Motorcycle Repair: Idle problem, cb900 custom, vacum leaks

Question
I recently purchased a 1981 CB900 custom which had been garaged for some time.The previous owner had removed the carb assembly but did not completely drain the gas.I have had the carbs disassembled,thoroughly cleaned and re-assembled.The bike is back together now and will start fine but will run smooth only when the choke is about halfway out.Once pushed all the way in ,it runs rough and will not idle.I have checked for vacum leaks and I am running some cleaner through the tank to take care of any lingering sludge.I have also installed new plugs and battery.Thanks in advamce for any insight.

Answer
Kevin, poor idling is usually rooted in blocked idle jets or air passages, defective air cut (anti-backfire) valves, petcock diaphragm failures and intake manifold leaks at carb or head.

If you have good even compression, have done the carbs properly and put in fresh plugs, a blocked petcock screen, inside the fuel tank is a potential problem area, if it is really clogged up. This would cause low fuel levels, due to the lack of sufficient fuel flow to all 4 carbs at the same time. Drain the fuel tank, unscrew the petcock from the tank and then remove the nylon screen from inside the fitting. The automatic fuel cock system, mounted on the carb rack has a diaphragm in it, that will fail, leading to fuel flow restriction AND a vacuum leak, all at the same time.

Double check the diaphragm on the left carburetor. That is the air cut valve diaphragm and they often fail after so many years. You can get replacements from K&L dealers, instead of going straight to Honda for an OEM part.

Make sure that the gas cap venting is all functioning, as well, otherwise it can vapor lock the fuel supply.

In cases of weak spark, a rich mixture will be required to keep the engine running, as well, but that is a fairly uncommon problem. There are spark modules back by the battery, look to see if any look melted or have bad connectors. Be sure to have a fully charged battery and that the charging system is fully functioning, as well. That is another trouble spot on the DOHC fours. There are carbon brushes under the right side crankshaft cover, which are expendable and need to be checked/replaced every 10k miles or so.

Bill Silver