88 Mercury 100 hp spark plug fowling

[1969 boatguy]

I have an 88 model Sea Ray 17 ft boat with a 88 model Mercury 100 horse power outboard that keeps on fowling the spark plugs after running and shutting it down. Once plugs are fowled out, it will not restart until I have pulled the plugs and cleaned them. Then it will run fine until it happens again and again. Please help!

 

[dlplost]

Re: 88 Mercury 100 hp spark plug fowling

ONLY use Johnson/Evenrude or Quicksilver injection oil.
We had same problem with ours when it was NEW.
Took it back to the Dealer 3 times before the OLD Mech working on the boat next to ours over heard our problem.
He walked over pulled the cap off of the oil tank, smelled it, put it back. Came over to us and said He could fix it, go have lunch and come back in an hour.
When we came back, He handed Me 4 new plugs and a gal jug of the Johnson/Evenrude oil. (it was a Merc Dealer)
He said "It will foul the plugs one more time, when it does put these in. Then it will NEVER do it again as long as you use this oil in it and NOTHING else."
He then opened the oil jug and made me smell the oil. It has an amonia smell to it. Then He said "NEVER use the store brand oil it WILL foul the plugs"

We followed the instructions and NEVER had a fouled plug after that. Eventually sold the boat to my uncle, told Him the story at the time. YUP, He got the cheap oil and had all kinds of problems, till His Buddy that was here with Him when I sold it to him reminded Him aout the oil. he changed it and it straightened right out.

 

[chum1]

Re: 88 Mercury 100 hp spark plug fowling

I use pennzoil from walmart tcw3 never had any problems, use the flat style plugs.

 

[1969 boatguy]

Re: 88 Mercury 100 hp spark plug fowling

What is the correct plug to use in this outboard motor and should I increase the heat range with a hotter plug? I will try the Johnson/Evinrude oil. Thanks.

 

[CharlieB]

Re: 88 Mercury 100 hp spark plug fowling

Check the fuel pump diaphragm for a pin-hole, it will flood one cyl and the recirc hoses will spread the excess fuel throughout all the cyls.

Disconnect the fuel pump vacuum hose and see if it is wet inside, it should not be.

Pump the primer buld and hold a little pressure on it while watching the pump, looking for a fuel leak to the crankcase side.

 

[1969 boatguy]

Re: 88 Mercury 100 hp spark plug fowling

Thanks Charlie, I will try this. Do you know what plugs works best in this outboard?

Check the fuel pump diaphragm for a pin-hole, it will flood one cyl and the recirc hoses will spread the excess fuel throughout all the cyls.

Disconnect the fuel pump vacuum hose and see if it is wet inside, it should not be.

Pump the primer buld and hold a little pressure on it while watching the pump, looking for a fuel leak to the crankcase side.

 

[CharlieB]

Re: 88 Mercury 100 hp spark plug fowling

When in doubt, look in the NGK book. The listed plug is most probably the surface gap design which work well for WOT running. Some have changed to a standard electrode plug but without checking for adequate piston clearance and careful consideration of the way you run your boat, heat range could be an issue.

Like I said, if any doubt, check the book and you can't go wrong.