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Strange Q100B carb problem

Old 03-19-2006, 10:45 AM
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Aerobat
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Default Strange Q100B carb problem

My Q100 started running very fast with the throttle closed and I had to run the airplane out of gas in the air to get it down[X(]. Naturally I thought it was a throttle linkage problem, but the linkage was fine and throttle butterfly was closing completely. I took the carb apart, cleaned it with carb cleaner, reassembled it, and it still does the same thing. Putting a different carb (Wallbro SDC80) on it solved the problem.

I called Bennett and they said to look for wear on the butterfly due to vibration - but there is none. The butterfly closes completely.

Any ideas? I guess I'll send the carb away for a rebuild.

Mark
Old 11-21-2006, 05:51 PM
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w8ye
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Default RE: Strange Q100B carb problem

The main thing is that you solved your problem and got back in the air.

Most likely there was an air leak around the carb base?
Old 11-21-2006, 06:48 PM
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Default RE: Strange Q100B carb problem

It took me a long time to solve this problem, and to be honest, the online forums were not much help! A friend of mine in Delaware R/C, who is very good with gas motors, immediately said it had to be a vacuum leak. It turned out that the rear crankshaft seal was bad. I now know a lot more about these motors, including how to tear them down and re-assemble, make gaskets, install seals, etc. He even helped me sandblast the cylinder, and the motor now looks and runs great, after 5 seasons of flying - pic attached.

Bottom line, if a 2-stroke won't shut down with the throttle closed, air is getting into the crankcase on the upstroke.

Mark
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Old 11-21-2006, 09:56 PM
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Default RE: Strange Q100B carb problem

Most often the rear seal fails because someone wrapped the starter spring up too tight. One of the spring coils will get over the edge of the one-way bearing and rub on the crank. It will get the crank hot and ruin the rear seal.

If the seal get bad at all the engine will be difficult or impossible to start and will most likely dead stick on you in flight never to run again.

On one of these if you prime the cylinder it will pop but never start.
Old 11-22-2006, 09:15 AM
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Default RE: Strange Q100B carb problem

The real seal on my motor was missing the coil that wraps around it and squeezes it against the crank. A motor will run with air leaking in through the seal - just very lean on the low end.

Mark

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