Spark plug diagnosis

[LagunaJay]

What does it mean when the spark plug’s gap is normal (.035), electrode/insulator is normal grey/tan color, but the rest of the plug has sooty black deposits? I thought initially it meant an overly rich mixture, but others smarter than me say either bad gas or low compression. Any suggestions? The engine is Yamaha 8HP, 4 cycle, 2 cylinder, carbureted and it starts fine, seems to run fine at all RPM’s and gas consumption seems normal. Next step I think is a compression test? See attached pictures if you’ve got an idea...

 

[Rich Stidger]

I'm not sure you have a problem here. Are you bored and want something to fix? IMHO if the engine starts and runs fine leave it be.

 

[LagunaJay]

In the short term, you are right. It’s the longer term health of the motor I’m concerned about.

 

[jviss]

Those plugs look fine.

 

[rgranger]

I'm guessing you are using gas with ethanol in it and it is a little old so now it has water in it.

 

[kloudie1]

I agree.. no real problem.. The darker deposits look to be from the edges running cooler than the electrodes. The water cooling is doing its job.. If anything, check the thermostat and its bypass passages ..

 

[LagunaJay]

Thanks everyone for the replies. The consensus seems to be all is good and my mechanic friend agrees...

 

[MikeHoncho]

I’m thinking the darker deposits are due to short uses and not sustained running like you would see in a vehicle. I would not be worried.

 

[SycloneDriver]

I’m thinking the darker deposits are due to short uses and not sustained running like you would see in a vehicle. I would not be worried.

That's exactly what I was thinking.