1987 Johnson 9.9 Keep Fowling the Plugs

ArtificialB8 · Dec 13, 2004

Father-in-law has a 1987 Johnson 9.9HP outboard that keeps fowling the plugs. Runs great for about 4 miles then starts to sputter due to carbon buildup on the plugs gap. Running 50:1 gas/oil mix (double-checked it wasn't overmixed). Any ideas how to solve problem?

umblecumbuz · Dec 13, 2004

Re: 1987 Johnson 9.9 Keep Fowling the Plugs

First question - what plugs are you running?

ArtificialB8 · Dec 13, 2004

Re: 1987 Johnson 9.9 Keep Fowling the Plugs

Champion Marine plugs, part number 5931 (L77JC4)with the correct gap.

jim dozier · Dec 13, 2004

Re: 1987 Johnson 9.9 Keep Fowling the Plugs

What kind of boat is it on. If its on something big like a sailboat maybe he needs a sail prop. He might be lugging the engine. Or if he is operating at low speeds a lot maybe his idle mixture is set too rich.

ArtificialB8 · Dec 13, 2004

Re: 1987 Johnson 9.9 Keep Fowling the Plugs

Uses the motor for duckhunting in a 14-ft jon boat so weight usually isn't a problem. As far as the idle mixture, there is a small knob on the front of the motor that I think (not sure) can be used to adjust the idle. Could that be part of the problem?

Dave Abrahamson · Dec 13, 2004

Re: 1987 Johnson 9.9 Keep Fowling the Plugs

Tried synthetic oil in it yet? Might help with the plug fouling issue. Although...I guess if it's fouling plugs that badly, there must be a reason. Just my $.01 (I'm keepin a penny) Dave

jim dozier · Dec 13, 2004

Re: 1987 Johnson 9.9 Keep Fowling the Plugs

If the heat range of the plug is correct and the oil/gas mixture is correct and the plugs are still fouling it suggests an overly rich mixture. The high speed jets are fixed but the low speed or idle mixture is set by the knob you mention. If he is spending a lot of time at low engine speeds, lean the mixture out (hopefully there is a arrow printed over the knob, because different engines lean in different directions) a quarter turn. Sometimes if its just the one plug on the cylinder that is connected to the fuel pump its an indication of a leaking fuel pump, but you said both were fouling right? While I certainly have no problem with synthetic oil, a properly running 9.9 should not foul plugs on TCW3 oil.

steelespike · Dec 13, 2004

Re: 1987 Johnson 9.9 Keep Fowling the Plugs

Could be a hole in the fuel pump diaphram this allows extra gas into the crankcase and thus the cylinder(s). There is also a chance the float is not controling the gas into the bowl though this isnt as likely at speed. Would usually show more at slower speeds.

bcfryer · Dec 13, 2004

Re: 1987 Johnson 9.9 Keep Fowling the Plugs

I am no expert but as I remember , low compression will cause a two stroke to foul plugs. I suggest a compression check with engine warm, both plugs removed, throttle in wide open position. pull it over several times per cylinder. both should be close in the reading and both should not be to much below 100 psi at the lowest.

seahorse5 · Dec 13, 2004

Re: 1987 Johnson 9.9 Keep Fowling the Plugs

The QL82C is the factory recommended sparkplug along with the QL77JC4. The '82 is a warmer plug and more resistant to fouling. If you are getting carbon flakes on the plug gap, your combusion chamber is badly in need of decarbonizing. Use the Engine Tuner spray available at any Johnson-Evinrude dealer. Make sure also that the thermostat is warming the motor above 125 degrees at idle and that the carburetor is adjusted correctly.

OBJ · Dec 13, 2004

Re: 1987 Johnson 9.9 Keep Fowling the Plugs

I'd bet on seahorse and the T-stat....

umblecumbuz · Dec 13, 2004

Re: 1987 Johnson 9.9 Keep Fowling the Plugs

As far as the asdjusting knob you mentioned goes - this governs a variable needle for slow running. Warm the motor, screw the needle home (pull the knob off the spinde to do this) and wind it out half a turn. Wait a few seconds for the motor to respond. Keep winding it out a fraction more each time until you hit the most even idle. Then push the knob back on the spindle, making sure it is at its midpoint of travel (pointer on knob straight down) - this gives you maximum adjustment if you need it later. That's the right procedure for the idle knob. On the left of the motor, looking towards the stern from outside the boat, your year should also have a 'trolling' knob. This is just a fast idle adjustment for trolling purposes, and doesn't affect the mixture.

the machinist · Dec 15, 2004

Re: 1987 Johnson 9.9 Keep Fowling the Plugs

I will bet with the low compression. I also had a 78 that was doing the same thing, checked everything & when I finally tore it down the compression ring was so soft that I bent it to look like a "3" before it broke. Re-ringed it & has ran like a champ since.

umblecumbuz · Dec 15, 2004

Re: 1987 Johnson 9.9 Keep Fowling the Plugs

I would question low compression. It's an obvious posibility, except that it might also show up in poor starting, and ArtificialB8 specifically mentioned that the motor runs great for first few miles. Presumably that means it starts easily also. Sounds more like carb to me(only one on this motor) - particularly float setting not right. Other real possibility is choke flap not backed off correctly, or even - as one guy here discovered - a lump of rag in the plastic air intake causing a restriction! (I didn't believe it either until I saw it.)

umblecumbuz · Dec 15, 2004

Re: 1987 Johnson 9.9 Keep Fowling the Plugs

Of course, if it's 'fowling' it could be guano in the mix!

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