CruzinImage BS38 carburetor floats -- Review

The floats are cylindrical, barrel-shaped, and will demonstrate a non-linear flotation/force curve. Unlike linear fixed-rate springs. As the float starts to submerge, it only displaces a small amount of fluid. When it has submerged halfway, it's displacement rate is maxed, since that's the largest part of the float. After that, continuing to submerge, it's flotation rate reduces since its width decreases. Being a circular shape, its flotation/force curve will be somewhat sinusoidal.

This graph shows what the expected buoyancy curve would look like for our 30mm diameter floats, from zero to 30mm fully submerged.
Buoyancy01.jpg
 
If you assume the oem float is ideal, which you have to do unless you bring up some fundamental principles about how it needs to operate and under what conditions, then any deviation from oem can only be considered a problem.
 
My OEM floats show 13mm of exposed float when float tested in gasoline. That means that 30-13 = 17mm of float is submerged, just to float the float. As shown on this vertical depth line.
Buoyancy02.jpg

At the intersection of that depth line with the buoyancy curve, we'll find the upward flotation force.
Buoyancy03.jpg

Note that we've passed the halfway point, in depth and buoyancy, just to float the float.
 
Sometimes I feel like I'm beating my head against the wall
650 central
Carb float-Genuine Mikuni
XS1/2, TX650/A, XS650 B/C/D/E/F/2F/SE/SF '70-79

#CSUDmi002.114 ..... $18.25 each
I just ordered a bunch of mikuni brass from mike this morning, he had a good price too , mains and pilots @ $4.15 each.
 
The thinner floats have only 90% of the displacement volume of the wider floats, and will demonstrate a slightly reduced buoyancy/force curve.
Buoyancy04.jpg

These thinner floats show 11mm of exposed float when float tested in fuel. That means that 30-11 = 19mm of float is submerged just to float the float. Draw this depth line and find the flotation force.
Buoyancy05.jpg
 
Show the flotation lines for both the wider and thinner floats.
Buoyancy06.jpg

The little green "*" points are the static flotation starting points, where the floats are supporting themselves only.
Buoyancy07.jpg

By shifting the curve of the narrower float such that the green "*" origin points align, we can start to see a comparison of the 2 floats "residual buoyancy" curve.
Buoyancy08.jpg
 
Up to now, we've been looking at the buoyancy/force/depth for just floating the floats. Additional force is required to get the floats to lift and close the float valves. That additional force is acquired by submerging the floats even more.

When I have my OEM floats set to the factory recommended 25mm, carb body base to top of float (inverted carb), the initial submerged depth of 17mm means that the fuel level would be at 25-17 = 8mm from the carb base.

My "clear tube method" measurement shows my fuel level to be 2mm below the carb flange, which is 4mm below the carb base, consistent with the manual.

BS38-2M-XS1B-Carbs-FuelLevel-25mm.jpg BS38-Floats.jpg
 
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Okay, so we've got an initial fuel level of 8mm below the carb base just to float the floats, and 4mm below the carb base with the float shutting off the fuel flow. That means that 8-4 = 4mm of increased fuel level is used to produce the needed "valve shutoff" force.

This means that the float is submerged even more to get this force. It won't be as much as the full 4mm level difference, since the float is pressing on the float valve spring, allowing the float to rise a small amount. But, if we were to show this possible max depth increase of 4mm, and the resultant flotation force at this new depth, we can see the demand on the thinner float.
Buoyancy10.jpg


Of course, because of the valve plunger spring, that new depth value will be less than what's shown on the "valve close" red line. By how much I don't know. But, I hope that showing these graphs reveals by how much the diminishing flotation of the thinner floats is occurring.

Edit: In addition to adding about 2mm to the float setting (25+2 = 27mm) for the thinner floats, just to account for their increased submerged depth, another additional 1 or 2mm would need to be added to shut off the valve, just because of the diminishing buoyancy curve, shown above, making the float setting upwards to 28 or 29mm...

*** End of excruciating graphs ***
 
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Now, have a rethink about members reporting rich conditions, even with their floats set at the factory 24mm or 25mm settings. Could thin floats (or even heavier floats) be involved here?
 
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Both of these new floats have a bit of twist in them.
View attachment 115271

Installed as-is on my carb.
One side shows 26mm float height.View attachment 115272

The other side shows 23mm float height.
View attachment 115273

This will take a bit of doin' to get them straight.
Oops, my fat fingers touched the screen too hard whilst scrolling thru the pages. I'm about to reset my floats today because I did something stupid. I changed too many things while rejetting.
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Just plowed though this whole thread, a usual 2M masterpiece of precise measuring and [re]engineering so parts meet the original specification along with an incredible ability to sus out the gotchas that ad-hoc manufacturing changes may create.
Since it wasn't mentioned in this thread; a typical reason you are here researching "float replacement" is the ravages of time may cause perimeter cracking in the brass, allowing fuel to enter, letting the floats "sink" raising fuel level and causing problems.
My most recent carb overhaul found 3 of four float chambers cracked and leaking.
cracked float1.JPG
 
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Yep! I had that problem 2 years ago. But it seemed like the brass just deteriorated. Oh, hi everyone. I'm back. Been trying to figure out how to kill a contractor and get away with it. Don't tell anyone though. All is good, now.:gun:

...but aren’t you from New Jersey?

Don’t you know “a guy”?
 
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