Ever since I bought this engine a year and a half ago ,OS 46 FXH, I never have a good flight. I have replaced all my fuel line and have one of our experienced member adjust it for me but with no luck. The only thing I have not done yet is to drill hole to the carb and I have hesitation to do it. Do you think I should buy a new carb or just give up.

How old is your engine? The original O.S. Max .46 FX-H had a difficutly with being a bit lean in the midrange, near the hover throttle setting. This was fixed by O.S. 1997, one year after the engine was introduced.

If your engine was purchased after that, it has the "new" carburetor, which has been unchaged since.

What difficulties are you actually having? Your note doesn't say.

Well, the engine will run fine until it reaches half tank and then it will start to increase it's RPM. Then, the engine intermitently will sound like it's choking and loses it's power. This occurs both on hover and foward flight. I adjusted/tune it on half tank hoping to make it rich enough so that it will not lean when it reaches half tank. In addition, it have a new plug and fuel line on it. It works well when it's on a test stand or on an airplane fuse. None of my OS engines have this problem, what am I missing here ? Even the best in our club is puzzled by it.

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The high-speed needle is the 'master' control of the carburetor, and must be set for full-throttle operation. The idle mixture is then set, along with the idle RPM. Many people try to idle too slowly. In a heli, a 'good' idle is any low RPM where the clutch is not engaged.

With the high-speed needle properly-leaned, and a good idle mixture set, the midrange should take care of itself. If it's a bit too rich or lean, that is usually adjusted with the idle needle, but not too much, or the idle will go sour.

If your engine runs well on the test stand or other aircraft, but not in the heli, then the problem's not in the engine, itself, but in the entire system of heli/engine/fuel system. The fuel system's a likely culprit, with possibilities of air leaks, vibration-induced fuel foaming, and so on. Also, cooling is different in the heli than in other installations, and this can cause difficulties.

One thing you didn't mention was the fuel you're using. If the engine's running hot, you can try a fuel with a higher oil content. Add some castor to your existion fuel. about 1.25 ounces increases a gallon by 1%. More castor helps cooling. Many fuels are deficient in oil, which can make for troublesome operation in hot environments (helis are one).