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TOPIC: Mikuni VM 33 Carbs

Mikuni VM 33 Carbs 15 Nov 2016 08:58 #748028

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I agree on tightening them by hand.... I recently tightened my plugs (for the first time) to the min using 2 diff wrenches and they felt too tight for me (damn near a full 1/3rd).... same with the filter bolt, drain plug and valve cover bolts. Pan, oil pump, cam gears, cam cap bolts, starter clutch bolts etc I carefully went with the min on the wrench (sneaking up on them with a 3 inch wrench then the torgue wrench) but will go back to a good feel on most. Do you use a small amount of anti seize on the plugs.... some don't but I pull mine quite a bit too and I think its really protecting the threads from wear.
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1980 KZ750 cam, ape springs, stock clutch/ Barnett springs.
Vance and Hines Header w/ comp baffle and Ape pods, Dyna S and green coils, copper wires.
29MM smooth bores W/ 17.5 pilots, 0-6s and 117.5 main
16/42 gearing X ring chain and alum rear JT sprocket....

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Mikuni VM 33 Carbs 15 Nov 2016 10:35 #748036

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I don't use anti seize on anything except my old Ford truck heads or things like that. I use a stubby ratchet or box end wrench. I've broken and stripped plenty of bolts in my time and have developed a feel for them. Others may prefer to use a torque wrench and I do use one for head bolts and such.
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Mikuni VM 33 Carbs 15 Nov 2016 12:10 #748041

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Well, that explains why my spark-plug crush washers are flattened out after one or two removals. I thought the FSM torque spec of 20 ft. lbs. was to be taken as gospel, but such is the result of my experience. :pinch:

Back to my jet question... :S

My RS36 carburetors are setup with 17.5 pilot fuel jets and 0.7 air jets.
Does that have exactly the same effect as using 20 pilot fuel jets and 0.8 air jets?
:unsure:
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Mikuni VM 33 Carbs 15 Nov 2016 13:11 #748045

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I would suspect the air jets are for the mains not the pilots. Pic?
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Mikuni VM 33 Carbs 15 Nov 2016 21:12 #748061

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swest wrote: I would suspect the air jets are for the mains not the pilots. Pic?
Steve


Steve, hope this helps.
Pic: Note Pilot Air Jet is the small one on the right.
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Mikuni VM 33 Carbs 15 Nov 2016 21:24 #748062

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Main air jet plugged? Is that a accelerator pump nozzle? Have you tried looking on the jetsrus charts like they have for the Micunis? Maybe try Sudco for the jetting for your machine. Also the float level looks a little high. That would make it rich on the bottom.
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Mikuni VM 33 Carbs 15 Nov 2016 22:07 #748063

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swest wrote: Main air jet plugged? Is that a accelerator pump nozzle? Have you tried looking on the jetsrus charts like they have for the Micunis? Maybe try Sudco for the jetting for your machine. Also the float level looks a little high. That would make it rich on the bottom.
Steve


Main air jets plugged, yes as exploded part list states and what looks like a jet is actually a plug.
Yes, the accelerator (pump) nozzle is shown vertical in the bore in front of the throttle slide, directly below the blue P.
All good on the instructions for jetting, except for the Mikuni application chart indicates Pilot screw setting range from 1/4 - 1/2 turn (off lightly seated I believe), not 2 1/2 - 3 turns. Runs well at 1/4 turn out currently.
Float levels measured and set correctly at 17 mm per Mikuni instruction manual. There is no measurement information on which to base using the clear-tube float setting method that works on the BS34's so well.
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Mikuni VM 33 Carbs 16 Nov 2016 03:34 #748069

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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.
I believe its called "Oppositional Defiance Disorder"

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Mikuni VM 33 Carbs 16 Nov 2016 08:27 #748080

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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.





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Mikuni VM 33 Carbs 16 Nov 2016 10:53 #748084

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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
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Mikuni VM 33 Carbs 16 Nov 2016 12:24 #748086

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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.
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Mikuni VM 33 Carbs 16 Nov 2016 12:57 #748088

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Is there a port on the bottom?
Steve
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Mikuni VM 33 Carbs 16 Nov 2016 16:56 #748104

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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.

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Mikuni VM 33 Carbs 16 Nov 2016 18:10 #748110

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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.
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Mikuni VM 33 Carbs 16 Nov 2016 22:06 #748126

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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:
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Mikuni VM 33 Carbs 17 Nov 2016 06:12 #748145

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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.
I believe its called "Oppositional Defiance Disorder"

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Mikuni VM 33 Carbs 17 Nov 2016 07:00 #748147

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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! :)
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Mikuni VM 33 Carbs 17 Nov 2016 08:25 #748152

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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. B)
Steve
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Mikuni VM 33 Carbs 17 Nov 2016 09:47 #748153

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Mikuni VM 33 Carbs 17 Nov 2016 11:49 #748157

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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
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