Help Srange Plug Fouling Problem with 50hp 1996 3cyl. Mariner

I am considering purchasing a boat from a friend, and he insists, and I believe him, that the engine ran beautifully just two weeks before he took me for a test ride.

On the test ride, the engine would only run on the low speed circuit, and just bogged when the throttle was opened further. I suspected that the carbs were gummed, so I purchased the kits, and rebuilt them. The middle and bottom carburetors had gum clogging the main jets, but everything else was spotless. After reinstalling the carbs, we took it out again, and thought all was well, but the engine began to misfire, and stopped firing on one of the cylinders.

We then tested the compression on the engine, and found that each cylinder developed 125 psi. The plugs were changed with another set of NGK units, and we headed out to the lake again. This time the engine started easily, ran smoothly, and developed great power, but after fifteen minutes or so, it dropped a cylinder again.

We inspected the plugs again, and found that one of the three was again fouled. The interesting thing is that we have had this occur three times now, but never on the same cylinder. Each of the three cylinders has fouled its plug after the engine is warmed, but never the same one twice.

I did not mess with the low speed adjustment on the carbs, but this fouling seems to occur during high speed travel, or immediately after cutting the throttle from a high speed run.

Would you suspect that this is a further carb issue, or should I look into the trigger, stator, or switch box?

Put a timing light on each cylinder( one at a time ) and take it for a run.----------Observe the flashing light and see how spark behaves.

Thanks Racerone,

Will follow your suggestion, and report back.

I suspect that the enrichener circuit on the carbs is not closing properly, which floods the engine. Check to see that all of the linkages come back all the way. I'd even remove the linkage arm after the motor warms up to make sure they are all off.

Jeff

Fastjeff,

That is an excellent suggestion. The enrichment circuit on this particular engine is activated by a solenoid, which transfers fuel from the top carburetor fuel bowl to a fitting on the intake manifold behind the middle carburetor. It is possible that something has become stuck in the solenoid, and is holding it open.

Thanks again,
Brad

Check the oil recirc system including the 2 check valves found in the intake manifold , behind the carburetors.--------------These check valves use a plastic ball and these are very abrasive and wear things out.

Yesterday, I installed a new switch box, and headed out to the lake, and as luck would have it, the switch box was not the problem. From cold, the engine started great, idled fine, and immediatly jumped up to plane. It produced good power for a run or two, then dropped a cylinder again.

I pulled the cover, and then observed the fuel flow from the carbs into the engine. I found that the main fuel circuit was again not flowing fuel to the bottom cylinder. After getting home, I removed the carburetors again, and found that the main jet in the bottom carburetor was yet again clogged with a very light weight greasy like substance. Upon examination, this stuff was about the consistency of a light lithium grease.

After reassembly, we fired it up on a water hose, carefully adjusted the low speed needles, and it ran great. It had excellent response, and seemed just fine. I know that running it on a hose is not the same as in the lake, but this seems to have been the problem. After this cold snap, we will test it on the lake again, and will know for sure.

I know the carbs were spotless after the first cleaning, and the tank & main fuel line were new, so it is just a mystery where this "gum" came from.

Thanks for all your help, and I will keep you guys posted on the progress.