Fouled Spark Plugs


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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.

4/5/2019 7:10:34 PM

I've got a 1979 honda cb650,it is hard to start with choke pulled out,I'll run it for a while and shut it off and go back out to crank it an hour or two later and the battery is dead. I've bought 2 new batteries and 2 new coils. Any idea on what it might be? Thanx David

1/8/2015 10:09:01 AM

I like a solid dose of rust buster in plug holes and down the valve stems, intake, exhaust ports let it sit, add some oil to the holes. Will it start? Run it if it will, start gentle, then head out and RUN it. See what happens, repeat the sequence if needed. You have little to loose trying this if the engine isn't making any funny noises and doing this has avoided a lot of unnecessary overhauls.

12/31/2014 6:03:25 PM

First question, are you testing compression with the throttle held wide open? If not, you are not getting a proper reading. If so, then next I would check valve operation and clearances, though it would be unusual for all cylinders to be equally affected. Looks like you've already at least looked at the valve train. It is possible that the rings are stuck in the piston grooves and not sealing against the cylinder walls, but your oil test should have shown that.

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