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Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.

Fouled Spark Plugs


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.

Each area of the power band is controlled by a different part of the carburetor circuit. Idle is controlled by the idle jet and the air screw. On Amal Concentric carburetors like you should have, you screw the air screw in until it lightly seats, then back it out one and one-half turns. That’s your starting point. Continuing to turn the screw counterclockwise leans out the mixture by admitting more air into the idle circuit. Your goal is the smoothest idle with no stumble as it transitions from idle to just open. If you have a smooth idle, but the bike hesitates just as you open the throttle, turn the air screw clockwise one-quarter to one-half turn to provide just a little more gas for acceleration.

The second circuit to adjust is wide open. Find a good straight road with a slight incline and light traffic and open the throttle up in third gear. Does the bike accelerate smoothly or does it stumble or surge? If it doesn’t accelerate smoothly, does slightly closing the throttle improve the running? If slightly closing the throttle improves running, your main jet is too small. Go up by 10 to the next largest main jet and repeat. Reading the plugs by doing a plug chop will also tell you about your mixture. Repeat the test above, using the kill button to stop the engine about two-thirds of the way up and coast to a stop. Remove the spark plugs and see what color the center electrode shows. Dry sooty black is too rich, stark white is too lean, tan to chocolate brown is just right.

After you have both the idle and wide open settled down, you can then work on just off idle and mid-range. Just off idle is controlled by the cutaway of the slide.

Amal expresses the cutaway in numbers like “3” or “3½.” The higher the number the leaner the mixture. Mid-range is controlled by the needle in the slide; raise the needle to richen the mixture, lower it to lean the mixture. Repeating the plug chop as mentioned above will tell you what you need to do. It’s a long, tedious process, but the end result is a bike that responds as it should through its full rpm range.

It can be harder to find the cause of oil in the combustion chamber, although there are really only a few sources; worn rings or worn valve guides, and possibly leaky or torn valve seals on the intake valves. A compression check and leak down test can sometimes give you an answer without taking the head off. You can also remove the carburetors and intakes and look in the intake to see if oil is visible on the intake valve or guide. There’s usually not a problem with the exhaust valve as that side runs pressurized while the intake has a vacuum, trying to suck oil down from the valve train. MC 


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.

12/30/2014 12:58:02 PM

This is a three generation project. A 1974 Honda CB550F. This bike was purchased after an extended storage. The plugs were sooty fouled. We pulled the carbs and completely cleaned and rejetted and mechanically synchronized the carbs. After remounting the carbs we tested for compression. All four cylinders read 40 to 50 psi! Oil squirted into the cylinders made no difference. Pulled a couple tappet covers to observe the valves. No apparent problems. Oil in crankcase looks OK (no blow by symptoms). Why such uniformly low pressure? Are all the valves not sealing? Could the soot be preventing proper closing? If so, how do we clean the combustion chambers of soot? Do we have to remove the head? Previous owner says bike was running when he put it away ... (they all say that). HELP PLEASE