bs38 float height?
- section8joe
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bs38 float height?
01 Jul 2012 00:05
I'm trying to measure float height with the carbs off the bike and I'm measuring them with the specs from an xs650 carb tuning guide for bs38's. Is it safe to assume that even though these carbs are on two different bikes that float height will be the same? Here's a few paragraphs I copied from the xs650 carb guide.
If the float bowls are fitted with drain plugs, fuel level can be inspected directly by fitting
a piece of clear hose to a plug and inserting the plug in the float bowl. Some float bowls
are fitted with drain valves opened by a screw, with a hose nipple on the outlet. To read
fuel level, attach the sight tube to the outlet and set vacuum petcocks, if fitted, on
"Prime," and open the screw until fuel flows into the sight tube. With the motorcycle
level, hold the sight tube upright beside the carburetor body. Fuel should rise in the tube
no higher than the lower lip of the carb body where it overlaps the float bowl, and no
lower than 2 mm. below it.
A second procedure is to read float height. This is measured from the surface on the
carb body on which the gasket seats to the top of the float, with the gasket removed. If
the gasket cannot be removed without damage, reduce the specification given by 1 mm.
and measure from the gasket. Holding the carburetor with the floats up, lower the floats
gently onto the float needle, taking care not to compress the float needle spring. Since
the reading is taken with the carb upside down, raising the float level will lower fuel
level, and conversely.
All BS38 carbs are set at 24 mm. +/- 1 mm.
P.S. The bike is question is a 79 kz750.
Thanks
Joe
If the float bowls are fitted with drain plugs, fuel level can be inspected directly by fitting
a piece of clear hose to a plug and inserting the plug in the float bowl. Some float bowls
are fitted with drain valves opened by a screw, with a hose nipple on the outlet. To read
fuel level, attach the sight tube to the outlet and set vacuum petcocks, if fitted, on
"Prime," and open the screw until fuel flows into the sight tube. With the motorcycle
level, hold the sight tube upright beside the carburetor body. Fuel should rise in the tube
no higher than the lower lip of the carb body where it overlaps the float bowl, and no
lower than 2 mm. below it.
A second procedure is to read float height. This is measured from the surface on the
carb body on which the gasket seats to the top of the float, with the gasket removed. If
the gasket cannot be removed without damage, reduce the specification given by 1 mm.
and measure from the gasket. Holding the carburetor with the floats up, lower the floats
gently onto the float needle, taking care not to compress the float needle spring. Since
the reading is taken with the carb upside down, raising the float level will lower fuel
level, and conversely.
All BS38 carbs are set at 24 mm. +/- 1 mm.
P.S. The bike is question is a 79 kz750.
Thanks
Joe
79 kz750b4
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- Motor Head
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Re: bs38 float height?
01 Jul 2012 00:09
I would think that would be just fine, just below the gasket surface is normal for these mikuni's.
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- Patton
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Re: bs38 float height?
01 Jul 2012 00:18
The "correct" float height is whatever it happens to be when producing the proper fuel level as shown by the clear tube test.
Where the fuel level is really screwed up, setting the upside-down on-the-bench dry-measured float height should get back in the ball park.
But even with float heights set exactly the same, measured fuel levels will likely vary among the carbs.
When the fuel levels are within specs, the measured float heights will likely all be slightly different.
Carbs care about actual fuel level. Carbs don't care about what the measured float height happens to be.
The objective is to achieve matching fuel levels -- not matching float heights.
Good Fortune!
Where the fuel level is really screwed up, setting the upside-down on-the-bench dry-measured float height should get back in the ball park.
But even with float heights set exactly the same, measured fuel levels will likely vary among the carbs.
When the fuel levels are within specs, the measured float heights will likely all be slightly different.
Carbs care about actual fuel level. Carbs don't care about what the measured float height happens to be.
The objective is to achieve matching fuel levels -- not matching float heights.
Good Fortune!

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- bountyhunter
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Re: bs38 float height?
01 Jul 2012 01:57 - 01 Jul 2012 02:00IMHO, the "bench adjust" is just a gross ballpark setting. Clear tube is mandatory. Correct fuel level is 4.5 - 6.5 mm below the lower "edge" of the top half of the carburetor.section8joe wrote: I'm trying to measure float height with the carbs off the bike and I'm measuring them with the specs from an xs650 carb tuning guide for bs38's. Is it safe to assume that even though these carbs are on two different bikes that float height will be the same? Here's a few paragraphs I copied from the xs650 carb guide.
If the float bowls are fitted with drain plugs, fuel level can be inspected directly by fitting
a piece of clear hose to a plug and inserting the plug in the float bowl. Some float bowls
are fitted with drain valves opened by a screw, with a hose nipple on the outlet. To read
fuel level, attach the sight tube to the outlet and set vacuum petcocks, if fitted, on
"Prime," and open the screw until fuel flows into the sight tube. With the motorcycle
level, hold the sight tube upright beside the carburetor body. Fuel should rise in the tube
no higher than the lower lip of the carb body where it overlaps the float bowl, and no
lower than 2 mm. below it.
A second procedure is to read float height. This is measured from the surface on the
carb body on which the gasket seats to the top of the float, with the gasket removed. If
the gasket cannot be removed without damage, reduce the specification given by 1 mm.
and measure from the gasket. Holding the carburetor with the floats up, lower the floats
gently onto the float needle, taking care not to compress the float needle spring. Since
the reading is taken with the carb upside down, raising the float level will lower fuel
level, and conversely.
All BS38 carbs are set at 24 mm. +/- 1 mm.
P.S. The bike is question is a 79 kz750.
Thanks
Joe
1979 KZ-750 Twin
Last edit: 01 Jul 2012 02:00 by bountyhunter.
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