Motorcycle Repair: High Idle CB77

Question
Hello Bill, When my Superhawk is warmed up she tends to hold her high rpm's when coming to a stop,  I just let out the clutch to get some drag and drop the revs and she drops to normal idle and holds there.  If I were to rev in neutral she may not come back down to idle.  But a little friction can get it back down to idle.  Throttle untouched when I use friction to get rpm's down.  Thanks in advance.

Answer
Kelvin, several systems interact when the high idle condition shows up. Loading the engine reduces the rpms which causes the spark advancer weights to pull the timing back enough to slow it down again.

The usual cause is that the spark advancer weight springs are getting weak or breaking at the ends. Grab the point cam on the end and twist it back and forth to check for any excess play in the advancer mechanism. There should be minimal loose play and the springs should want to twist it back into the retarded position. Premature spark advance is what causes engines to rev up like that and hang there, until they are loaded down a little, as you have noticed.

Also look for sideplay on the point cam where it runs inside the right side camshaft. If you have lateral movement, then that affects the point gap, which changes the ignition timing. When the timing starts advancing the engine's vacuum signal to the carburetor metering circuits changes the way the carburetor wants to feed the intake port.

You can setup the ignition timing statically with a 12v test light, so that the points open at the F (and LF) marks, but it is necessary to then check the ignition timing, including full advance by using an automotive timing light with the engine running. You never want to see the spark timing advance beyond the II marks on the rotor. Those advance marks are at about 43-45 degrees before TDC. Beyond that the pistons will start to enter the "seizure zone" when put under a full load out on a hot highway situation. I also recommend using #140 main jets in the CB77 carbs now, due to the diluted fuel we have to use here now.

You do want to check the intake system for air leaks... carb flanges warp, o-rings flatten out and the insulators can chunk out around the o-ring recesses. Any air leaks will lean the engine out which sends the idle speeds up, as well.

Bill Silver