Spark Plug Half-Fouling

[guitarplaya39]

1994 Merc 75hp 2 stroke, 3-cyl. I've been having issues with surging at mid-throttle and discovered the bottom plug appeared half-burned. What would cause this?
I decarbed the engine, put new plugs in, and it did the same thing to the new plugs after a quick outing.

 

[racerone]

Fuel pump diaphragm should be checked.

 

[guitarplaya39]

Fuel pump diaphragm should be checked.

So could this be caused by a lean condition in that cylinder? Perhaps it's only fouling closest to the intake?

 

[cyclops222]

Why are you using those....Surface Firing sparkplugs ? They are way too easy to foul-up. Change to a Standard Style of sparkplug. I had the same problem with both old Evinrude engines. Check that the 3 carburetor linkages are closing the same amount both positions.

 

[Dukedog]

surface gap is fine ina merc... omc's not so much... jmo

 

[guitarplaya39]

Why are you using those....Surface Firing sparkplugs ? They are way too easy to foul-up. Change to a Standard Style of sparkplug. I had the same problem with both old Evinrude engines. Check that the 3 carburetor linkages are closing the same amount both positions.

I've always just used stock components. It's been fine for years so I'd rather just stick to what Mercury recommends.

 

[Dukedog]

 

[cyclops222]

Check that the sparkplug wire is FULLY inserted at both ends. Possible poor connection or failed wire.
Can you swap that bad cylinder H V wire with another cylinder wire to see if the wire is bad ?

 

[merc850]

Looks like a weak spark caused by failing stator or trigger. See if the spark will jump a 7/16" gap. Do not use regular reach plugs unless manual says so, they will hit the piston top.

 

[guitarplaya39]

Check that the sparkplug wire is FULLY inserted at both ends. Possible poor connection or failed wire.
Can you swap that bad cylinder H V wire with another cylinder wire to see if the wire is bad ?

Definitely fully seated on both ends, but I'll try swapping wires or testing the resistance.

 

[Texasmark]

Why are you using those....Surface Firing sparkplugs ? They are way too easy to foul-up. Change to a Standard Style of sparkplug. I had the same problem with both old Evinrude engines. Check that the 3 carburetor linkages are closing the same amount both positions.

I totally agree on surface gaps. The Mercury service manual for my engines lists surface gaps and a conventional gapped plug alternate. On the location of the carbon buildup it depends on the fuels position within the combustion chamber when the plug fires. Fuel enters in a swirling motion.

 

[racerone]

No such thing as ---" half fouling "---Do some trouble shooting on the motor.

 

[guitarplaya39]

Meant to follow up earlier, but I wanted to run the boat a bit to make sure.
It turned out to be the fuel pump. No more bogging and the plug looks normal. I'm guessing it was just enough to cause a lean condition in the bottom cylinder.
Thanks for all the advice!