I have a OS 40LA and it seems to idle rough can any one tell me why. It rebs up ok but when I idle it back it seems to shake the airplane alot.

Quick guess. Your prop needs balancing
Walt

I'm guessing you're hearing a rattling sound at idle? If that's the case then don't worry, the LA is a plain bearing engine which means the crankshaft can move backwards and forwards very slightly. This only happens at idle when there's not enough thrust from the prop to keep the crankshaft pulled forward.

It could also have loose engine mounting bolts... Combine a small imballance of the prop with loose bolts, and the engine will shake like crazy at low rpm. Check the bolts and the prop balance. (and if you've got a spinner.. check that its ballanced... and not cracked.)

While you're at it, check all the nuts and bolts; carb, muffler, etc.. Something you thought was tight, might be 'rattling loose'..

yeah, chec teh balance of the spinning bits; prop and spinner.

I have a 46 la, and is sounds like a buzzsaw through plastic at idle..

Thank to Waltaz,Downunder,Fhhuber and Dr Wogz your input is greatly appreciate.

I think probably what you are seeing is merely the power pulses reactions at low speed causing the whole airframe to skake. I'll bet as you increase the throtle from idle it smooths out. If this is the case it is something that just happens generally at low speed and there is not much can be done. The power pulses plus the compression reaction at these slow speeds will cause the airframe to shak considerably. I don't look at them as vibration, butjust a shake that must be endured till rpm's are increased to begin taxiing when it should go away.

Speaking of loose nuts and bolts, pick up some locktite at an autoparts store. A drop on those nuts and bolts will save a lot of headaches. One warning though - don't put locktite on plastic. I did that and it melted the plastic.

Roodester