Poor Idle Issues

My 76 Kz900 has the original 26 mm carbs, stock air box, and a V&H Exhaust.

It starts easy and run through the RPM range flawlessly.

However, it has not idled well for several years. When riding, pulling up to a light requires that the rpm does not drop below 1500.

I have cleaned an been through the carbs recently without any improvement.

It ildes poorly and hunts up an down, if set below 1500 rpm then stops.

Yesterday, I warmed it up to operating temp, turn off the engine ,cooled the pipes with a fan. Restarted and felt the pipipes at a 1500 rpm idle. The tempertures varied from pipe to pipe. I turned the air screws out on the hotter pipes and saw a slight improvement.

Suggestion would be greatly appreciated.

Have you checked the carb holders to make sure they aren't letting air in? Ed

Making sure as 650ed said, there are no air/ vacuum leaks. Compression/ valve adjustment/ Ignition timing, dwell adjustment/ Ignition advancer properly operating/ Good quality of spark/ Using a Manometer to Sync the carbs. If not already installed a in-line fuel filter.

I have tried spraying WD40 on the carb holders and didn't see any change. I might try that again.

Thanks, Motor Head

I'll go back and check the basics. Particularly with a manometer.

WMKZ900 wrote:

My 76 Kz900 has the original 26 mm carbs, stock air box, and a V&H Exhaust.

It starts easy and run through the RPM range flawlessly.

However, it has not idled well for several years. When riding, pulling up to a light requires that the rpm does not drop below 1500.

I have cleaned an been through the carbs recently without any improvement.

It ildes poorly and hunts up an down, if set below 1500 rpm then stops.

Yesterday, I warmed it up to operating temp, turn off the engine ,cooled the pipes with a fan. Restarted and felt the pipipes at a 1500 rpm idle. The tempertures varied from pipe to pipe. I turned the air screws out on the hotter pipes and saw a slight improvement.

Suggestion would be greatly appreciated.

A abnormally high idle or a fluctuating idle is almost always an air leak somewhere stemming from an overly lean condition.

WMKZ900 wrote:

I have tried spraying WD40 on the carb holders and didn't see any change. I might try that again.

When you try this be sure to idle the engine down as low as possible first. That's when it' s most likely to cause a change. Ed

With a good hot engine, poor idling is generally air leak/seep anywhere in the carburetion/intake system.
Condition of individual cylinders.
Mixture screw setting.
Synchronization of carbs.
Ignition timing/condition.

When's the last time you did a compression test? More than a 10% + difference between cylinders will start to cause a small variance in smooth idle, that increases with further degredation of engine condition.

Don't forget to maintain your valve clearances - this is considered regular maintenence.

Sometimes, there can exist a very small air seep that can be occuring in the old carburetors themeselves, that requires very fine tuning of the idle mixtures. This has virtually no detectable effect on throttle inputs, but makes idle unstable and have a tendency to drop when coming to a stop after a quick hot run.
You will need an experienced ear/colortune plug to find the exact proper mixtures at idle.

If the pipe temperatures varied at 1500rpm (significant idle input), then your carbs are deffinately suffering from an out of sync condition, or pilot/idle mixture system clog.

Properly performed vacuum sync, and verification is absolutely necessary to achieve a low/slow idle of 900rpm or less, if desired.

Check ignition timing with a known good timing light.

GL!