Topic: Question about CB750 K8 float levels

[chicagobob]

I've had trouble getting my 1978 CB750 carbs not to overflow.  The manual says to set the float level at 12.5mm from the bowl flange, with the carbs on end and the floats just touching the valves, but on my bike (with new floats and valves), this results in massive overflowing. 

Using the clear tube method, I find I have to set the floats at about 24mm above the flange to get the level below the bowl gasket, and at that extreme float angle, it's difficult to adjust the fuel level accurately. 

Is this unusual?  Does anyone have any suggestions?

[madmtnmotors]

Sounds like your floats are upside down.

[bryanj]

Or the data sheet is different to the carb type. 12.5 relates to PD type carbs

[HondaMan]

12.5mm is the correct float height for the PD42b carbs. For the PD42a carbs, use 14.5mm instead. These carbs came on both the K7 and K8 (and some F3) versions of the bike. the "a" or "b" is right on the carb itself.

This said: the 12.5mm float height is the lean-burn setting for the emission-controlled versions of these bikes (most often from California sale), and using the 14.5mm setting usually reduces their cold-bloodedness.

[chicagobob]

Well, the bike runs with the floats at 24mm (and they are not upside down).  Clear tubes show the level to be 3-4mm below the float bowl gasket, in spite of the srane static measurement - and these are new floats, so they don't have distorted tangs.  Revs hang a little, but the weather here is so cold I haven't been able to run the bike long enough to warm it up or to balance the carbs. 

[madmtnmotors]

Make a few more posts and then you will be able to include pictures. I'd like to see what a 24mm float setting looks like. All three of my K8's and all three of my F2's thrive on the 14.5mm float setting...

This said: the 12.5mm float height is the lean-burn setting for the emission-controlled versions of these bikes (most often from California sale), and using the 14.5mm setting usually reduces their cold-bloodedness.

The float height is measured from the carb body. Wouldn't a smaller number (12.5mm) result in a higher float level and richer setting?

[chicagobob]

The 12.5mm setting, done with the carbs on their sides and the floats just touching the float valves, resulted (for me) in a very high fuel level, causing the bowls to overflow.  I had to set the static float distance from the flange to about 24mm, which leaves the floats at an angle, so that the point farthest from the hinge is the high point from which I'm measuring.  Once the carbs are reassembled and back on the bike, this leaves the fuel level at about 3mm below the level of the float gaskets, which seems go me to be correct, and which does not leak. 

I am still confused by that 12.5mm figure in the manual - it was just so far off for my application.

[madmtnmotors]

You indicate new floats, so did you replace the float needles too? Where did the new parts come from? I had brand new aftermarket float needles and bowl gaskets that turned to goo and began leaking profusely within one week of exposure to 10% ethanol laced fuel. Original Honda parts are best. If aftermarket parts are used then the rubber bits must be made of Viton in order to stand a chance.

[HondaMan]

Make a few more posts and then you will be able to include pictures. I'd like to see what a 24mm float setting looks like. All three of my K8's and all three of my F2's thrive on the 14.5mm float setting...

This said: the 12.5mm float height is the lean-burn setting for the emission-controlled versions of these bikes (most often from California sale), and using the 14.5mm setting usually reduces their cold-bloodedness.

The float height is measured from the carb body. Wouldn't a smaller number (12.5mm) result in a higher float level and richer setting?

Normally, yes.
The PD42/b carbs also use #35 pilot jets, though, and #110 or #112.5 mainjets for leaner burn, despite the float height change. The higher float levels also improve the accelerator pump performance a bit, which helps with the leaner pilot jet.

It also encourages overflow...

[chicagobob]

You indicate new floats, so did you replace the float needles too? Where did the new parts come from? I had brand new aftermarket float needles and bowl gaskets that turned to goo and began leaking profusely within one week of exposure to 10% ethanol laced fuel. Original Honda parts are best. If aftermarket parts are used then the rubber bits must be made of Viton in order to stand a chance.

I used new needles and floats from 4into1.  The needles looked solid when I removed them to change the floats a couple of weeks after initial installation.

[flybox1]

If you've clear tube checked them, and you're at 3mm fuel level, you're good to go.
Your variable is likely (aftermarket) float needle length.  With such a small part, if its off .5mm, that could change measurable fuel level by several mm.
I start w/ 14.5mm, test it, and adjust from there. 
The final float height measurement is irrelevant if the fuel level is correct.

[Don R]

I know I'm stating the obvious but clean fuel is a factor too. I struggled with my gl1000 before acid washing the gas tank. Good luck.