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Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.


Fouled Spark Plug

Motorcycle Classics tech expert Keith Fellenstein

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 email with “Keith’s Garage” as the subject.

Fouled spark plug

Q: I have a question about my Yamaha XT500. The spark plug keeps fouling. Whether I ride it for a short time or a long time, it doesn’t matter, it still becomes black. I took the carburetor apart and cleaned the jets. I adjusted the idle screw, but after running the bike again, the plug is black. I bought the bike from someone who lived in Sacramento, California, and where I live the elevation is around 2,200 feet. Do I need to run smaller jets? How do I fix this problem? – Marvin Schaaf/via email

A: I’ll give you the easy answer first: Your being at 2,200 feet elevation shouldn’t have a major effect on carburetion. Now as for your black spark plugs, the first thing to determine is if they are fuel- or oil-fouled. Oil fouling is black, like fuel fouling, but fuel fouling is characterized as more of a fluffy deposit where oil fouling is smooth and glossy. Too much gas is easily rectified by smaller jets, but too much oil is a more costly and time-consuming repair. Let’s start with the jets and see if that improves things. You didn’t say what year or variation you had so I’ll cover them all. The standard main jet for an XT500 varies by model: 210 for a C, 240 for a D, 230 for E and F, and 250 for the G and H. Compare that with what you have. Check the idle jet also. The standard idle jet varies by model also: 35 C, 30 D, 25 E and F, and 20 for G and H. It’s always possible that the previous owner felt the machine was set up too lean, and rather than purchase bigger jets he or she decided to drill out the ones they had. If that’s the case, the numbers on your jets will say one thing while the orifice will be delivering something else. Jets are pretty cheap; you could just buy another stock set, and at the same time, one step leaner set. Try the new stock set first, and if you still have plug fouling, try the one step leaner set. MC