91FX FAQ

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My engine seems to be bogging, running 'heavy' or hot, and does not hold a needle setting well. What's wrong?

Assuming the engine is fully broken in, you are using good quality fuel with at least 18% oil, and that there is no fuel foaming occurring, it is possible you are overpropping the engine. With use here in the U.S. at our altitudes, fuels, etc, we have found that this engine prefers slightly smaller props - 13x8 or 13x9 specifically - and turning at higher RPMs. If your engine's max RPM is under 10,500 then the engine is working too hard and not making peak performance.

Remember, this is a 2-stroke engine. It cannot be expected to turn the same enormous load of prop as a far more expensive, pumped or super-charged 91 4-stroke engine.

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