1977 Johnson 85hp fouling plugs low speed

My '77 Johnson 85 will occasionally foul the plugs so badly as I run down the river from the launch ramp that it won't run well enough to put the boat on plane. When I swap out the plugs for the spare set, they're soaked with black oil. Usually it will run fine then -- pulls strongly all the way to redline.

My Johnson dealer says it's the off-brand of 2 cycle oil I've been using, but I kind of have my doubts. I guess using the Johnson/Evinrude brand of oil is worth a try, but I was wondering if anyone else has come across this?

Is it just what can be expected from a 30-year-old motor, or is there some tuning I can do? The carbs have fixed jets...

Re: 77 Johnson 85 fouling plugs low speed

Are both sets of plugs the same type?

Re: 77 Johnson 85 fouling plugs low speed

Yep, L77JC4 Champion.

I'm starting to wonder if I need to de-carbon this motor...I don't think it has ever been done.

Re: 77 Johnson 85 fouling plugs low speed

A de-carb wouldn't hurt but I don't understand why one set of plugs would foul & not the other...unless you're running at a much slower speed with the ones that foul. Is that the case?

Re: 77 Johnson 85 fouling plugs low speed

I don't think it's really one set of plugs vs the other. It's how the boat gets used. I launch at a ramp that has about a mile of no-wake river before you get to the lake and can open it up. The engine is running at it's coldest during this river run, and of course at it's slowest. I think it just loads up. Sometimes not badly enough that it doesn't burn off when I get to the lake -- sometimes it runs fine the whole day -- but sometimes enough that it won't run well when I put a real load on it. When I swap the plugs, the engine is warmed up and probably burns the fuel/oil mix more efficiently? Dunno.

Dad bought this boat/motor new in '78, and it saw only light to moderate usage before being parked in the garage in '92. He gave it to me a couple of years ago, so I went through the 'waking a sleeping outboard' process of rebuilding carbs, changing water pump impeller, etc. Since then it has gotten used for maybe about 25 hours or so -- hard to make it out onto the lake more than 3-5 times a season here in MI -- just not enough time.

For a big outboard, it probably spends more than half its running time < 1500RPM. When it is allowed to run on open water, it runs good, but it's occasionally unreliable due to the fouling, and that's what I want to fix.

Re: 77 Johnson 85 fouling plugs low speed

Excessive trolling will contribute to plug fouling...also weak ignition (check for a strong spark) or a too rich low speed mixture adjustment (if memory serves, you can't adjust the air/fuel mixture on that carb). I presume you're running a 50:1 mix.

Re: 77 Johnson 85 fouling plugs low speed

I think you're right, it's "excessive trolling". That thing never did like to run slowly. The carbs are not adjustable, so I'll double-check the spark and Seafoam it. Hopefully that will help. Thanks!

Re: 77 Johnson 85 fouling plugs low speed

Hi there. I am having a simalar issue with my 70 hp johnson. how did you end up resolving this? Do you know what the spark plug gap should be. Thanks for your time.

Thermostat in place and working ?---------No reason for one of those motors to foul the plugs.----They will idle all day long if tuned /set up correctly !

Where does the thermostat go? Is there a diagram somewhere online? it will idle all day long no problem. Thanks racerone. I appreciate you.

On a 70 HP the thermostat and poppit valves are in that housing at thye top of the cylinderhead.

Parts and pieces do not know how big a motor they are in and that they are not meant to troll.-------I used to troll with one of my V-6 235 HP motors all day long.-----Sometimes had to fire up the second motor to troll properly in big waves.

i have a 75hp johnson and the local boat mechanic told me to pull the thermostat out and leave it out... im confused now cause im under the impression that my motor is a cold natured lil girl... whats the right choice?

Hey Jody. I'm in the same boat. I spoke with two different marinas and they both said that the thermostat was not needed. It all depends what region you are in. I live in the north east where lake water is 75+degrees (currently). I'm sure the water in ontario is much cooler. You are thee third person that has aid no thermostat needed. good luck.

If you do not " know " then of course you can learn about outboard motors.----------Many motors today have a system called " quick start " and the timing is advanced electronically untill motor warms up.---------Many motor have an ECU that monitors engine temperature and injects the proper amount of fuel based on a temperature signal.--------Running an outboard without a thermostat is a " mistake " ----and most outboards do not get a chance to wear out, they end up in the scrap pile because owners makes " mistakes "

Wasn't trying to upset you. --------- but are you saying that the all the boat mechanics in mass are hacks because they all agree.------This is an older motor as you already "KNOW" so i don't believe your ecu theory applies. -------Is it possible that you were wrong? -----Are you a certified mechanic? Your not a very smooth talker. Just sayin.

jimpin, i live in north carolina, the water temp here is around 85+. and my motor is a 1977 75hp stinger

Well let's look at this logically since you will never know who is right on the thermostat issue. First of all, quit looking at the temperature of the water. The T-stat in an outboard is designed to maintain a 145 F temperature, so unless your water is that hot, it's temperature is irrelevant. Warmer incoming water just means that the t-stat will stay a little more open for longer then if the water was cooler. That's all.

Now. OMC have been using T-stats for about 50 years or more. Do you really think that they would have gone to all the trouble in design, material costs, and the addition of one more component that could fail, if it was not needed. Do you not think that they have any access to Mass. mechanics or wherever. OMC put that in there because they found that a motor that runs at 145F works more efficiently, fouls less spark plugs, builds up less carbon and will last a heck of lot longer, then an outboard without one. Why do you think that outboards that were manufactured over 50 years ago are still around and working well.

So think about it for a while, and then go get a t-stat for your motor.

`Run without a thermostat and throw away 65% of the engine life...

I've read comments on this forum from people who use a separate tank with a less than 50 to 1 mixture exclusively for extended trolling. When they are done trolling, they connect to the main tank with a 50 to 1 mix. You should not have to do this if the motor is in good condition and working properly but....it would seem like an easier solution than changing spark plugs. It would not be good if you were to forget to switch tanks and run at full speed onthe reduced lubrication.

Thanks bobbyc. how do you feel about the thermostat thing. just seems as though all the professionals i speak with outside this form say its not necessary on these motors. what do you think?