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CB77 Float Height Redux

Fuel System: Gas (Petrol) tanks, Carburators
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redblk63 Member
Posts: 101
Joined: Mon Oct 15, 2007 5:16 pm
Location: pasadena, california

CB77 Float Height Redux

Post by redblk63 » Fri May 15, 2020 12:08 pm

There has been a lot of discussion on this site regarding the proper setting of the carb floats:

1. lm used to have a description of this in the FAQ section, although I cannot find it now. In short, he says to set the floats so that the bottom of the floats are lined up with the bottom of the main jet carrier* when the tab makes contact with the tip of the float valve.
*The main jet extends below the jet carrier. Use the jet carrier to set the float height.
You can stop here if you wish, because at least for my carbs this coincides with Honda's recommendation given on page 117 of the Jan 1960 Shop Manual. (see 4. below)

2. Thanks to whoever warned that the idle jet fuel inlet must be below the fuel height. You can easily test what happens when this is not so by turning off the fuel tap. After about 1/4 mile, the engine will not idle, and you must open the throttle whereby the engine still runs fine on the main jet which is set below the idle jet. By the way, I'm surprised more people have not mentioned shutting off the tap as a diagnostic tool. I have found it most instructive in diagnosing "too rich" conditions in several bikes.

3. Much thanks to the person who attached clear hoses to the drain valves (not on my carbs) to serve as a height gauge so we can see the actual fuel level. His pictures were very instructive!

4. Page 117 of the Shop Manual says to set the Pw 26 carb float so there is a 0.1 mm gap to the float valve when the bottom of the floats are 22.5 mm below the top edge of the carb flange. The diagram makes it clear to me the top edge is used, not the notch.
Since the ratio of the distances from the fulcrum to the float and tab are 5 to 1, when the tab just touches the distance from the flange should be 22.5 - 0.1*5 = 22.0 mm.
It is more convenient to use the notch for measurements, so I measured the notch depth to be 1.5 mm +/- 0.2 mm. My large measurement error is because this was all done with the carbs installed.
The result was the bottom of the float should be 23.5 mm +/- 0.2 mm below the notch.

Guess what. For both of my carbs from a 1965 engine, this adjustment wound up right where lm recommends, see item 1. Since it's much easier to sight to the jet carrier, I'm using lm's way from now on.

Finally, I'm sure I have missed others who have contributed to the float height discussion, sorry!


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