Carbon build up on spark plugs

A

azaviator08

New Member
What causes carbon to build up on spark plugs? Why does this give us a drop in RPM on mag check? What are we actually doing when we try to "burn" it off?
 
Carbon is a byproduct of combustion, but it will only build up on the spark plugs when they are below a certain temperature. Burning it off the plugs raises the temperature high enough to remove the carbon fouling from the plugs. The rough excessive mag drop that you get when you have a bad mag drop is because the fouling has shorted out the spark plug. With the spark plug shorted out that cylinder is no longer firing. You don't get the roughness on both mags because the other spark plug will still ignite the fuel in the cylinder.
 
N18040 said:
Carbon is a byproduct of combustion, but it will only build up on the spark plugs when they are below a certain temperature. Burning it off the plugs raises the temperature high enough to remove the carbon fouling from the plugs. The rough excessive mag drop that you get when you have a bad mag drop is because the fouling has shorted out the spark plug. With the spark plug shorted out that cylinder is no longer firing. You don't get the roughness on both mags because the other spark plug will still ignite the fuel in the cylinder.
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Your answer is partially correct. Carbon buildup is a condition when the mixture is excessivly rich, and the temperature is simply the by-product. It is possible to foul a spark plug when running it at the proper temp. range, but it isn't very common. One good example would be oil fouling. Usually when an engine is burning oil, the cumbustion temps. will be higher. Oil is a much lower octane and causes more heat, and more carbon. These are usually the bad mag drops that you can't clean up by "leaning and heating."

And you get that carbon buildup not just on the plugs, but also on the rest stuff inside (pistons/valves/cly head/etc.)
 
Also carbon isn't the only thing that will foul plugs. The Tomahawk I fly has issues with lead fouling. Even though we have fine wire plugs on the bottom the bottom ones are still full of lead every 50 hours when I pull them out to clean them.
 
Lead fouling is common problem with my Lycoming O-235 in the lower plugs...Lean to taxi, lean above 3000, work the mixture / watch the EGT for best results. We also had some lead fouling with the O-360 last fall.
 
N18040 said:
Also carbon isn't the only thing that will foul plugs. The Tomahawk I fly has issues with lead fouling. Even though we have fine wire plugs on the bottom the bottom ones are still full of lead every 50 hours when I pull them out to clean them.
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pnwchief22 said:
Lead fouling is common problem with my Lycoming O-235 in the lower plugs...Lean to taxi, lean above 3000, work the mixture / watch the EGT for best results. We also had some lead fouling with the O-360 last fall.
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Heat will almost always clean up lead fouling. In our 152, if you don't lean aggresivly on a taxi, you are going to foul a plug.
 
If I don't clean the bottom plugs from the O-235 in the Tomahawk every 50 hours typically the bottom plug on cylinder three will build up enough lead that it won't burn off. At least I haven't been able to burn it off.
 
N18040 said:
If I don't clean the bottom plugs from the O-235 in the Tomahawk every 50 hours typically the bottom plug on cylinder three will build up enough lead that it won't burn off. At least I haven't been able to burn it off.
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Well, it doesnt look like you have a case of "almost always." The 235 in our 152 doesn't like bottom plugs either. But I find that if I make students bring the RPMS up to 1500 and lean for a drop, then begin their taxi at normal RPM settings, it doesn't do it any more.
 
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