1983 KZ750 F1 Carbs driving me crazy!

Long story, I will try and keep it as short as possible. I bought this bike last year, had been “restored” and then purchased by a young man whom I bought it from in a non-running condition. The seller indicated he had problems with the petcock and leaking carbs.  I bought the bike, replaced the petcock with a standard / non- vacuum petcock which works great. I pulled the carbs. They appeared very clean already but went through them anyway.After cleaning them, I bench-tested the float levels (clear tube method) and everything seemed fine.

I was missing two of the pilot jet plugs, or had two I didn’t need (I wasn’t sure at that point). There were no leaks and the fuel levels looked good. I installed the carbs on the bike, hooked up the gas and the bike fired right up, but kept trying to idle at around 4500rpm no matter what I did. Eventually, I guessed that this was perhaps do to missing pilot jet plugs. I manufactured some plugs from brass rod and o-rings. I had checked for vacuum leaks etc.The left two carbs started leaking fuel at this point as well. I stripped them off the bike and went through them again. I will skip detailing how many times I went through changing out needles / floats and testing everything.

I finally bench /clear tubes tested each carb individually and I discovered that if the vent towards the top of the carb, the one with the black plastic tee fitting was closed / blocked, gas would again rise even over the top of the carbs. If I left the vent open, the gas levels would stay at the correct level.I reassembled the carb rack, bench tested again and everything worked great. After placing them back on the bike, it fired up and idled at about 2,000 rpm and sounded very good. Within a couple minutes, I was leaking gas from all four carbs.

I am absolutely at my wits end with these carbs and looking at replacing them with a set of Mikuni carbs from a GPZ or something similar. I realize I will need to likely re-jet and tune.I have been working on and rebuilding all kinds of carbs for 40+ years and this is the first time I feel completely stumped. I don’t recall ever having this strange of an issue ever before. I would appreciate any help or ideas.

Those pilot plugs are 100% needed. Are they all in place?

chopper1963 wrote:

Those pilot plugs are 100% needed. Are they all in place?

They are in place. I could not find any so I made some from brass rod with o-rings. 

Do you know where abouts on the carbs that the fuel is coming from ?

Wookie58 wrote:

Do you know where abouts on the carbs that the fuel is coming from ?

Not really. They will overflow out the tops of the clear tubes even when I have end of them above the tops of the carbs. If I let it go for a little while, the carbs will start puking gas from inlet side, including the very top oblong port in the inlet. 

According to the parts diagrams, the F1 carbs are the model without the secondary main jet, unlike most other versions of CV34's.  They have simple pilot and main, and they do NOT use pilot plugs.  Does this match your bike?  

And if your carbs are the type with a brass overflow tube sticking up in the float bowl, check these tubes for cracks.

Certainly sounds like a float issue ! have you checked that the floats do indeed "float" and that they are not fouling on the bowls which could hold them open !

Wookie58 wrote:

Certainly sounds like a float issue ! have you checked that the floats do indeed "float" and that they are not fouling on the bowls which could hold them open !

I did check them all. they all float at the same height in a bowl of gasoline. I even swapped floats 1+2 with 3+4, 1+2 still leaked. Swapped the needles as well, 1+2 still leaking. Replaced the needles and still leaking. 

Tested with them right side up and upside down with air pressure (mouth) moving the floats by hand (also did this with just the needles), they shut off fine while bench testing but not on the bike. 

I am thinking at this point I would be ahead just replacing them with Mikuni carbs.

Nessism wrote:

According to the parts diagrams, the F1 carbs are the model without the secondary main jet, unlike most other versions of CV34's.  They have simple pilot and main, and they do NOT use pilot plugs.  Does this match your bike?  

And if your carbs are the type with a brass overflow tube sticking up in the float bowl, check these tubes for cracks.

I believe you are correct, the diagram does not show a plug and I could not see any hole between the main jet tube and the pilot tube. I decided to try a plug, to address the idle issue, because there were two of them in two of the carb bowls and frankly I had checked everything else I could think of such as vac leaks etc. looking for an explanation for the high idle. 

Honestly, it is the carb overflow issue that really has me stumped. I assume I could figure out the high idle if I could get them to stop overflowing. 

I really appreciate the assistance from all of you. 

Have you tried blowing with your mouth and a piece of pipe as they fill with the bowls on (it appears all the testing so far has been with the bowls off)

If you are in the US, I'll look them over for you.  I charge $250 for a rebuild, with all new O-rings.

If you want to continue on your own, you might want to make a leak test stand, with all the carbs ganged together, and using a remote tank.

And I don't remember mention of where they are leaking?  From the carb throat?  Are you filling the fuel via the center Tee?  Is the fuel tank free of rust?  Did you replace the needles?  

A video of the leaking would be useful.  

Wookie58 wrote:

Have you tried blowing with your mouth and a piece of pipe as they fill with the bowls on (it appears all the testing so far has been with the bowls off)

If I am following your question correctly, I have not done that. Are you saying to blow into the black plastic tee fitting vent?