912 S Sooty Plugs

[Dobbs]

Looking for help on possible causes for sooty plugs.

I replaced the plugs on my 600 hour engine this past week end and was surprised to see how sooty the plugs were on the number 1 cylinder (both). Number three cylinder plugs, also fed by the same carb were perfect ( tan colour), as were the other two cylinders.

The engine is not using oil, or at least not excessively, less than half a pint per 100 hours.

I have had the engine from new, it flies around 80 - 100 hours per year, flies at regular intervals, gets serviced as per the manual and I use the oils as recommended in the manual, and run avgas less than 5% of the time.

Can anyone maybe shed some light on possible causes.

[vernon11]

Dobbs beat me to the post. Yes I was very surprised to see the amount of soot.
Vernon.