Technical Q and A for classic motorcycle maintenance and repair.
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, Motorcycle Classics, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as the subject.
Fouling spark plug
Q: My motorbike is a Bajaj Caliber. The spark plug on it is a light brown on long trips, but if I check the plug after a short journey or when it’s just been idling for a bit, the plug tip is black. Why is this and how can I overcome this problem? — Ricardo P./via email
A: If your plug is black and sooty at idle or low throttle settings, I’d try leaning out the idle mixture by 1/4 to 1/2 a turn. You may have to increase the idle speed if you do that, and you may find it stumbling a bit as you open the throttle. Oftentimes, I’ll adjust the idle a little rich so that acceleration is smoother as you open the throttle. If the plug is just black and not fouling, that may be the best compromise. (Note: A follow-up email from Ricardo says this worked.) MC