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1975 15 hp Fouled plugs

D

dawnfogg

Contributing Member
Ok, I rebuilt this '75 15 hp Johnson, and quite frankly I was a little shocked when I put it on the boat and it actually worked. Ran great for three days and then all of a sudden lost most of its power. I limped home and pulled the plugs and they were black and oily. I was using Champion L7J per the manual. So I am assuming that is my problem. My question is simple, how can I prevent the plugs from getting this way? I really wasn't idling or trolling- actually spent most of the time wide open. Should I run a hotter plug? Will adjusting the idle needle help lean out the mixture.

As always, look forward to hearing the advise.
 
Re: 15 hp Fouled plugs

That is what it says to use in the servuce manual. A buddy who was with me said it looked like the gap was too wide. Didn't measure, but could that be a problem?
 
Re: 15 hp Fouled plugs

The currently recommended plugs are UL81C gapped at 0.030" ... measure it!


The original plugs were L7J Did not think were still available!
 
Re: 15 hp Fouled plugs

The guy at the autozone said the L7J's were not available but gave me ones that were comparable. I will go get a set of the UL81C's and see if it makes a difference.

Thanks
 
Re: 15 hp Fouled plugs

Plugs really only foul if you are running too rich or you are running too cold.

First, ensure your low speed valve on your carburetor is adjusted to the proper level. Search for Joe Reeves carburetor adjustment on this site for the procedure. 2nd, if it is not too difficult, confirm that your thermostat is working properly and is not stuck open. You can do this easily, on your motor, since the T-Stat is closed when you first fire it up and the majority of the water spray out the exhaust vent is restricted by it. At the very beginning (1st 15 seconds of run time) the water spray out the back exhaust port is almost unnoticable. You pretty much need to put your hand under it to see if it gets wet. In about 15 seconds it will start to spray out the back exhaust port pretty good if the water pump is working properly. If that same spray amount comes out right away (in the 1st 5 seconds, lets say), your T-stat is stuck open and needs to be fixed. If it opens like I said in about 15 seconds you are good to go with that. The last thing to verify then are the plugs. Your motor specs call for Champion UL81C and many here will tell you to use those since OMC knows best. I personally find that these motors run best on NGK B7HS. I had quite a few problems with the Champions on this motor but have used them on other motors quite successfully. If your problem persists, you may want to go try a pair of NGKs just to be sure. Since your fouling is happening at higher RPMs and not from continuous trolling, I would not suggest going to a hotter plug. This would just mask the problem and probably create a whole slew of new a bigger ones.

A decarb doesn't hurt either, but probably isn't your problem, but since carbon is getting on your plugs (that is what fouling is) you can be pretty sure it is getting on your piston rings as well, so I would want that cleaned out once the problem is resolved.

Good luck.
 
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Re: 15 hp Fouled plugs

Joe Reeves' instructions

(Carburetor Adjustment - Single S/S Adjustable Needle Valve)
(J. Reeves)

Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.

Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.

Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.

Note: As a final double check setting of the slow speed valve(s), if the engine has more than one carburetor, do not attempt to gradually adjust all of the valves/carburetors at the same time. Do one at a time until you hit the above response (die out or spit back), then go on to the next valve/carburetor. It may be necessary to back out "all" of the slow speed adjustable needle valves 1/8 turn before doing this final adjustment due to the fact that one of the valves might be initially set ever so slightly lean.

When you have finished the above adjustment, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.​
 
Re: 15 hp Fouled plugs

Thanks for this advise. As I turn the slow speed needle in, am I leaning the motor or making it more rich. I assume I am leaning because I am restricting the flow of gas, right?
 
Re: 15 hp Fouled plugs

Thanks. I will change the plugs and reset the slow speed needle and let you know if that cures the problem.
 
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