Kz750 carb issues + balancing

Last month my carbs on my '81 kz750 E four cyl started overflowing and I'd come back to my bike, lift it off the kick stand and when it straightened up it would dump fuel out of the airbox on the ground.

I narrowed it down to the carb closest to the choke and decided to order 4 new float needles. Those should be here tomorrow, fingers crossed.

In the meantime I've been reading about making a carb balancer and I made the version with bottles to avoid sucking oil back into the engine. I know on some carbs the intakes have a screw to tap into, but mine is rubber, so where do i stick these 4 vacuum lines to on my carbs and get the correct readings?

You should replace the seats with the needles. The carb holders should also have nipples to hook the vacuum lines to. Do you have a manual?
Steve

Each of your carburetors (CV34) has a vacuum nipple, usually #2 goes to the petcock at the gas tank and #1, #3, #4 are plugged up. I would also do a clear test tube test to see if the floats are within specs also.

Great looking bike. Welcome to the site.
In your photo above it looks like you have the vacuum nipple on the #1 carb hooked up to petcock. Look for similar nipples on the #2, 3 & 4 carbs.
If the glass jar apparatus does not work out check out Morgan Carbtune.

The brass needle seats on those carbs are not replaceable but its probably worth cleaning them using a Q tip and carb cleaner or better yet a Q tip and Brasso.

Like mopguy suggested the clear tube test to check fuel levels is a good idea. Getting the levels in spec was the single best thing i did with my carbs.
Good luck

Thank you guys. I'm still having an overflow issue on carb 2 (second from choke) where i have my fuel vacuum to petcock. Carb one is hooked to the valve going to the valve covers and the airbox, some egr relates items. I replaced the valve needles and cleaned the carbs well. I build a bike per month as a hobby so ive done it a lot, but usually single pumpers that don't need a balancer. As far as my issue goes I'm going to remove the carb and plug the needle valve, then put gas in and see if the fitting is leaking anywhere.




Update: I've checked the overflow with actual gas to see if it leaks and it doesnt.



I cleaned the needle seat again just to be sure. Its fine.



These floats do not appear to be hollow. If they are solid then it's not the floats that is the issue.

A couple of things to try
1. Put the float and float needle back in place but not the bowl. Turn the carb upside down and blow into the fuel line. If the float needle is not sealing you will know it fairly quickly. If it is sealing the way it should then it takes a lot of lung pressure to lift it and let air escape.

2. Attach a rubber tube to the overflow outlet. Fill the bowl with water (safer than gas). Put a finger on the top.open end of the brass overflow tube and blow in the rubber end. The brass overflow tube may have a hairline crack (not uncommon, mine did) and if so look for tiny bubbles in the liquid.

The leak has to be either from the float needle not sealing and excess gas overflowing or through a hairline crack in the brass tube. Easy fix if either way.

I forgot about the egr/clean air stuff as we never had that in Canada. I think many people in the US ditched it

You cant use water. The gas is a micropenatrant. Same reason blowing doesn't help find small leaks. You can pour gas in a float bowl and the overflow will leak if it has micro cracks, do it with water and it won't. Half the reason is what i said earlier, the other is surface tension related to water.

Anywho, i have decided the float on this carb is just a tad bit higher than the others and when i put the bike on its kick stand the bowl goes into the overflow. I'm going to remedy this by making the float lower slightly than the others are. They are all even right now so idk why it's doing that, but it'll fix the issue.

It's on the life support system being balanced right now.

You cant use water. The gas is a micropenatrant. Same reason blowing doesn't help find small leaks. You can pour gas in a float bowl and the overflow will leak if it has micro cracks, do it with water and it won't. Half the reason is what i said earlier, the other is surface tension related to water.

The point in using water is not to watch it leak but to watch for bubbles from the brass overflow when you stopper the top and blow via a tube through the bottom. Like looking for a tiny leak in an inner tube.
I've had my 1980 750 E1 with the same Keihin CV34 carbs for 14 years and had the carbs off many times and can say from experience that both suggestions I made above work.
If you are setting float height search here first for "clear tube method"
Been there done that also

Thank you, I appreciate it.

Are you sure the petcock is functioning properly? It should prevent overflow. Also, if the diaphragm has a tear or hole, fuel can pass through it and travel down the vacuum line into the carb.

I've rebuilt several sets of those Keihin's and they are a degree of magnitude more fiddly to set up than the more common Mikuni BS CV family of carbs.

On paper the rubber tipped float needles should lend themselves to a better seal than a metal needle but I've not found this to be the case. I think one of the issues is because the float needle spring is quite weak, particularly if you purchased aftermarket needles. Jetsareus.com sells the real Keihin needles but they are not cheap. I recommend cleaning the stock needle and seeing if you can recycle them as opposed to purchasing cheap junky aftermarket "carb kit" parts. Regardless of which way you go it's critical to perform the clear tube test on these carbs to set the float level and verify you have a good seal. I've had to bend the float tang on every single set of carbs I've redone to raise the float before the fuel level fell within spec.

A few other things I've learned is that it's critical to ungang the carbs in order to replace the fuel T and interconnect fuel tube O-rings. On every set of carbs I've touched the O-rings on these critical parts were hard as a rock and brittle. Typically when trying to remove them they just crumble. Another O-ring that must be replaced is those on the float bowl. In the photo above there is a bunch of gasket snot on the float bowl which is a bad sign. You can get new O-rings for all these parts easily enough. I've got all the sizes documented in case anyone wants to order in bulk from an O-ring store, otherwise you can get most of these parts from Z1 or ebay, or even a dealer.

Oh, and I had one set of carbs with a crack in the fuel overflow tube as mentioned previously. It was easily fixed by soldering but finding the issue took some doing since I was not expecting this.

Lastly, in order to ungang the carbs at least two of the choke butterflies must be removed. When replacing these screws it's critical to use red loctite on the screws, and I also recommend flaring out the end of the screw per the guide in the service manual. You can make your own flaring tool easily enough though.

Bottom line is that short cuts are long cuts when it comes to carbs. Doing the job right the first time saves time in the end.

Nessism wrote:
Bottom line is that short cuts are long cuts when it comes to carbs. Doing the job right the first time saves time in the end.

These words should be attached to the title of every discussion thread no matter what the topic.