Idle sticks high - kz650d1a '78

After the bike warms up there's a tendency for the idle to be quite high. 3 out of 5 times, if I blip the throttle a little it drops back down to acceptable rpms. As the bike bike gets warmer, this "fix" works less and less. Confirmed there is no obstruction with the cable and throttle return. Carb has been cleaned and all fuel lines have been confirmed clean.

I have pods on the bike right now. But this problem was around before the pods.

Jets are 115 main, 7.5 pilot, stock needle position. Right now my mix screws are 1 full turn out. Any more out and the idle gets even higher. Did a bench sync but haven't done a sync while the bike is running.

The header pipe on piston 1 is cooler than the rest. Water doesn't dance and sizzle off it. All power seems to be present when 3/4 to WoT. Little groggy in the 1/4 though. But not so bad when the bike is warm.

Spark is healthy. Compression isn't amazing but it's there. about 100psi-110psi in all cylinders.

I have a friend with the same carbs. Was going to just try swapping our carbs just to rule out any other causes. Electrical, etc.

Any thoughts? Thanks guys.

Sounds like a possible vacuum leak. Try this test:

Make sure the vacuum caps are tight on the carb holder nipples.

Warm the engine up to normal operating temperature.

Temporarily set the idle as low as possible without stalling the engine. It should be down below 1000 rpm. The lower the better because when it is set low the carb slides are closed (or nearly closed) and this raises the vacuum level in the carb holders. The combination of the increased carb holder vacuum level and the very low idle amplifies the effect of the leak test.

Once the idle is set low, spray carb cleaner around each of the carb holders where they bolt to the cylinder head and where the carbs attach to them. If there are leaks, the vacuum in the carb holders will suck the carb cleaner in and this will affect the idle. It may cause the engine to stall. If the leaks are at the carb holder / cylinder head mating surface you need to replace the holders. If the leak is where the carbs attach to the holders you may be able to tighten the clamps enough to seal the leak. Some folks use starter fluid or other substances for the test, but carb cleaner works best as it doesn’t evaporate too quickly like ether and leaves no residue on the engine like WD40.

If you do need new holders just bite the bullet and buy them. Trying to seal things up with some kind of sealant or goop will only lead to frustration as it will at best only last a short time.

Ed

Thanks a lot Ed. I'll give this a try.

Mixture is lean. Typically caused by a vacuum leak but other factors could contribute as well.

Not to go against Ed here but the "spray stuff on the carbs" technique to find vacuum leaks will only find gross leaks, not small ones. It's a worthwhile test, but not definitive.

Pods drastically affect mixture so that's something to consider.

The fact that one cylinder is not as hot as the others suggests a problem. Hard to know what to suggest other than go through all the basic maintenance and see what happens. Valve adjust, vacuum carb sync, check spark plug wires and cap for proper resistance, etc. The problem could be related to one or all of these things. You can't diagnose running problems on a bike that's in need of basic maintenance.

Thanks, Nessism. I'll keep that in mind.

I did the valves last year so they should be good. Plug leads are brand new and the dyna coils are 3 years old, bought new. I do need to do a vacuum carb sync though.