1989, I50 hp force outboard motor spark plugs

[Zeeky]

I have a 150hp force engine. What type of plug do the motor call for: a conventional gap plug or a surface gap? My Clymer shop manual doesn't specify. This manual indicates UL18V (QL76V) for years 1889-1991A
Prestolite CD ignition.

How do I know if my motor is Prestolite CD or Thunderbolt ignition

When I purchased the motor it had L76V (QL76V). After examining the motor I noticed marks on top of the piston that look like the piston came in contact with the spark plug. Surface type plug.

 

[pnwboat]

Re: 1989, I50 hp force outboard motor spark plugs

Your motor uses surface gap plugs regardless of ignition system. Pretty much impossible for the piston to make contact with the spark plug if it's a surface gap plug.

Prestolite ignition system uses UL18V. Thunderbolt Ignition uses L76V or QL76V (resistor plug).

Prestolite ignition has Blue coils and 2 Blue CD Modules.

300 888788 F684475 1 Blue Ignition Module Force 125HP 4 Cylinder Outboard | eBay


Thunderbolt ignition system has Dark Blue coils and a Black Switch Box.

1992 120 HP Force Outboard Ignition Assembly Stator Switch Box Coils 120HP | eBay

 

[Zeeky]

Re: 1989, I50 hp force outboard motor spark plugs

Thanks. Great answer. Do you have any possible reason for the imprint on top of the piston. I'm a little fearful that it may be something serious like maybe bearings. I removed the crankcase and the main and connecting rod bearing are good. Should I explore the internal motor farther? What ever happened, the marks on top of piston is circle shape and the same size as the spark plug area.

 

[Jiggz]

Can ypu post a pic of the piston in question?

 

[pnwboat]

Some of the factory cast pistons have a circular machining mark on the top center area of the piston. Maybe that's what you're seeing?