20 hp Mariner running bad

I have a 20 hp mariner-about 90 model--runs good at wide open speed but dies atlower speed-after some running new plug inspection showed one plug looked almost asif it hadn't been used-only slight gray/smoke color in one small spot-rest of plug still looked brand new--any thoughts

Re: 20 hp Mariner running bad

Welcome to iboats,Gp.<br /><br />If water gets into the cylinder, the heat will make steam, and steam clean the plug. I would check the compression before doing anything else.<br />Of course, water can also get into it from the gas tank,so check that too.

Re: 20 hp Mariner running bad

You don't say which plug looked however.Is this engine new to you?By all means do the comp. check as suggested and take a little fuel sample to check that possibility.There are alot of conceivables here so more info might help.

Re: 20 hp Mariner running bad

It was the bottom cylinder that looked as if the plug had never been fired except for one very small spot-the rest of the ring looked brand new--Actually this motor belongs to my Dad-he is very peculiar about clean gas etc..he assures me it has no water in it-I had some info wrong also--he fishes the Mississippi River and a day or so ago it ran up the river 20 minutes wide open fine then just suddenly shut down--like to have never cranked but finally did--ran lines running fair-then later almost ran battery down trying to crank it-got it cranked and ran 20 minutes or so wide open down the river-seems asif something might be heating up and shutting down-I told him to replace the plug which appears different and I also told him to check to be sure his tank line is not cracked or leaking at a fitting.any other suggestions appreciated..

Re: 20 hp Mariner running bad

Now I know a little more-I went and tinkered with it some..Compression on bottom cylinder is 140 and top is 120--did finally getit cranked and it ran reasonably well (for a Mariner)killed it and it would hit but didn't want to run-kept trying and finally cranked again-many times when I hit the starter it would not turn over(like it was vapor locked or something)-I removed the bottom plug and it would turn over free-I replaced the bottom plug and removed the top plug and it would be "vapor locked" again-something going on with bottom cylinder-I'm just not sure what it is..fire looks strong..

Re: 20 hp Mariner running bad

Somebody mentioned possibly a leaking exhaust gasket-they recommended replacing the two gaskets-facing the front of the motor,where are these gaskets.Easy to do??If same person sees this post please reply to gprestridge@netdoor.com--I've had computer problems and may have llost your previous reply.Thanks