Fouled Spark Plugs

Reader Contribution by Keith Fellenstein
Published on November 29, 2011
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Editor’s note: If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as your subject. 

Fouled spark plugs

Q: My 1973 Norton 850 Commando has the annoying habit of fouling spark plugs. What can I do to prevent it?
-Kenneth Peters/Lawrence, Kan.

A: If this is the first time you’ve fouled a set of plugs, I’d change to projected tip spark plugs of the same heat range and try again. Projected tip plugs have the same thread depth as regular plugs, but the center electrode projects farther out into the combustion chamber. Not far enough to collide with the valves or piston, but out in the fire a little more than a regular plug. Sometimes that’s all it takes to stop fouling plugs. If that doesn’t work, you may want to try one heat range hotter plug.

Finding out why the plug is fouling is also a good idea. Is the fouled plug sooty dry black, indicating a too rich condition, or sooty wet black, indicating oil in the combustion chamber? Sooty dry black can be remedied by adjusting the carburetors to lean out the mixture as needed. You’ll need to determine where in the power band the fouling is taking place; idle, just off idle, mid-range or wide open.

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