QuestionMy 1981 Honda CB750C starts and warms up normally. If I ride for about an hour, the idle speed starts to increase, and eventually runs at 3000 RPM. If I roll the throttle, the speed drops to normal, but goes back up within a few seconds. Any ideas?
AnswerTony, Erratic idle speeds can be caused by inaccurate valve adjustments, ignition timing out of spec or the spark advancer springs being stretched or broken. The advancer runs in oil in the left side, so they are generally not a cause for worry, but don't forget about it if nothing else will work.
From there, you have other issues to consider concerning the carburetors, intake tract and fuel system. Heat can cause the rubber intake manifolds to expand and cause air leaks. The vacuum petcock can get a pinhole in the diaphragm, causing both an air leak and a reduction in the fuel flow, which can cause the fuel levels in the carbs to drop, leaning out the mixtures.
The carburetors should be synchronized when the engine is warmed up, anyway, not cold. As the pistons expand, the compression increases slightly and the oil viscosity changes as it heats up lessening internal drag, which adds up to more rpms at idle. Check all the small vacuum hoses connected to the carbs and intake for damage or loose fitting ends.
If the engine has a lot of miles on it, the compression could be uneven, especially noticible at idle (cold) and it will increase slightly as it heats up, which again raises the engine speed as it produces more power when the compression is higher.
Do check the compression first, have the valves checked for proper clearances, then you can go into the timing and carbs after that. A lot of it depends on how many miles it has on it and how it was maintained and how long it was stored..if it was stored properly.
Finally, check the free play in the throttle cables and the choke cable to be sure that nothing is binding during operation.
Bill Silver
