'81 KZ750 leaking gas from carb 1 & 3

I recently purchased an 81' KZ750E that hadn't been running for a couple years. It didn't need much besides some thourough cleaning and a tune-up. I had the carbs apart and cleaned everything with carb cleaner and compressed air--jets, airways, etc. It got all new fluids, fresh gas, plugs, filter, battery, etc. Started up instantly--I was actually a bit surprised at how quickly it started. Everything sounded and looked fine besides gas coming out of the overflow on carbs #1 & #3.

First i thought maybe the needles were bad, so i switched them out with some spares that i had from another similar bike that didn't leak whatsoever, but I still got the same result. I even took the float bowl off carb #1, turned the petcock to PRI and manually pushed the float/needle into the seat to see if fuel would leak by and it didn't. The float needle and seat appeared to be doing their job.

Then i thought maybe the floats were bad. I let them sit in gas for over an hour and none of them had sunk or absorbed any gas for that matter.

I had heard of people having issues with the petcock leaking while in the ON position. I removed the tank from the bike, filled it with gas, and turned it to the ON position. No fuel leaked as it should because it is vacuum operated.

In a bit of a last ditch effort I raised the float level substantially higher on both carbs. Carb #1 had stopped leaking and carb #3 had almost stopped entirely--it still leaked a little bit, but it was progress.

I rode the bike for a bit while frequently checking the overflows for leaking. There was no/very little leaking from the carbs. I stopped to top of the tank at a gas station, rode home, and when i got in the driveway, gas was pouring out of the overflows.

I'm honestly at a dead end with this problem and I'm running out of patience very quickly with this bike. If anyone has had a similar problem or has any suggestions/help it would be very much appreciated.

Thanks

You need to check fuel levels with a clear tube.

IIRC many of the Keihin CV34 carbs do not have an overflow tube in the bottom of the bowl. Rather they have a drain and drain screw which will drain the bowl but it does not provide overfill protection.

If you carbs do not have the tube in the bottom of the bowl then it may just be the o-ring on the drain screw that is leaking.

I suspect your carbs are the same as mine - Keihin CV34
If the float valve/needle valve is good and the brass seat into which it goes is clean ( Q tipwith carb cleaner and or Brasso) and is not the source of the leak then the only things left are the drain screw O ring like gd4now suggested or the brass overflow tube in the carb bowl. These do develop hairline cracks which can be hard to find


You can just see the tip of the brass tube in the schematic above. You can test for leaking by filling the bowl with water, connect rubber overflow tube to the nipple on the bottom, put a finger over the top of the brass tube, blow in the rubber tube and look for bubbles in the bowl -the same technique you might use looking for a puncture in an inner tube.
I have had to repair one of mine. If this turns out to be the source of the problem the JB Weld allowed to fully cure will work or what I did was toe get a piece of slightly wider copper or brass tube at a hobby shop, slip it over the existing brass tube and solder at the bottom.

+1 on bountyhunter's advice

Good luck

Thanks for the replies guys,

The carbs are Keihin CV34s identical to the diagram JR posted. I'm doubting it's the o-rings because I've had to drain the bowls frequently during this whole process and I would think that if the o-rings were bad they would drain completely? I could be wrong, but that's just my guess.

Anyways, I'm planning on checking the float levels with a clear tube and I will also check for cracks in the drain tube itself within the next couple days.

What still has me confused is the fact that it was barely leaking with the gas tank nearly empty, and as soon as I filled up it started pouring out again. If anyone could maybe explain why this would be that would be great.

Thanks again

My carbs tend to leak if I leave the petcock on Prime. I know it shouldn't but it does and the more gas in the tank the more leak. You only had the petcock at ON so that can't be the reason.
Do you have on in line fuel filter between petcock and carbs ? If not then perhaps small particles of rust or crud are messing with the needle and seat. I didn't see a fuel filter mentioned. These are practically mandatory on old bikes.
Good luck