Surging Throttle: Honda CB350K3


| 12/26/2013 8:46:00 AM


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 e-mail with “Keith’s Garage” as the subject.

Surging throttle

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

GERALDE
1/2/2014 5:27:27 PM

know idea the seals issue; from the dr ad index > replace the throttle spool.




The sound and the fury: celebrate the machines that changed the world!

Motorcycle Classics JulAug 16Motorcycle Classics is America's premier magazine for collectors and enthusiasts, dreamers and restorers, newcomers and life long motorheads who love the sound and the beauty of classic bikes. Every issue  delivers exciting and evocative articles and photographs of the most brilliant, unusual and popular motorcycles ever made!

Save Even More Money with our RALLY-RATE plan!

Pay now with a credit card and take advantage of our RALLY-RATE automatic renewal savings plan. You save an additional $4.95 and get 6 issues of Motorcycle Classics for only $24.95 (USA only).

Or, Bill Me Later and I'll pay just $29.95 for a one year subscription!




Facebook Pinterest Instagram YouTube Twitter

Free Product Information Classifieds