Motorcycle Repair: slow to return to idle CB77, air leaks, cable housings

Question

CB77   My '63 restored Superhawk has about 200 miles on it and I'm finding it a litttle slow to return to idle from around 2500-3000. This was not here in the beginning and has just come about in the last 100 miles or so. I'm certain it is not the cables or slides hanging up, looking into the carb throats show them snapping shut. I've checked for air leaks around carb to aircleaner . Adjusting the air mixtue screws out seems to help very slightly. Weak advance springs? The bike runs great otherwise. Thanks for any advice.

Answer
Kevin, nice job on the 1963 CB77.

Yes, the most common issue with slow returning idle symptoms is often the spark advancer springs being weak or stretched a little. I always crimp the ends of the springs slightly to help snug up the weight return function. Otherwise, the idle/spark timing interfaces continue to feed each other, resulting in the high idle speed.


Carbs can warp a little when they warm up, but it is hard to detect that with air filters in place, although you can reach in and tug on the throttle cable housings to check for excess slack when the engine speed is still above 12-1400.

I have seen some decaying carb insulators recently, where they break out around the o-rings and cause an air leak. I assume that you have replaced all the o-rings on the insulator and carb base flange. Make sure that the carb flanges are pretty flat. Don't go crazy tightening down the carb nuts, either, as that contributes to the air leak issues and sometimes warps the carb body, too.

Make sure that the spark timing at idle is dead on the F mark/LF marks, but always check to see if the spark timing is still coming in before the II marks on the rotor for each side. Over-running the advance marks can cause piston overheating and piston seizures.

Check the slides for a decently tight fit in the carb body bores. Use the later, coated alloy slides, rather than the old chrome-plated brass slugs that the bikes came with, originally.

If you are close to sea-level, I suggest running #140 main jets with the needles at #4 or #5 clip position from the top. I have the #140 mains, if you need them.

Bill Silver
www.vintagehonda.com