zinger 20X8 and 18X12. Last week I broke my 18X12 and tried to use the Testor 18X12 and never was able to get the engine to run anywhere near correct with that prop. Went back to the Zingers and it tuned right up and ran smooth again??? All I can figure is the Testors are way lighter then the Zingers and the engine likes A heavy prop?? Guess that's why we are supposed to try different props and see what the engine/plane likes best. Still odd though.