What would different float heights do?

Been having some trouble with my recently purchased 1979 KZ750. The problems are hard to describe, especially for a noob like me, but they were mostly with stable idling, occassional stalling and occassional backfiring.

I knew the bike had sat for a couple months before I bought it (with gas in it and no fuel stabalizer). The previous owner said the carbs had been cleaned last summer (prior to it sitting).

It seemed like some varnish build up somewhere in the carb was pretty likely so I went ahead and took it off today.

I didn't find much gunk. The passageways all seemed to be clear, but I did my best to clean them anyway.

When I was putting the float bowls back together I noticed the floats were set to dramatically different heights. One was at about 30mm (spec for my bike is 31 +- 1) and the other was at about 20mm.


I set them both to 31mm. I'm wondering though what sort of trouble this would have caused with the bike.

Can anybody tell me what likely symptoms I would have seen as a result of this?

The height (amount really) of fuel in the bowl determines how much is moved into the engine under a given vacuum.

Lower a fuel amount = leaner operation overall, higher = richer overall.

The fuel height is best tested via the "clear tube method" of attaching a hose to the bottom of the bowl, and holding it up against the bowl to see where the fuel level fills up to.

It is very important that all fuel levels are the same, if either a little lean or rich. Incorrect fuel levels can mock many other symptoms like syncronization, jetting, mixture screws, sometimes air leaks, etc.

Spray carb cleaner through the pilot passages in the carb while looking in the carb bore, so as to verify that cleaner is squirting through all the small holes into the bore, else you will never truely know if the passages are clean.

Good luck.

You are right to suspect that radically different fuel heights in the carbs will result in odd engine behavior. Measuring the float height is a good way to get them in the hunt, but as TeK9iNe mentioned, the clear tube method is the most accurate.

Ok that seems straight forward enough. Although measuring, disassembling, adjusting, reassembling, measuring, lather rinse, repeat, sounds like a PITA.

I'm now wondering if I've misinterpreted the fuel level spec. The closest I could find in my manual for float height was this:

Fuel Level (from bore center): 31 +- 1

Is this using the clear tube method you described? If so I'm probably way off on where I set my floats.

Topper wrote:

Ok that seems straight forward enough. Although measuring, disassembling, adjusting, reassembling, measuring, lather rinse, repeat, sounds like a PITA.

Guess what - it is a big PITFA, but if you dont do it, you'll end up with herky jerky operation, possible cylinder damage and an impossible to tune bike, but hey! Its up to you! :laugh:

Measure with the carb upside down from the bowl gasket surface at the center of the carb side, to the top of the float in that position. Set them to roughly 24mm on both sides.

Put the bowl on with 2 screws. Flip over. Hookup rig. Attach hose to drain in bottom of carb and hold hose up against side of carb. Turn on gas and check the height of the fuel in the bowl. Mark with a pencil on the side of the bowl where the fuel is. Measure down 4mm from the bowl gasket on the bowl and make another mark. Now measure the distance between the 2 marks, and pop off the bowl and change the float height by the difference between the two marks, then when you recheck it will be perfect, and voila! Your done one carb!
It should only take you about an hour to do them all properly, then you can forget all about it!:laugh:

You're off a considerable amount, assuming we're talking about a 79 750 twin.

I was bored one day, so I tried running the fuel level of my 750 twin at different heights, results were basically as described by TeK9iNe.

The fuel level is specified by a distance from the carb float bowl mounting flange to the fuel level, I think it's 2-3mm, but I'm not sure and my manual is not available at the moment.

I built a fixture with a remote tank, so I could adjust the float level, flip them over and measure the actual fuel level, then flip them back over upside down to adjust if needed.

My experiments showed 1 inch (25.4mm) between the highest part of the float and the surface of the float bowl mounting edge, made the fuel level exactly in the middle of the specified range when testing with the plastic hose.

Worked that way on mine anyway, ymmv.

An illustration of the clear tube test is shown in the illustration below, with side located drains.

With version of CV carbs where outlet nipple on bottom of float bowl serves dual purposes of both overflow and drain, be sure to use the drain outlet function and not the overflow function when allowing fuel to flow into the clear tube.

[Click on image to enlarge view.]



Good Fortune!

Been having some trouble with my recently purchased 1979 KZ750

There is only one 1979 KZ750, a twin.

Two carbs, no drains on bottom, I have a couple of modified fuel bowls I use.

Good illustration of measurement though!

steell wrote:

The fuel level is specified by a distance from the carb float bowl mounting flange to the fuel level, I think it's 2-3mm, but I'm not sure and my manual is not available at the moment.

The only spec I could find in my manual was on page 114, table 11: Fuel Level (from bore center): 31 +- 1

Measuring the outside of the carb that actually puts it right about at the level in Patton's illustration.

steell wrote:

There is only one 1979 KZ750, a twin. Two carbs, no drains on bottom, I have a couple of modified fuel bowls I use.

Yup it's a twin. So I can't do this without some modified carb bowls????!!!

Topper wrote:

Been having some trouble with my recently purchased 1979 KZ750. The problems are hard to describe, especially for a noob like me, but they were mostly with stable idling, occassional stalling and occassional backfiring.

If this is a 750 twin, the spec for the fuel level in the bowls is 5.5mm below the lower edge of the upper body of the carburetor (where the bowl mates against). If you don't set it with clear tube, it will be wrong and will run terrible.

If this bike sat with bum gas in it, the only way to get it to run right is:

1) Take off the tank and rinse it out

2) Take off the petcock assembly and clean it

3) install filters in the fuel lines

4) clean the carbs and set the fuel levels

5) not mandatory, but I rcommend changing the pilot jets to size #50. It runs a lot better than the stock 45 jets. This assumes you have the stock BS-38 carbs.

Topper wrote:

Yup it's a twin. So I can't do this without some modified carb bowls????!!!

The stock bowls have drains at the bottom. Find a clear tube and trim the end so it jams into the hole and you can jury rig it well enough (guess how I know)...:laugh:

bountyhunter wrote:

the spec for the fuel level in the bowls is 5.5mm below the lower edge of the upper body of the carburetor (where the bowl mates against).

Where did you find that spec?

Also still looking for an answer on whether or not I can do the clear tube test without modified bowls.