Carbon soot plug fowling

[daveinrenton]

Don:

I cleaned my plugs yesterday and they had a nice black dusting of carbon. It was easy to brush off (soft metal wire brush). I checked my plug inspection visual guide and it indicated that the the fuel/air mix could be a little rich. These engines run cool and that is another factor, for sure. I adjusted the carb for running smoothness according to the MM overhaul manual when I last goofed around with the carb. Is the carbon dust fouling just an inevitable result of cool running or should I try leaning it out a bit more?

Dave

[Don Moyer]

Dave,

We have actually seen plugs clean up slightly by leaning out the idle mixture as much as possible, which (assume you're talking about a late model engine) is the only mixture control you have, and it is the adjustment on top of the carburetor.

Don

[daveinrenton]

So, Don, sounds like a dusty carbon fouling is par for the course and an occasional brushing is sufficient maintenance. Agree?

I'll play with the carb some to maximize toward the lean without affecting smooth running.

Dave

[Don Moyer]

Dave,

I agree to a point, but I think we have to be careful to separate sooty
plugs as a function of infrequent and short periods of engine operation
versus sooty plugs at the end of a 6 hour motor trip. I'm attaching a pdf
file of items to consider when interpreting plug condition, and even at risk
of splitting hairs a bit, here are some other thoughts to consider:

1) I don't think you should have to clean your plugs periodically. If the
condition of your engine coupled with your motoring habits results in a
minor sooting condition, it should level off at a constant level below the
point where it causes any problems.

2) To use an expression I've used many times in the past: "In the face of
otherwise good consistent performance, the condition in question is probably
not serious enough to warrant serious maintenance action". If on the other
hand, the condition in question (in this case - sooty plugs) reaches the
level of causing a power loss or a missing condition that requires
maintenance action, then we ought to figure out what's causing that level of
sootiness, even if it simply means forcing yourself to use your engine more
than you're accustomed to.

3) Please do not confuse this concept as a case against good preventative
maintenance actions, or as just another version of: "If it ain't broke,
don't fix it". If you've been replacing your plugs every 3 to 5 seasons as
a preventative maintenance action, you should continue to do so. However,
if you have to clean the plugs between changes due to a sooting condition
that rises to the level of causing a problem between your reasonable
changes, we need to look into what's causing that level of sootiness.

Don

[daveinrenton]

Don,

I change plugs annually as a regular thing because they are inexpensive and I figure subject to difficult conditions because of the cooler operating temperature and relatively infrequent and low rpm (slow motoring) use of the engine. I checked the plugs recently not becuase of poor performance but because I had seen an article in a recent Boat US magazine that had a great picture of various plug conditions and I was curious to see where mine fell. After examining them and brushing them off, I decided to send a note to see if this might be a common thing. The light carbon sooting was not deemed extreme by you or the article (as a melted or oil-soaked plug might be) but I am in a constant push for perfection to make the very most of what I have. If a small adjustment in the fuel-air mix can clean things up without messing up starting or running performance, why not? I'll keep my eye on performance and, if it diminishes between service periods, I'll look again at the physical evidence to find the clues to improve it. Thanks for your detailed reflections.

Dave

[Don Moyer]

Dave,

Good for you on all counts! Let us know how your plugs react to your efforts. Since you're willing to experiment a bit, you might consider an adjustable main jet. Many of the subtle variables in an otherwise OK carburetor can be fine tuned closer to the perfect mixture by being able to adjust the high speed mixture as well as the idle mixture.

Don

[daveinrenton]

Will do, Don. I'll make the adjustments on the idle and pull the plugs in a month or so of weekend use and take a reading. I think the adjustable jet makes sense given my general attitude about clean burning and generally making thing last.

Dave

[lgmuth]

Don
How do you feel about using an autolite 308 compared to autolite 306?
Lou

[Don Moyer]

Lou,

We have no information on an Autolite 306 or 308. We chose the Autolite 86
in order to offer a slightly higher in heat range than the Champion RJ12C
provided without having the center electrode protrude much further into the
combustion chamber.

Don