Mikuni VM 33 Carbs

JOE MKII wrote: I agree with Swest , that fuel level looks high, even if the floats are correct lowering the fuel level a mm or less at a time is worth a try if your having problems.
Float level is a viable and accepted way to fine tune a carb , within reason of course.

Not sure how to adjust the floats to lower the fuel levels, other than excessively tweaking the float tab, and then whether it would make much difference or not.

When I first got the carbs, I set the floats to 19 mm, concerned that fuel would drain when the bike was left to rest on the side-stand (when these pics were taken).
Subsequently I reset them to the Mikuni spec, and they don't leak at 17 mm either. :S

BTW, after checking the fuel levels with the special clear tube (pic 1), I found that neither setting affected the fuel height more than 1 mm, so I reset them to 17 mm. It seems strange that the levels are above the bowl top, considering BS34's were easily set 3-4 mm below that.

Pic 2 shows 7 mm measured from the top of the bowl to the fuel drain-tube hole, 2-3 mm above the fuel per pic 3.

I'd set them even with the bottom of the bowl flange and see how it runs. I don't care for it being past the gasket. Might be too rich that way and could leak.
Steve

No leaks yet, but your viewpoint makes sense. Not sure how to get the float tabs bent enough to drop the level 4 mm though.
Are you confident its not supposed to be this way? Take a look at this pic, and consider if the pump plunger needs to be immersed with fuel to work properly.

Thanks again.

Is there a port on the bottom?
Steve

swest wrote: Is there a port on the bottom?
Steve

Are you referring to a hole near the base of the bowl (directly below the plunger), where the three other bowls have none?
I have one lousy photo that shows the inside of the bowls on that side, but that hole is hidden from view behind the brass drain tube.

Yes, is there only one pump? Is it on that one? My carbs has one bowl with a nipple at the bottom for a external pump. The nozzles have been removed. I'd like to find one but can't.
Steve

The bowls are connected together with fuel hose at the bottom, through which the fuel is pumped to each carb nozzle.

There's an incredible variety of tuning features on these carbs. I've learned a lot about them by simply experimenting with different jet combinations. I'm blown away by how well a few simple engine mods have improved my engine performance, but these carburetors bring them all together to make incredible music. :woohoo:

Flatslides are awesome carbs, and smoothbore flatsides are really cool and at idle sound really wicked! Lots of power and throttle response to be gained when jetted right.

I know this is getting WAY off topic so maybe no answer is needed here, I will do a site search, but has anyone tried running the CV flat slides from a giixxer.
Those carbs are great too if all there internal parts are not worn.

daveo wrote: ...Not sure how to adjust the floats to lower the fuel levels, other than excessively tweaking the float tab, and then whether it would make much difference or not... after checking the fuel levels with the special clear tube (pic 1), I found that neither setting affected the fuel height more than 1 mm, so I reset them to 17 mm. It seems strange that the levels are above the bowl top, considering BS34's were easily set 3-4 mm below that....

When the 33 smoothbores were brand new, the 23 mm float height shown in Sudco's tuning manual would presumably produce a "correct" service fuel level of approximately 3 mm below the float bowl gasket, this being with perfect float assemblies.

Over time, normal wear plus degraded condition after multiple disassemblies, tang bendings, perhaps less than delicate re-installations, and other possible previous ham-fisted handling by others, the prescribed "float height" imo has become increasingly unreliable as a means toward achieving the far more critical fuel level.

Imo, when now dealing with these old carbs, the "correct" float height is whatever produces the far more critical service fuel level as measured by the clear tube test, which should be approximately 3 mm below the float bowl gasket.

The float tang adjustment is the proper method to achieve the correct service fuel level (i.e., 3 mm below the float bowl gasket).

When adjustment of the float tang fails to change the fuel level (as measured by the clear tube test), something is probably wrong with the float function, which could possibly include a bent or hanging/sticking float, or floats installed upside down in certain carbs. But could also be due to installation of a "wrong" float needle, or worn or damaged float needle.

Am unaware as to whether or not floats may be installed upside down in Mikuni 33 smoothbores, but if it's possible, would double-check to confirm that the floats are correctly installed. And, of course, that the floats are indeed the correct floats for 33 smoothbores (not pulled from some other style carbs).

Imo, the service fuel level as determined by the clear tube test should never rise above the float bowl gasket, and should be set within spec (say 3 mm) below the float bowl gasket.

Good Fortune!

The floats can be installed upside down and won't work correctly. Various floats can be used in these carbs and will work correctly. New ones can be obtained but I like the later style solid foam type. The hollow brass type can develop leaks while the solid ones seem to last forever.
Steve

Here are links to some threads with posts by wiredgeorge regarding float height and fuel level:

www.kzrider.com/forum/3-carburetor/25463...evel-question#254644

www.kzrider.com/forum/3-carburetor/23203...el-adjustment#232622

www.kzrider.com/forum/3-carburetor/12468...-float-height#124956


Good Fortune!

daveo wrote: The bowls are connected together with fuel hose at the bottom, through which the fuel is pumped to each carb nozzle.

There's an incredible variety of tuning features on these carbs. I've learned a lot about them by simply experimenting with different jet combinations. I'm blown away by how well a few simple engine mods have improved my engine performance, but these carburetors bring them all together to make incredible music. :woohoo:

I suspect if you lower the float levels using the clear tube method, you will have to start over. The cutout at the gasket is not for fuel but to vent the pump assy.
Steve