Small Engine Troubleshooting Problems? Ask a Mechanic Now.
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400 hours is really not many for one of these machines. There are a couple things that you can check that would cause this. If the engine is overfilled with oil it can cause it, if the carburetor is leaking fuel into the crankcase it would cause it, if the crankcase breather reed plate is bad it would cause it. The first two are easy to check, the crankcase reed plate is a bit harder to check. It is normally built into the valve cover on these machines. I would check these possibilities.