VM33 smootbores

the air bleed he is talking about are the jets in the bell opening.They lead to the emultion tubes,or needle jets.
There is a tip on the old cope site(A.P.E now)that suggests to use .6 air jets with vm33's to improove throttle responce . it says when u do this u can go down on the main jets 4 sizes.why would u go down on the mains?i thought the air jets afected the needle jets mostly?i havent tryed this yet.i bought the jets but i will change my cams first.can u give me some advice on tuning the air jets?how they affet and where they affect?i cannot find any info on airjets for some reason .the mikni tunning manual dosnt say anything either.

Post edited by: mark1122, at: 2007/08/10 08:38

Looking down the throat of the VM33 from the intake side, there are four or five round holes arranged around the lower edge.

The middle hole (from memory) is threaded and should contain a removable air bleed (Mikuni # BS30/97). Originally the VM33s were fitted with 1.0, and the 29s were 0.9 (nominal bore size in mm).

Mick

If the ports or engine are too small ,at
lower RPM the air velocity in the carbs
probably drops too low to pull and atomize
the fuel from the jets.Adding an accelerator
pump may cure this.I have seen some added to
the 29s years ago but don't know about the 33s.

tj,wouldnt that be the other way around?if the ports (just jike the bigger carb)are bigger u get more volume but loose velocity . so at lower rpm u dont have enough velocity to pull fuel up through the jets when u crack the throttle?i supose this is why the rs34's have accelorator pumps.but i could be wrong.

Mark, I agree with you.

An engine with small ports that flows 100cfm will always outperform an engine with larger ports offering the same 100cfm.

It's all about air speed and velocity. To a point. You can have too much of a good thing.

Air bleeds are used to tailor the fuel curve.
For example, let's say that your VM33s are running 1.0 air bleeds and 120 mains but you feel that it's a bit rich in the very top end. You could fit 1.1 air bleeds which would gradually lean the fuel curve more as rpm increases.

Alternatively, your same 1.0 and 120 combo is lean at the very top end, you fit 0.9 air bleeds and richen it at the top end.

As a general rule, air bleeds working on the main circuit offer more effect towards the top of the rpm range.

They are a very useful but often misunderstood, tuning tool.

Personally, I like to run larger air bleeds and introduce as much air as possible to atomise the fuel as finely as possible. When tuning an engine, I'll keep increasing the air bleeds until it tells me that it's lean and then go up another size on the mains and re-start the process. Yes, it's slow but the results are worth it.

From my experience, it can make a big difference to throttle response, especially handy for road racing and for street bikes.

Smaller fuel droplets burn much more efficiently than larger droplets.

Dosnt the air jet feed the emultion tube?wouldnt this meen it would affect the mid range just as much as the mains?my bike pulls hard on the mains but lacks bottom end.i'm hopeing the new cams will help hear.they are similar to the ape k410's.

Hear is the quote from Greg Cope:
Mikuni 33mm Smoothbores
For those of you who still own these carburetors a little jetting trick. In order to get better throttle response install 0.6 air correction jets. This allows you to run a smaller main jet while still giving you approximately the same air fuel ratio as before. Good starting point is 4 main jet sizes smaller than you previously had. Also installing a O-6 needle jet will help lean out the mid range.
when he says 4 main jet sizes,does he mead go from a 140 to a 134?

Air bleeds are the holes in a tube that allow gas to come out in a mist instead of in drops... at least, that is the terminology I have used and heard in the past. I can't say that much of the carb terminology is standard... There are THREE air jets on any VM series carb... the one back in the venturi and off the edge connects to the choke (actually an enrichening circuit on a VM). Since the slide is closed with the choke on, air flow is fairly fast and gas is pulled in from the brass choke pickup on the bottom that sticks down into the bowl and this gas is atomized with the air from the choke air jet. The other two air jets are the main and pilot air jets. On a VM33, the pilot air jet is fixed... it is a brass deal that is pressed in. The MAIN air jet is the only one that can be removed. It screws out. Most documentation says it is supposed to be a #110 but in real life almost all of them are #100 air jets. This jet connects directly to the main jet well where the main jet holder (emulsion tube) is screwed in. (The pilot air jet connects to the pilot well and pilot jet).

The main air jet draws in air to be mixed with gas that comes out the needle jet opening in the emulsion tube. This draw is governed by the venturi air speed which is pulling gas and I guess air from the bowl and the air jet. This would richen the mixture a bit... Throttle response? This is the reaction of the throttle to changes in its position and I am not really clear how richening would make a difference in throttle response when bumping up the size in the main jet would theoretically have the same effect, if this is what is happening.

Sudco does have replacment air jets in .5mm to 2.0mm and the type someone mentioned BS30/97 is correct. Guess I will need to get a set and see how they work. Will let ya'll know after I get to it.

Mark.. as far as Mikuni main jet sizes, they run in 2.5 increments. If you went from a #140 to one size down, that would be a #137.5 or at least that is the way I know to characterize ONE size.

Post edited by: wiredgeorge, at: 2007/08/10 15:38

Thanks George. i agree on the 1 size ,and also on your analogee that things are discribed in differant ways.and that lead me to wonder what 1 size meant.

Mark, I have always done mechanical work by myself and when I pick up a term, it often is the term I picked up from someone else... who knows where that person got it from. There is no standardized carburetor terminology; look at a few carburetor tutorials and you will see few, if any of the terms that are the same from one to the other. Step sizes in jetting are also based on the manufacturer. Most Mikuni jets are in 2.5 increments. This is not the case with Keihin jets. I have 123, 124 and 125 Keihin main jets in my jet box. All are genuine Keihin jets so I am not sure if they use any incrementing. Then you get aftermarket stuff from Keyster... they will clone a Mikuni jet and if the jet is #17.5, they will just call it a #17 sometimes and other times, #17.5. Can you sub a #17 for a #17.5 Keyster pilot jet? I suspect you can as they were probably drilled by the same bit.

G'day George,

I agree with you on the different names for different parts.

Us Aussies tend to name parts a bit different to the rest of the world.

Will be interested to hear the results of your testing. My own experiences have shown that the better atomised the fuel mixture, the easier it is to get it to draw when the throttle's opened. By introducing more air via the screw-in air bleed, this can be fine-tuned.

How many people are aware that round Mikuni main jets are numbered by nominal bore size and that their hex main jets are rated by flow?

Personally, I hate aftermarket jets. Too may variables. Mind you, if using a Dynojet kit, and only Dynojet jets are used, the results are good.

I also hate jets that have been re-drilled by someone else. With such fine tolerances, a slight miscue with the pin vice can make a big difference to flow.

This is a good thread. Lots of info flowing around.

George,
Have you had a chance to mess with the air bleeds yet? I was also wondering how you sync the VMs as the J model carb holders (that is what you use, correct?) don't have the vacuum tubes molded into them like the earlier Z1/KZ1000 type.