The bike was running rough and could not get a constant idle speed. I checked the air filter and it was pretty well shot (contamination, etc.). As I awaited delivery of the new air filter, as a test I ran the bike without the air filter and it had instantaneous throttle response and constant idle speed. Great!

I fitted the new air filter and the throttle response was good but the idle speed was totally erratic. It was OK at 3000 revs but anywhere below that it would run a bit and then drop down and stall.

The plugs, etc. are all good, just this idle speed. Is the petrol feed on a vacuum and not getting proper mix?

Can anyone advise and help, please?

  • Is this carb'd or fuel injected? If carb'd, I'd suggest the vacuums need to be equalized between carbs ... but I'm not a bike guy. This is probably a question for @DukatiKiller to answer. Mar 17, 2015 at 17:59

1 Answer 1


Possible Answers

We were hoping for a little feedback regarding this issue prior to attempting an answer. A few things come to mind regarding the problem description and we'll run through a few of them for you.

Regarding Stalling

  • Fuel Filter - Your fuel filter may be impeded and require replacement. Since you have not indicated this happens at higher RPM's and only at high idle this is unlikely. Replacement is cheap and considered a maintenance item. It may be in your best interest to replace the filter regardless.

  • Petcock - You queried about the vacuum line. If the vacuum line is not attached to the petcock, the vacuum cannot open the diaphragm driven valve within the petcock and you would not get any fuel. If the diaphragm is malfunctioning, entirely possible, you may be getting a fuel starvation issue but considering the lack of knowledge regarding high RPM operation it's difficult to determine if this may or may not be an issue.

  • Air Leak - Air leaks and lean conditions can lead to some of the symptoms you describe. An air leak would be a leak between the carbs and the head. The area of vacuum from the carb butterfly on into the head. If the idle is high after you rev the engine and then it suddenly falls off and dies is the primary symptom. You can get a contact or brake cleaner and squirt it around suspect areas, have a fire extinguisher handy and don't squirt vast quantities. Just little bursts onto manifold joints etc. Smell the exhaust while you do this. This will be your breadcrumb. The exhaust will smell different and very strong if you have an air leak and the carburetor cleaner is getting sucked into the hole and going through the combustion process. Additionally, you should see the idle stabilize a bit as the carburetor cleaner fluid blocks the unmetered air leak. You can also use soapy water as a medium to troubleshoot this.

Regarding Rough Idle

  • Valves - The 1995 ZZR600 has a shim and bucket valve train. There are no rocker arms and the cam lobe rides directly on a bucket. Beneath bucket is a shim for changing valve clearance followed by the valve stem, keepers and springs, etc. A shim and bucket setup will lead to zero clearance situations where the valve can actually hang open and there is a tiny gap between the valve seat and valve face in the head. When this condition arises the vehicle is hard starting, poor idling and general runs like a bag of hammers. One valve hanging open can lead to a very poor running/idling condition that improves with higher RPM. Running a compression test can give you breadcrumbs and hints but a leak down test will truly determine if this condition is occurring. It's a very common issue and is considered general maintenance to adjust your valves.

  • Carburetor Synchronization - When you added in the new air filter you increase the vacuum in the venturi of all the carburetors. This additional vacuum will accentuate any issues that are present from valve adjustment, air leaks between the carbs and head and out of synch carburetors. Out of synch carbs will lead to rough idle conditions. Carburetor synchronization is considered a normal maintenance task and requires mercury sticks (old school dangerous) or dial vacuum gauges. Adjustment is in between the carbs on the butterfly shaft running through the set of four carbs. If the synchronization is terribly out you will see the symptoms you have described.

  • Out of tune - If your air filter was so clogged that the vehicle ran much better once it was removed there is a high chance that a prior owner or low quality mechanic adjusted the air screws to a lean condition because of the clogged air filter. The clogged air filter creates a rich condition and if extreme could potentially foul the plugs. Someone may have leaned out the idle mixture to compensate and once the air filter is replaced....now the bike runs lean.


We would presume that you have a combination of issues.

  1. Lean condition, as described above. The air screws affect idle and just off idle.

  2. Out of synch carbs - you can eyeball and listen to adjust the synchronization. Keep reducing the idle as you change the balance between carburetors. When you are getting closer to synch the idle will increase. When that happens you are on the right track. Reduce the idle and keep playing with them. Some auto stores will 'loan' vacuum gauges that you can use for this, you pay a deposit and get it back upon tool return. This is the better option.

  3. Valve Gap - Ensuring you don't have a gap between the valve face and valve seat will prevent gasses escaping your combustion chamber during compression as well as your power stroke. The bike will start, idle and run better with significant performance increases.


Here are some additional links that can be beneficial to your troubleshooting.

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