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Jeremy R
Jeremy R, Technician
Category: Motorcycle
Satisfied Customers: 2
Experience:  20+ years experience. Factory trained and certified
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Hi. I have a 1998 Kawasaki zx9r I recently experienced running

Customer Question

Hi. I have a 1998 Kawasaki zx9r I recently experienced running problems it won't rev past about 6500 rpm, I have replaced the fuel filter, air filter and stripped and cleaned the carbs, replaced the diaphrams . It will rev cleanly in the workshop but not when under load, also it runs pretty well without the airbox.......any clues......please.
Submitted: 3 years ago.
Category: Motorcycle
Expert:  Doug E replied 3 years ago.

So this problem happened all of a sudden?

It won't run over 6500 in any gear? What does it do when it reaches that RPM? Does it cut out or just won't go any higher?

If you hold it as high as it will go, kill the engine and pull the spark plugs, what do they look like?

Customer: replied 3 years ago.

Hi Doug.

Yes I was out on a ride. I went to overtake a car and there was a slight hesitation before the bike picked up and from there it wouldn't rev. It is ok at lower revs but as soon as you try to push it on a bit it starts loosing power almost as if it has carb icing or is out of fuel. Haven't tried the spark plug test, but we put new plugs in when we stripped the carbs. When it is on the stand in the shop it runs up through the revs with no hesitation.

Expert:  Doug E replied 3 years ago.
I would normally expect a problem in the fuel system, but you have been through that. One thing you might want to check is to check the fuel level in the carbs while running.

Level the bike and connect a clear hose to one of the drain fittings on the float bowl. Run the hose up alongside of the carb body and open the drain screw. Your fuel level is going to be slightly above the level of the float bowl. Start and run the bike and rev it up. The level should remain close the whole time so watch to see if anything odd happens. Repeat for each carb. There may be an obstruction or something going on with the tank vent/petcock that doesn't flow enough fuel under power.

A failed igniter can cause an RPM related issue but it usually won't rev past that point in neutral either. If you have a similar bike in the shop, swap out the igniter and try it.

I would take a compression test as long as you are pulling the spark plugs just so I can rule out a valve issue.

Knowing how the plugs are burning at the RPM may shed some light on what's going on.

Customer: replied 3 years ago.

Hi Doug.

Sorry for the long delay, been a hectic few days.

A compression test was done, and as far as I am aware everything was okay.

The bike will run pretty well without the air box fitted but as soon as you put it back on it won't run properly it just won't seem to rev past 6500-7000 rpm.

Expert:  Doug E replied 3 years ago.
That indicates either there is an obstruction in the tube to the box or the carbs are jetted way too rich.

Were any mods done to the bike prior to this?

This problem started all of a sudden, correct?

I would assume the plugs are burning dark being it's running rich but are all burning the same?

Customer: replied 3 years ago.


No mods have been done they are the same carbs that have been in the bike for the last eight years.

All that has been done is the diaphragms, sliders and needles have been replaced.

Yes the problem started whilst out on a ride.

I will check to see if what you say about the spark plugs is correct.

Expert:  Doug E replied 3 years ago.
Is your exhaust sounding the same?

Customer: replied 3 years ago.


Expert:  Doug E replied 3 years ago.
Did you replace the slides in the carbs? There was an issue with them wearing and not rising. With the airbox off you should be able to watch each one rise as you open the throttle.

I would also verify the fuel level in each carb and check the hoses that go into the airbox from the fairing. There was an issue with those if they aren't installed correctly, but that's not something that would cause a problem on a ride, only after working on it.

I would also do a compression test cold and again hot.