Any ideas on what may be wrong with my 1976 KZ400?

Hello knowledgeable folks of KZrider. 

I have been trying and failing to get my KZ to ride nicely, and I am hoping for some pointers.

It starts easily, but idles slightly erratically, though this doesn't bother me too much. When accelerating it stutters as if it is running out of fuel- this is infuriating! It starts stuttering around 4000 RPM, and if you push through to around 7000 it stops. 

So far I have cleaned the carburettors, checked float heights and ensured there are no air leaks.
Replaced spark plugs.
Set ignition timing to the specs in the manual.
Replaced the condenser.
Made sure the timing advancer is working.
The air filter looks brand new.
I have completely drained the fuel tank, cleaned the petcock and refilled with new premium fuel. 

I posted the bike in the new members section and it was suggested there that maybe I need to look at carb jetting. Does this sounds like a good next step, or if there is something I may have missed to try first? I haven't re-jetted a carb before, is it simply a case of replacing all the jets with the next size up?

Any suggestions would be very welcome.

Thanks, Jack.

It's not the jetting, unless the bike is modified in ways not visible, or if the current jets are not stock.

Typically, problems like you describe are caused by either air leaks, or carb problems.  Realize that spraying stuff on the outside of the carbs doesn't always find air leaks if they are small ones.

I have cleaned carbs before and then really cleaned them. Sometimes it takes more than one pass at them. There are many small orifices and passageways that need to be inspected and cleaned thoroughly. 

If the engine, intake and exhaust are stock, the stock jetting will work brilliantly. What you describe is commonly caused by clogged circuits in the carburetors. To truly clean them, they need to be COMPLETELY disassembled and soaked in carburetor cleaner. An ultrasonic tank helps, too. Then, each jet and tiny passageway needs to be carefully inspected to insure they are spotless. For the pilot circuit, a steel e-string from a guitar works really well. It is a good idea to then take a can of carburetor cleaner with its spray straw installed, and spray in every passageway to insure there are no blockages. Be sure to wear safety goggles or a face shield as carburetor cleaner will hurt or even seriously damage your eyes.

Also, the pilot screws on your carburetors are supposed to have an o-ring on them. If the o-rings are damaged, hard, or missing, you may never get the carbs adjusted. See reference numbers 19-22 in the diagram. Also pay  close attention to the order they must be installed in.
www.partzilla.com/catalog/kawasaki/motor...ts-kz400-d3-76-77-kz

A common problem is the missing pilot passage plug after a carb rebuild. Your engine will run very rich from idle to full throttle!

 

You should check/adjust the valve clearances as well

wrote:

If the engine, intake and exhaust are stock, the stock jetting will work brilliantly. What you describe is commonly caused by clogged circuits in the carburetors. To truly clean them, they need to be COMPLETELY disassembled and soaked in carburetor cleaner. An ultrasonic tank helps, too. Then, each jet and tiny passageway needs to be carefully inspected to insure they are spotless. For the pilot circuit, a steel e-string from a guitar works really well. It is a good idea to then take a can of carburetor cleaner with its spray straw installed, and spray in every passageway to insure there are no blockages. Be sure to wear safety goggles or a face shield as carburetor cleaner will hurt or even seriously damage your eyes.

Also, the pilot screws on your carburetors are supposed to have an o-ring on them. If the o-rings are damaged, hard, or missing, you may never get the carbs adjusted. See reference numbers 19-22 in the diagram. Also pay  close attention to the order they must be installed in.
www.partzilla.com/catalog/kawasaki/motor...ts-kz400-d3-76-77-kz

Thanks Tex.
I learned the safety goggle thing the dumb way a few years ago! Thanks for the tips, I think I'll have another crack at carb cleaning in case I missed something.
 

Thank you to all who commented, and directed me back inside the carbs.
After taking the carbs back apart and really cleaning them I noticed the O rings on the main jets weren't sealing (43 on the diagram Scirocco helpfully posted).
Replaced the Os and the bike is now in fine fettle.