Blackened spark plug

[davidm]

I'm changing out spark plugs on my 2002 BF50A. The old ones from cylinders 1 and 2 look "healthy" but #3 (the lowermost cylinder) looks much blacker, with only a trace of tan showing on a small portion of the ceramic part of the plug. It is not fouled with crusted on carbon, but is blackened nonetheless. What might be the cause of this? Motor seems to be running well and capable of WOT. Pictures below may help tell the story. Thanks for any ideas! Dave

 

[jgmo]

Well, it certainly looks like the lower cylinder is a bit too rich. As to why, it might be tricky to figure out. I would probably start by making sure that the carburetors are synchronized or "balanced". If you troll or idle quite a bit, it could make a big difference if they were not phased to each other properly. It takes a special tool and procedure for syncing the carbs but you can have it done fairly reasonably at lots of shops.

The choke for that carb could be hanging closed a small amount.

Or, it could be that you're not getting complete combustion in that cylinder due to the valves being out of adjustment or that plug is misfiring some.

If it were a thermostat allowing the engine to run too cold, I would think the problem would show in all cylinders, not just the one.

It really doesn't look all that drastic from here so I wouldn't get real excited about it right away. Depending on how long the plugs have been in there, I think I would throw some more plugs at it and then monitor the situation.

Make sure that you do run the engine a good amount at the higher throttle settings to keep the cylinders cleared out.