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.
Q: I have a 1988 Suzuki Cavalcade 1400 that spits back through the carburetors. I took the air cleaner off and could see small flashes of fire in each of the throttle throats when it would spit back. It only does it when idling or when the throttle is held at a steady speed. The spitting back quits when the choke is on. It does get better when the engine is warmed up. I don’t think it does it when accelerating or cruising at higher speeds. What do you think might be the problem? — Morris/via email
A: Be careful, those backfires can take on a life of their own and really ruin your day. I have two ideas. I think that either the timing is off or the carburetors are set too lean. Lean settings were a common problem with many bikes from the 1980s. The fastest way to EPA and/or CARB certification was to set the idle so lean it hurts. Gasoline formulations have changed since then and modern fuels don’t burn as well when run lean. You say it gets better as it warms up? And gets better at bigger throttle openings? That’s a good indication the mixture is too lean at idle, either from clogged circuits or because of an air leak. I couldn’t locate a service manual, but you’ll need to get to the idle mixture adjustment screws, which can be difficult: They’re often sealed or adjustment-limited in 1980s bikes, again due to emissions regulations.
Update: Morris later informed me that he pulled the carbs off and found that the rubber intake pipes were cracked through, plus two of the four O-rings where they mount to the head were missing. I suggested that he also clean out the idle circuits while the carbs were off, if possible. Those passages are incredibly small and any gummy gasoline or rust flakes can easily clog them up. MC