clear tube test/Float-Fuel level

Hello, I have been reading on this forum about using the clear tube test over the manual caliper measurement for Float/Fuel level adjustment on Carbs.

Bike: 84 Zn750. I don't know the Float height.

Question Clear tube test:

Screw clear tube in Carb bowl Drain screw and measure fuel level to bowl screw or Gasket?

Do I do this with Cars on the bike. I have 4 carbs. seems like a long enough process and messy.


Please let me know if these are correct steps.

1- empty carb bowl.
2- connect clear tube on drain screw hole.
3- Fill call with petcock on Prime.
4- examine Fuel level in Tube. take not.
5- take carbs off bike adjust Tang to get proper level.
6- put carb back on. clear tube test again.
7- keep doing till Clear tube level is perfect.


Step 5. I was wondering If i have to take carbs off every time or can this be done with carbs on the bike, " Adjust tang".

Thank for any tips..

also if anyone has the zn750 float level height that would be great.

Can anyone tell me the Diff between kz750 and zn750?
since I was looking into getting the shop manual. but could not find any for zn750

Did you try attaching clear tubing in place of the carb overflow tubing, then opening the drain screw? That's how it's done on my 81 650-CSR. On the earlier models, the drain screw is removed - I'd be surprised if yours is like that.

It's easier & faster to do them off the bike.
I made a carb stand. Others use a bench vise.
If you happen to have 4 old/spare carb holders, you could make a carb stand using those.

I use a funnel & tubing as an auxilliary fuel supply.
To speed things up use only 1 or 2 bowl screws (I use 2).

The fuel level measurement is usually at some point below the bottom of the carb body.
On my 81 650-CSR it's 3mm +/- 1mm below the carb body.

To keep from mangling the float I place them on a support. Instead of actually bending the tang, I flex it a number of times, then check. Flex, flex, flex, flex, flex, flex - check. etc etc.

When checking the fuel level, hold the tube up high next to the carb body, wait until the fuel level in the tube settles & wait 'til no more air bubbles can be seen rising, then slowly lower the tubing & stop. Note the level. If you raise the tubing after lowering it, start over.

The checking process is easy.
The adjusting process involves disassembly, bending the tang, reassembly, and re-checking.

Checking process -- carbs should be level whether on bike or bench.
When on bench, wood blocks may be used to hold the carbs level.

The drain screw is fuel source for the clear tube.
On carbs where same bowl outlet serves dual purpose as both drain and overflow, the outlet is used as fuel source for the clear tube.

With both bowl and tube empty (before fuel is supplied), I hold the tube firmly in position against the float bowl, and continue holding it there without allowing any movement while turning on the fuel and watching the fuel flow into the tube and rise to its final level.

While the fuel fills and seeks its final level inside the tube, imo the tube should remain steady and not be allowed to move. Applicable whether carbs are on bike or bench during the test procedure.

The above is as experienced using Mikuni carbs with manual slides (non-CV) and simple single-purpose float bowl drain screws. And haven't noticed air bubbles in the fuel as it rises inside the clear tube.

The special tool shown in the pic below isn't really needed, when using an appropriate sized clear tube "self-threaded" into the drain screw hole.

Good Fortune!

[Click on image to enhance view.]

Without the special tool, and using an appropriate sized clear tube "self-threaded" into the drain screw hole, it looks more like this.

[Click on image to enhance view of beautifully hand-drawn tube section.]

Don't know what carbs you have. My '82 750 has Keihins. Here's how they are done....


testing the inside carbs on the bike is a bit tricky because of space.

Patton wrote:

...The drain screw is fuel source for the clear tube.
On carbs where same bowl outlet serves dual purpose as both drain and overflow, the outlet is used as fuel source for the clear tube...Mikuni carbs with manual slides (non-CV) and simple single-purpose float bowl drain screws....

polkat wrote:

Don't know what carbs you have. My '82 750 has Keihins. Here's how they are done....

testing the inside carbs on the bike is a bit tricky because of space.

Yes, that is correct for CV carb where same bowl outlet serves dual purpose as both drain and overflow, with the bowl outlet nipple being used as fuel source for the clear tube.

In the Keihin drawing, the fuel should be depicted inside the tube as filling the tube all the way from the carb nipple to its level inside the tube. Should not be an air space inside most of the tube. Diagram H5 was reportedly drawn by a finatically dedicated ink saver.

Regarding limited space for testing inside carbs, it doesn't matter how long the clear tube must be to loop around whatever's in the way before final routing to alongside the carb body, so long as the clear tube isn't kinked.

Good Fortune!

Patton wrote:

...In the Keihin drawing, the fuel should be depicted inside the tube as filling the tube all the way from the carb nipple to its level inside the tube. Should not be an air space inside most of the tube. Diagram H5 was reportedly drawn by a finatically dedicated ink saver.

hee. Take a more careful look at the tube (& the carb) in the diagram. They're showing a partial cutaway view.

ahh xcellent that answers all my questions..thanks so much for this amazing info.