® Reasons for a hanging idle on the CBX1000

First the definition. A hanging idle is when the engine idles normally at 1000-1100 rpm until the throttle is used or the engine gets hot, then the engine rpm won't come all the way back down even after the idle speed screw is turned downward. Here are seventeen things that can cause a hanging idle on the CBX.

  1. Improper carb sync

This is not only the most frequent cause of a hanging idle, it's also the thing most often misadjusted on the CBX. Syncing on this model is not particularly difficult, it's just not very intuitive. If syncing on any multicylinder motorcycle is a learned skill, on the CBX it's several levels more an art that few can say they have mastered.

  2. Partly clogged idle jets

This is second in importance and again, so ubiquitous as to be endemic to the CBX. Due to their extremely small size, the carbs' idle passages are frequently dirty and the resulting leanness can cause a hanging idle.

  3. Incorrectly installed choke cable

You may not have thought of this, but the fact is the CBX choke cable installs in a way that looks incorrect. If you trust the visual, you'll get it wrong and the carbs' fast idle mechanism will prematurely deploy, making a good idle impossible to achieve. See my video on cable install.

  4. Vacuum leaks

This is the one forums often focus on but it isn't as likely as the ones above. It's still high on the list however. Vacuum leaks on the CBX are most often due to bad intake manifolds. But see number 8 also.

  5. Tight valve clearances

The Honda CBX is one of the worst engines in terms of valve recession. It's almost a given therefore that your valves have insufficient clearance. And tight valves greatly affect idle smoothness. Valve clearance adjustment, cylinder compression tests, and cylinder leakdown tests are tasks that are vital to the CBX.

  6. Incorrectly set pilot screws

Most CBXs need to be adjusted to a pilot screw setting different from that Honda recommended, to between almost 2 turns and 2 1/2 turns.

  7. Incorrectly adjusted fast idle

The CBX carburetors' fast idle linkage is important and unfortunately very quirky. It needs to be looked after when the carburetors are on the bench. Moreover this adjustment is very non-intuitive and is actually different on the 78/79 model versus the later models, and is virtually impossible to do on the bike. And of course the factory manual completely ignores it.

  8. Carbs not fully installed

This is easy to get wrong because mounting the carbs is a unique, not-very-fun, even strenuous procedure. Here's a tip: When the carbs are fully installed, the slotted aluminum vacuum sealing screws will almost be touching the rubber manifolds. Use this as your guide.

  9. Engine overheated during sync

The CBX is a very hot running engine. Even the most expert mechanic cannot sync these carbs without a fan blowing on the cylinder head. If syncing takes longer than five minutes, and you don't have a fan, the engine has overheated. This is hard on the engine, but it also makes syncing impossible. No matter what your gauges read, they're lying to you. The sync is faulty. Once this has happened, the engine must be allowed to cool down before syncing can resume.

10. Pliot screw broken off

The pilot screws in the CBX carburetors can have their tiny tips snap off inside the carb body. This will affect the smoothness of the engine idle and the ability to sync the carbs.

11. Throttle plates shifted

The CBX carburetors' throttle plates can shift on their shafts. This results in added friction and binding as the throttle is used, leading to throttles that are slow to close, resulting in a hanging idle.

12. Cam timing incorrect

More than one individual has got the cam timing incorrect on the CBX. This will result in engine rough running that can't be tuned out.

13. Linkage dampers missing parts

The CBX throttle linkage dampers have very tiny pieces inside. When the carbs are serviced on the bench, these pieces can get away from you. The incorrectly-assembled dampers will make the throttles not uniform in their movement, which can cause a hanging idle.

14. Throttle shafts binding

This is similar to number 11. When as part of a rebuild the CBX carbs' sync adjusters are re- zinc plated, their shafts' diameter increases. This inevitably results in binding in the carb bodies. Careful fitting of the replated shafts is the preventative.

15. Engine wear

The CBX engine due to its soft valves and very high running temps wears astonishingly quickly. By 15,000 miles the valves are already significantly receded and by 25,000 the cylinders worn to double their assembled piston-to-cylinder clearance. The effect of significant engine on carburetion can lead to throttle abnormalities.

16. Vacuum valve issues

On 80 through 82 models having the vacuum valve, the vacuum nipple on the #3 carburetor often loosens, creating a vacuum leak that precipitates a hanging idle. Moreover, the valve's diaphragm gradually fails, creating a lean condition that can also result in a hanging idle.

17. Ignition timing incorrect

After syncing the least intuitive task on the CBX, ignition timing is an odd out-of-the-box procedure and thus on most bikes liable to need adjustment. This is even more likely if a Dyna or other aftermarket ignition trigger assembly has been fitted. Engine tuning shortcomings such as this can result in idle inconsistencies.

Last updated May 2022
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