# 20 HP Onan Backfiring



## bradleyj (Mar 29, 2008)

I have a 1989 Wheelhorse 520H with a 20 HP Onan 2-cylinder engine. The engine backfires out the muffler pretty regularly after it warms up. I pulled both the spark plugs and found that the one in the front cylinder is covered with soot. I swapped in a new plug and the problem persists. Compression in that cylinder is 120 psi. The other plug looks like new. The engine runs perfectly other than this. Starts fine, runs solid at all rpms. Any idea where to look?


----------



## Restrorob (Dec 23, 2007)

You may want to make sure it's firing/running on both cylinders, It doesn't sound like it is from what your saying.


Start the engine then remove each plug wire one at a time with rubber handle pliers etc, If the engine dies when one is removed the cylinder on the other side is not firing/running.


Engine model and spec numbers could be helpful......


----------

