1990 Force 120hp 2 of 4 spark plugs clean

[bart98stang]

Greetings all. I have a 1990 Force 120hp which runs quite well on the ears. I removed the plugs to see what they look like and #'s 3 and 4 are clean. Left to right are plugs 1-4 respectively. Compression was #1 120psi, #2 115psi, #3 115psi, #4 120psi and all 4 produce a fat blue spark across 7/16 air gap. Can somebody please tell me what this means? Thanks.

 

[bart98stang]

Re: 1990 Force 120hp 2 of 4 spark plugs clean

I cleaned each spark plug and put each one back in a different cylinder then it came out of. I restarted the motor and let it run for about 5 minutes. I removed each plug again and all 4 were clean. Not sure what this means........ Please enlighten me! Thanks all!

 

[pnwboat]

Re: 1990 Force 120hp 2 of 4 spark plugs clean

Running on muffs is not really a valid test to read the plugs. You need to take it out on the water and run it under a load at normal operating conditions 3000 - 5000 RPMs to get a good read on the plugs. Running on muffs does not duplicate actual running conditions.

 

[I`mNotMe]

Re: 1990 Force 120hp 2 of 4 spark plugs clean

Shinny Clean Plug is often NO Spark, or a leaking head gasket or exhaust plate gasket (letting water into the cylinder).

Currently, my 1996 Force 120 has the same problem. re: 3 of its spark plugs are wet / oil fouled and its bottom plug is shinny clean. If wondering, Force 120 plugs will be wet oil foul if using lower engine RPMs too long. They need higher RPMs (and its higher heat) to burn the wet carbon away. Since my Force engine "turtle speeds" into harbour and onto its loading ramps area as well, its plugs are consistently wet black foul looking.

Regarding the shinny spark plug, I was told it can be possible Head Gasket leak or Exhaust Plate Gasket leak. Currently, I'm installing a new head gasket, new impeller (with its new gaskets) and new Starter Brush kits as well. Plan to perform head gasket replacement (since its a DIY task using < $50 gasket from iboats.com ), do a decarb process (using Mercury Tune Up juice), then take it on a long hard river run. If it still has a shinny bottom spark plug, I'm planning on replacing its exhaust plate gasket next.

Perhaps same DIY approach might work for your Force 120 with shinny spark plugs as well???

 

[bart98stang]

Re: 1990 Force 120hp 2 of 4 spark plugs clean

Running on muffs is not really a valid test to read the plugs. You need to take it out on the water and run it under a load at normal operating conditions 3000 - 5000 RPMs to get a good read on the plugs. Running on muffs does not duplicate actual running conditions.

That makes a helluva lot of sense, I should've known that. It's currently on a stand so I cannot run a load test.

It does have spark. It jumps over a 7/16ths air gap and I grounded each plug to make sure they fire and they all do. I'll have to look into the gaskets.