Excessive Carbon on Spark Plugs...

[DukesFin]

GM L6 250 cid motor pushing a 21' Grady White with a OMC Stringer Drive...

I pulled plugs yesterday and found excessive carbon building up on the plugs...

One friend was thinking that the cause was possibly due to the electric choke being wired from the coil, not letting enough power to spark, thus the carbon build up...

What do you guys (and gals) think? Any ideas of what could be causing this?

 

[bjcsc]

Re: Excessive Carbon on Spark Plugs...

I think the choke theory is unlikely. It's most often caused by a rich condition (too much fuel or too little air), but it is also the result of running at idle for extended periods (normal)...

 

[KaGee]

Re: Excessive Carbon on Spark Plugs...

The carbon shows a rich condition at idle.

To properly check your plugs condition at speed, you need to make a hard run and shut down immediately, then check your plugs. (Per DonS)

You probably need to lean out your idle a bit.

 

[DukesFin]

Re: Excessive Carbon on Spark Plugs...

Sounds good to me...

We have just rebuilt the carb (Holley 4bbl 390cfm). When we dismantled, we noted the mixture screws were set at 1 complete turn from closed... When we put everything back together, we were surprised to find that about 1/4 to 1/2 turn seemed to be best...

I'll get some new plugs today and test/adjust as needed.

I also do a LOT of trolling at idle speed.

 

[KaGee]

Re: Excessive Carbon on Spark Plugs...

Sounds good to me...

I also do a LOT of trolling at idle speed.

I fouled a plug doing the same. The oil companies are putting additives in the fuel in some areas that shorten your normal plug life. Especially in marine engines operating at lower temps.