1987 KAWASAKI GPX750
Q: My bike holds on to revs after the throttle is closed and then gradually returns to tick-over. I think I’ve checked all the usual suspects for air leakage, which I’ve assumed is the cause of the problem. Inlet stubs are properly clamped to the inlet manifold and carb bodies, are pliable and don’t appear to have cracks or splits. The throttle/choke cables are properly routed and move freely, and the throttle snaps shut when released. Removing the carb bank from the bike, each slide moves smoothly in its bore and returns to fully closed via the diaphragm spring. Butterflies open in unison via the throttle linkage. Removing the carb caps, the diaphragms are pliable, with no splits or pin holes that I can see, and are properly located in the seal register under the caps, which are fully tightened. Then, they are 31 years old. IIS there anything else I should be checking or may have missed?
Rob Neal
A: I think the only thing left is to test whether or not you can detect a leak somewhere. You can do this by getting the engine to idle at a normal speed and using a flammable fluid such as Bradex Easystart or lighter gas, which you can carefully spray around the carburettors – but avoiding the air intake – to see if the idle speed changes. Use some common sense when spraying flammable substances around a running engine, such as having a fire extinguisher nearby. The other cause of this problem used to be a sticking ignition auto-advance mechanism, although, of course, you have an electronic system on your bike. I think it might be worthwhile to check the timing with a strobe, to ensure that it is responding correctly to changes in engine speed.