1986 90HP fouled plugs mystery

I have a 1986 Johnson 90HP outboard, took it out on lake and ran fine almost all day, later it started smelling like gas and running rough and very shortly after that it died and would not start, leaving me stranded on the lake. I pulled the plugs and the two on the right were fine but both plugs on the left were soaked in wet gas, I replaced them with two spares I had and it ran for a few minutes before fouling them also. I thought that the lower cylinders were fed by lower carb and upper cylinders by upper carb. So now I cannot see how wet plugs could be caused by a flooding carb, since both plugs are on left. Am not sure where to go next.

Re: 1986 90HP fouled plugs mystery

they might not be wet with gas, might be water.
Compression test it, could be blown gasket.

Re: 1986 90HP fouled plugs mystery

you have 2 power packs, one for each side, sounds like you lost spark on one side. then the power pack cooled off, and worked until it heated up again. swap the packs for side to side, see if the problem follows the power pack.

Re: 1986 90HP fouled plugs mystery

Agree on the powerpacks thing. Does sound like it is dropping one.

Re: 1986 90HP fouled plugs mystery

OK thanks for the advice, I noticed 2 ignition coils on each side but am not sure, what is a powerpack?

Re: 1986 90HP fouled plugs mystery

OK I replaced the power pack with a new one and I still have the same problem, I did a compression test and all 4 cylinders were about 125 PSI, also I removed the 2 left plugs and the lower one dried out with a torch like you would expect - the gas burned off quicky with a yellow flame, but the upper plug acted like it was fouled with water as I saw the water boil off the plug insulator, can anyone give me any ideas what to check next?

Re: 1986 90HP fouled plugs mystery

You need to check your spark at 1/2 inch gap with a spark tester on all cylinders before you can rule out the electrical system I think.

Looks like you're right about the carbs... the top carb feeds the top cylinders and the the bottom carb feeds the bottom cylinders. Knowing that, I'm leaning toward electrical. I've got some other hair-brained ideas, but I'll keep them to myself until you've ruled out the electrical.

Re: 1986 90HP fouled plugs mystery

scrampbell said:

You need to check your spark at 1/2 inch gap with a spark tester on all cylinders before you can rule out the electrical system I think.

Looks like you're right about the carbs... the top carb feeds the top cylinders and the the bottom carb feeds the bottom cylinders. Knowing that, I'm leaning toward electrical. I've got some other hair-brained ideas, but I'll keep them to myself until you've ruled out the electrical.

Click to expand...

OK, I bought a spark tester and two carb rebuild kits, I replaced the needle valve and seat on each and verified that float level was correct, and also could not blow air thru gas intake barb with carbs inverted, so I put carbs back on and proceeded to check for spark on all 4 cylinders with plugs removed. All 4 had a strong blue spark at 1/2" gap but I did notice that the lower left cylinder just blew gas out of the hole, for many cranks too. So I am starting to wonder if float on lower carb could be bad. I believe that the left upper plug was firing this last time that I started it, as it looked just like the two right plugs. BTW the carbs were almost perfectly clean inside and believe they may have been rebuilt before, also replaced all of the fuel lines while at it. Also while priming the carbs after reinstalling them, the priming bulb never did get hard and I could always hear gas flowing somewhere, it would seem to me that after the floats rose up the gas would dead head. And I pulled the fuel lines from the priming solenoid and no gas was coming from there. I sure have learned a lot about boat motors in last two weeks. Any thoughts?

Re: 1986 90HP fouled plugs mystery

Well I found the problem and wanted to post it in case someone else runs into this as it had me stumped for a couple of weeks, the fuel pump was leaking gas into the vacuum hose that is plugged into manifold near lower left cylinder, this caused that cylinder to be totally flooded, and also was starving the carbs for fuel. So in a case of cylinders not firing one suggestion would be to pull the vacuum hose off and be sure no gas runs out. I am going to order a regular fuel pump and go to premix. Thanks for everyone's help.