Idle Revving

My bike behaves exactly like yours...

I just rebuilt the carbs, bench synched them and have great difficulty getting it started. When I do, the RPM's rev to about 6000 (or higher) unless I constantly play with the throttle.

It reminds me of a undamped oscillation I encountered in some of my engineering classes. By playing with the throttle I smooth out the oscillation and cause it to idle somewhat normally.

This problem should be fixed the moment I synch my carbs.
But, I cannot synch my carbs unless the bike is running. I cannot keep the bike running at normal RPM's unless I am adjusting the throttle. Can I synch carbs while I am adjusting the throttle?

It's true. It's a Catch 22. How do you sync the carbs to solve the problem when the bike acts erratically when it's running?

I've been wondering lately if maybe the tension that keeps the butterfly valves closed is too loose. if the pressure of the engine is just too much that it pulls the valves in the carbs open more and more as the idle increases, it may be that there needs to be more tension keeping the valve closed. Mind you, this is all just a theory. My next step was to make absolutely sure that the return cable on the throttle was tight enough to not allow any movement. But then I started to wonder, does the butterfly valve move independently of the throttle mechanism? Or is it tied to it?

Anyone have additional information on what might cause this behavior?

I can't figure out what year and model bike
you're talking about. Did I miss that info
some where?

Is it stock? Modified air intake?

Bob

Hey! I guess maybe that info never got into this particular post? Weird. I've got a 1976 KZ750-B1.

Standard air intake. Nothing different or weird.

I can't figure why the idle would change without any interaction with the throttle. Seems like the butterfly valves should only be able to be moved by the throttle cables.

So confused.

mind2find wrote:

...1976 KZ750-B1.Standard air intake....

For reference, Kawasaki.com shows the following diagram as oem carb on 1976 KZ750-B1.

[click on image to enlarge]



Don't know much about CV carbs, but wondering if some fault with the diaphram #19 might cause surging idle, such as a pinhole or improper fitment?

Good Luck!

mind2find wrote:

...Not sure what you mean by the vacuum line either....

The oem manual petcock doesn't use a vacuum line, regardless of carb style, whether manual-slide carb, or CV "Constant Vacuum" vacuum-slide carb.

On later models having a vacuum petcock and manual-slide carbs (non-CV carbs), there is a vacuum hose to the petcock from a nipple on the carb or carb holder.
Am guessing CV carbs would similarly have a vacuum nipple used to connect a vacuum hose to a vacuum petcock.

Good Luck!

Ah, that would explain the answers I've been getting. So I guess there's 4 overflow ports on the carbs then. one on the bottom of each bowl and one on the side of each carb.

As far as the diaphragm, that's an interesting theory! I did have a really really small tear in one of them when I initially got the bike. It didn't have this effect, but it indicates they're old and worn.

Maybe I'll pull those apart and shine a flashlight through them again. That was the only (and best) way to detect minute tears and holes in those things.

Any other input is welcome. It's so nice outside!

I figured out two pieces of my problem.
I found rips in two of my vacuum hoses, which I would guess are contributing to my idle issues. I will need to replace them.

But, more importantly... it seems as though my throttle cable needs replacing or needs to be lubed. The carb does not return to the same spot each time I let off the throttle. It doesn't surprise me since the bike was sitting for 4+ years before I bought it.

I initially needed to set my idle screw almost the entire way in to make contact with the throttle assembly. Now only about 1/4" of the screw is past it's "nut" and the bike seems to be running fairly normally at a steady 4000 rpm with the choke on.

Sorry, I thought my postings were including my bike model... 1980 kz 550.

fishy, congrats! let us know if it ends up clearing up your problems.

guess we've got a different setup and different problems since i don't have any vacuum hoses.

happy riding!

mind2find wrote:

It's true. It's a Catch 22. How do you sync the carbs to solve the problem when the bike acts erratically when it's running?

I've been wondering lately if maybe the tension that keeps the butterfly valves closed is too loose. if the pressure of the engine is just too much that it pulls the valves in the carbs open more and more as the idle increases, it may be that there needs to be more tension keeping the valve closed. Mind you, this is all just a theory. My next step was to make absolutely sure that the return cable on the throttle was tight enough to not allow any movement. But then I started to wonder, does the butterfly valve move independently of the throttle mechanism? Or is it tied to it?

Anyone have additional information on what might cause this behavior?

What I think is happening...

At some point in the process of things not adjusting out right, the idle screw has been turned up to get the bike to work while it's on choke and the butterfly is revealing too much of the xfer ports at the top and just upstream of the of the plane of the butterfly. The mixture might be all screwed up too.

Get a known starting position - with the bike cold, turn the idle screw out (toward lower idle speeds) until you see the butterflies don't react when you turn it any more. At that point, the transfer ports should be covered. Set your mixture screws to "first position" from the manual. Warm it up from there. You might need to balance the carbs too but try to start from a good first position with the transfer ports minimally exposed, and with known good boots on there.

Mike

Hey Mike, thanks for all the great detail.

I had reset the mixture screws to "first position" in my last tinkering round. i'll double-check the idle screw and check the rest. I've got to pull them apart to have another look at the diaphragms anyway.

My boots seem pretty soft, but I know they're a few years old at least, so I'm going to replace them at the minimum for preventative maintenance. I don't see them on Z1enterprises.com but I've got an email in to Rob there to see if they can dig them up.

mind2find wrote:

Hey Mike, thanks for all the great detail.

I had reset the mixture screws to "first position" in my last tinkering round. i'll double-check the idle screw and check the rest. I've got to pull them apart to have another look at the diaphragms anyway.

My boots seem pretty soft, but I know they're a few years old at least, so I'm going to replace them at the minimum for preventative maintenance. I don't see them on Z1enterprises.com but I've got an email in to Rob there to see if they can dig them up.

How about just putting a light coating of RTV on the inside and on the forward mating surface just to see if that does it for the boots? It's quick... Best of luck.

Mike