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, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as the subject.
Q: I own a 1971 Honda CB350K3 with 18,000 miles. The bike starts, idles and pulls well on full throttle. When cruising at enough throttle to maintain speed on level ground, it feels like a surge. The power comes and goes, but speed is maintained. I have cleaned the carburetors, set float height to spec, installed new O-rings on the jets, replaced the points and condensers, set the timing, checked advance, checked the intake boots for leakage, and verified ignition and charging. The cam chain has been tensioned. The air filters are new. I have inline fuel filters, which I replace yearly. I have used a minimum of 89 octane, although 93 octane made no difference. Compression is 180psi. The valves have been properly adjusted. I am at a loss where to go next. — Wayne Gloth/via email
A: I had a similar problem with my Triumph Trident at small throttle openings, such as cruising speed. The problem there is that the carburetor is running in the transition zone between the idle circuit and the needle jet/main jet circuit. At that point, most of the gas needed to run the engine is coming through the idle jet. As the engine vacuum pulls the slide up, more air is admitted, but not much fuel is being pulled through the needle jet. You may be able to correct it with careful adjustment of the idle mixture needle. Run the bike at that throttle setting for about a half mile, slightly uphill if possible so the engine is working. Hit the kill switch and coast to a stop, then remove the plugs and check for rich or lean condition. Since those are constant velocity carbs, adjustment outside of the idle mixture becomes more complicated. If it turns out the mixture is too lean you can always shim the needle using thin washers, but if the mixture is too rich I’m not sure there is a fix. MC