Setting Float Height on Smoothbores

Looking for some advice, again! I want to double check the float level on my new set of RS34 Mikunis and could use some clarification on the process. Here's what the Mikuni manual says:

"Invert carburetors and remove float bowls. Float
Assembly’s actuator tab should just begin contact with
Needle Valve Assembly when bottom of Float
Assembly is 17mm from carburetor bottom as shown.
Make required adjustments by bending actuator tab."

I inverted my carbs and measured the float heights before I installed them. They were all within the +/- 1mm tolerance. When reading the instructions again this evening, I was confused :S

Does this mean that you have to hold the float up by hand and just let it touch the needle valve assembly when measuring, as stated in the manual? Does the weight of the floats compress the needle valve spring enough to alter the reading when the carbs are inverted?

Anyone set these levels before?? Thanks!

Nobody has set the float level on smoothbores??:ohmy:

Well, I went ahead and pulled the carbs to check the float height again. It definitely makes a difference in the height when you allow the weight of the float to compress the needle valve spring, about 1-2mm. I set mine this time while holding the float up off of the valve - just allowing the tab to make contact with the valve as instructed. I'm just curious to know how everyone else does it??

Anyway, hopefully someone will chime in...

79MKII wrote:

...just let it touch....

Yes, just let it touch.

Consider this upside-down-on-the-bench measuring as a rough ball park adjustment subject to confirmation by the clear tube method which truly shows level inside the floatbowl regardless of variations in float buoyancy, etc.

Actually, many of us don't goof around with anything but the clear tube method which may be performed on the bench or while installed on a fully assembled bike.

Am maybe behind the times, but the term "smoothbores" invokes an image of the following carb:

[Click on image to enhance view]





Good Luck!

i always tip the carbs at a bit of angle just so the floats are making contact but not putting pressure on the valve and measure and set them that way if that makes any sense i hope this helps some i didn't see this post before now or i would have said something sooner.

Okay My curiousity has the best of me. And my ignorance can't stand it...LOL
Can someone please explain in detail the CLEAR TUBE METHOD ?
Sure would appreciate it

edspring1 wrote:

Okay My curiousity has the best of me. And my ignorance can't stand it...LOL
Can someone please explain in detail the CLEAR TUBE METHOD ?
Sure would appreciate it

To determine fuel level inside the float bowl.

Carbs level as possible, such as bike on centerstand, propped up or held level.

Turn petcock off and drain the bowl.

Connect a clear tube to the drain on the bowl (not the overflow nipple). My recollection is hazy in thinking some carbs (?CV?)may share a nipple for drain and overflow, which nipple I'm guessing could be used. But don't use the overflow nipple when there's a separate drain screw. Use the drain screw hole.

With correct size o.d., the plastic tube may be "threaded" into the drain screw hole. Or a plastic connector may be "threaded" into the hole, to which the tube is attached.

Run the tube up vertically beside the bowl. Hold it real still and steady.

Turn petcock on to allow fuel flow into bowl.

Fuel level in tube should match level inside bowl, which on manual slide Mikunis is typically supposed to be 2-4 mm below bottom of carb body (same as 2-4 mm below bowl gasket), but I don't know the specs on CV or other type carbs.

The test simply measures the fuel level inside the bowl and won't mess up anything inside the carb. The "messing up" part comes later when attempting to adjust the level by bending the float tang.

Here's the "official" tool. Available at Z1E.

[Click on image to enhance view]



There are slightly different techniques for smooth-bore carbs and others without the common side-located floatbowl drain screws.


Good Luck!

Patton - your image of smoothbores is the 29mm variety. The ones I have are also smoothbores but radial slide, 34mm. They are all "smooth" inside though which is what makes them different from regular carbs.

Anyway, I have been wondering about the fuel level and had thought that it would be better to set the fuel level instead of the float height. The float height setting is meant to set the fuel level anyway, so why not just cut to the chase and set the fuel level?

The problem I have is I can't find a fitting to for the bowl. I checked the Z1E site and only found the one that you showed, for the drain screw, which I don't have. I've also been searching everywhere else on the internet with no luck so far.

Let me know if anyone knows of a source for fuel level tool for the smoothbores (large hex plug on bottom of float bowl).

Thanks

timebomb33 wrote:

i always tip the carbs at a bit of angle just so the floats are making contact but not putting pressure on the valve and measure and set them that way if that makes any sense i hope this helps some i didn't see this post before now or i would have said something sooner.

That's pretty much how I set them this time - I put them on their side, standing vertically so the tab just touched the valve. Measuring them differently resulted in the floats being set about 1-2mm lower than they were, so we'll see what difference it makes if any.

Patton wrote:

...
[Click on image to enhance view]

....

With these, I use a cone shapped rubber gromet fitted snugley inside the hole at bottom of floatbowl, with the clear tube fitted snugly into the gromet hole.


Good Luck!

Patton wrote:

Patton wrote:

...
[Click on image to enhance view]

....

With these, I use a cone shapped rubber gromet fitted snugley inside the hole at bottom of floatbowl, with the clear tube fitted snugly into the gromet hole.


Good Luck!

Thanks - I never thought of that - does it work without leaking?

79MKII wrote:

...does it work without leaking?

Yes, depending on how firmly "threaded" into the hole.
Could actually just hold a sufficiently large gromet firmly against the hole during testing, without "threading" it into the hole.

However, a slight leakage usually doesn't alter the test results in any of the clear tube testing situations. Because fuel is entering the floatbowl to maintain the level faster than it's seeping out. Think of a running engine drawing fuel from the floatbowl (i.e., engine consumption being akin to seepage), where the fuel level inside the bowl never-the-less remains constant.

And here's a free tip. When removing and replacing the float bowl during repeated efforts to get the float tang bent just right, only need to use a single float bowl screw to hold the float bowl in position for the level test. It usually won't leak much, if at all, and saves some time.

Good Luck!

Good tips - thanks!

There was an adapter on eBay last week that I thought about buying it. It was advertised as being for early 900 carbs that had the drain plug on the float bowl (maybe same as smoothbores??). Anyway, the seller told me the threads were 18.0 x 1.0 and as far as I can tell the 900 carbs were all 17.0 threads anyway. Didn't want to chance it for $25.