Piet Le Roux

I recently bought an OS91FX. It is mounted inverted in a pattern plane with the standard muffler. The tank is as low as it can go but the carburetors height is about in the middle of the tank. I am using Cool Power 15% and the engine has had about 10 liters (2 gallon) of fuel through it. The prop is an APC 13X8. Problem is that it sounds like its missing badly while it is running at 50% throttle, when you open the throttle the missing goes away and it peaks at about 10800 with the 13X8 prop. Plug is a #8. Idling is good and transition is good, I have never had a dead stick. Will this mid range problem clear up when it's run in properly? How long does it take to run it in?

Bax

My Feedback: (11)

Less than smooth running in the midrange is very common in model engines. That's because the carburetors are not linear. If the top-end and idle mixtures are set correctly, and the engine doesn't quit when run for extended time in the midrange and also has a good throttle-up and throttle-down response, then don't worry about the midrange.

You may need to check your high-speed needle setting. It's possible that it's a bit rich, yet. We usually use a tachometer to set the needle when we're running large props.

Piet Le Roux

Thanks for the advice Bax. I am planning to “tie it up” in the back yard and check the top needle setting with a tachometer. I had a similar problem with my other ringed engine an OS50SX and when I flew it last Sunday I could not help noticing how smooth it runs (it’s a year old now). I hope that the 91FX will go the same way; the only problem is that we got a pattern meet in ten days time.