Idling issues

I've had this 1976 kz900 for a couple months now and I've been tracking down and fixing carb issues and spark issues that it had when I got it. I installed an electronic ignition, new coils, replaces a lot of wiring, rebuilt the carbs with new jets and filters,new intake manifolds and more. I think I have the bike jetted properly because the plugs look just about perfect and while riding the throttle response is very good through all the rpms.

The one problem that I can't seem to sort out is the idling. I have to turn the idle adjustment up when it's cold and then down when it's warmed up. Then if I rev it, the idle will stay up around 2500 rpms for awhile and then slowly drop down and almost go so low it dies. If I just let it idle on it's own the idle fluctuates between 1200-2500 rpms. I know this can be casused by a vacuum leak, that's why I changed the intake manifolds, but that didn't seem to help. Can and air leak be coming from somewhere else?

What else could cause this?

Also, when I synced the carbs the vacuum strength seemed lower than bikes I have synced in the past.

Would lube the weight arm pivots on the timing advancer.
May use a drop of motor oil on each of the two pivots.
May access the pivots via the timing inspection window.









Inspect the weight arm springs on the timing advancer to assure that the weight arms are consistently being freely pulled back to their fully closed position.

The advancer isn't supposed to look like this:




Erratic idle may also be caused by conditions other than an issue with the advancer.
If not already done, would check the valve clearances and assure they are within specs, because too tight valve clearances may contribute to the reported issues.

To test for air leak at carb holders, or a loose clamp, may spray WD40 about the carb holders and watch for smoke from the exhaust which would indicate ingestion of the WD40 into the intake tract. May also notice a change in the idle rpm caused by ingestion of the WD40.

Good Fortune!

Could also slightly enrichen the pilot circuit in each carb by turning the pilot air screw adjuster IN, say 1/4 turn, and test run.

Has there been any popping from the exhaust on deceleration?

Good Fortune!

Another possible issue could be adjustments of the throttle cables, which might lack sufficient slack, or be hanging for whatever reason, such as binding or kinked or frayed internally.

And maybe the throttle cables need lubing.


There's of course the time-honored "still dirty pilot circuits" possibility.




Good Fortune!

I started with the idle jet screws 1.5 turns out from gently seated and as I tuned the bike and checked the plugs I ended up turning them back in 3/4 of a turn. The screws and now 3/4 out from being seated. That has helped with throttle response but not the idling issue.

I have inspected and cleaned the carbs 3 times now and it has an electronic ignition so it can't be the weights.

SeattleKZ wrote: I started with the idle jet screws 1.5 turns out from gently seated and as I tuned the bike and checked the plugs I ended up turning them back in 3/4 of a turn. The screws and now 3/4 out from being seated. That has helped with throttle response but not the idling issue.

Seems excessively rich.

Does tuning the pilot air screw make a noticable difference in idle rpm?
If not, would suspect an imperfectly clean pilot circuit.

Would inspect the pilot screw tips, which might be damaged and thereby failing to properly regulate air flow into the pilot circuit.

Would also assure that the o'rings on the pilot screws are in good condition so as not to allow air leakage.

Good Fortune!

The idle screws and o rings are new and in perfect condition. I have been doing the adjustments while the bike is not running. I will adjust it while its runnin and check.

Which electronic ignition?

For example, the popular Dyna-S retains and requires the stock timing advancer.

Good Fortune!

It's the Dyna so maybe my memory is not to good. I'll check tonight when I get home from work.

Patton wrote: Another possible issue could be adjustments of the throttle cables, which might lack sufficient slack, or be hanging (for whatever reason).
And maybe the throttle cables need lubing....

When gently closing the throttle, should hear a faint but definite click from the carb as the throttle stop makes contact with the end of the throttle adjustment screw.

Good Fortune!

When adjusting the carb pilot screws while the engine is running, it's a good idea to have a fan on the floor blowing air over the engine to prevent it from over-heating.

Same applies when performing a running sync.

Good Fortune!