We were given a 1986 Honda Fourtraxx 350 4x4 that has a knocking noise when running. It appears to be coming from the bottom, but when the cover is off my husband can't see anything broken. The oil was very black before he changed it. I'm sure that regular maintenance wasn't done on the machine. Does anyone have any ideas?

Thanks!
Teresa

 

Sounds like it could be rod knock. In that case you won't be able to see the affected parts without tearing the engine down. At the very least you'd pull the head, then the cylinder, then pull up and down on the con-rod to check for play. If there is ANY up/down play, that's the source of the knock.

Before tearing into the engine though, check valve clearances. If for some reason the clearances are excessive, that would also make a knocking or tapping noise, and would be alot less involved to repair.

Diagnosing sounds over the internet is an exercise in futility, so make sure to carefully check any obvious external sources of the sound first, before tearing into the engine.