Spark Plugs

[Gezer]

I have a engine missing problem.
All 8 plugs in this EAB Flathead 52 Merc, I think) are carboned up. also noticed one bank has 215 autolite and the other has 216 autolite. Not sure why or what the difference is or which one , if either should be on this engine. Idles ok, but misses under load.
Have not checked wires, timing is ok.
I AM ALL EARS.

[frnkeore]

If you have a spark plug cleaner, use it or try to find one to use. Carbon builds around the porcilen (SP) until it arcs to the electrode and shorts. The higher the compression pressure, the sooner it arcs there.

Also, gap the plugs at .025, before reinstalling them.

If it burns oil at all, the carbon will build faster and you will need a hotter, heat range plug. That may be why it has two different heat range plugs in it now.

A good tune up (points and check the oil output), will help the voltage, also. The higher voltage will help fire and keep the plugs clean also.

Frank

[supereal]

Mismatched plugs should be replaced with a new set. If the old plugs have a black soot covering on the tip, it indicates that the fuel mixture is too rich. Missing under load is often caused by ignition problems, but a good dry/wet compression test is also advised, At least check with a vacuum gauge. That will provide info on the condition of the valves and rings. Your carb should be checked to find out why the engine is running too rich.

[Fordestes]

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The difference in the plug numbers is usually the indicated heat range , some manufactures the higher the number the hotter and others the lower the number the hotter , look at a plug chart for the brand of plugs and it should indicate the heat range numbers. I think the autolite numbers are the higher the hotter, hotter is a longer heat path back to the cylinder head where the heat is taken care of by ambient air and the cooling system. I use 216 autolites . the hotter the heat range the more heat is retained and the slower the heat is dissipated away from the plug firing tip , a longer heat path from the plug tips (hotter) allows less carbon buildup. be careful not to use too hot a plug ,that can cause engine damage.