I know there has been some talk of props here. Anyway I had what I thought was a strange experience with an ST60. I have a 3 blade 12 x 6 (closer to 5 pitch) carbon fiber prop that I bought Kaz. Beautiful prop and finished really nice. I put it on my ST60 and it ran terrible won't hold a needle setting and the engine sounds rough like a bad plug or fuel. So of course changed the plug and changed the fuel and cleaned the needle. It ran so bad that I refused to put it in the air until I got the problem sorted out. Waited until the following day and tried it again and had the same results. Pretty frustrating. I had never used a 3 blade before. But for no real reason I went to a Bolly 2 blade 12 1/2 x 5 1/2 carbon prop. Same fuel, same plug, same setup, and I fired it up and within 5 seconds the engine broke into a solid 4 stroke and ran as sweet as ever. Lifted the nose up and it broke into a 2 stroke and it sounded really great.......So I says thats weird, musta been dirt or something so now that's it's fixed I'll try that 3 blade again. Same fuel, same plug, same setup fired it up and it ran really crummy again. Didn't respond well to needle adjustment and sounded like it was missing definitly not right. Took the 3 blade off and stuck the bolly back on, same plug etc. etc. and it broke into a solid 4 stroke and ran as sweet as ever again.....I feel like I'm in the twilight zone or something?

Hi Greg,

Just "off the cuff" from miles away, sounds like it just loads the engine (set up) too much.  Might need a different venturi, different compression ratio, etc..  You could start cutting the diameter down a *LITTLE* bit at the time, but man, for the cost of a CF prop, I hate to do it! LOL!! (unless it has been done before and I know it works)

Funny thing is, it might work perfect on my ST .60, and maybe not.  I have noticed that for whatever reason, the same engines can run slightly different, even if they were reworked by the same person.  The modern purpose built engines show that phenomenon less, in my experience.

Bill

It sounds to me like the three blad prop is just more load then the engine can turn at its present level of tuning. A good rule of thumb was always to drop an inch in diameter when going from a two blade to a three blade prop.          Phil