Very Difficult Question About Mikuni VM29 Carbs

This "rebuilt" set of carbs I bought a while back has been a nightmare. Many parts were mixed and matched, some were broken, some were incorrect and now I've found another problem that might be the most serious yet. I stumbled upon this issue today and I think it could be why these carbs won't run correctly.

I discovered today that the air jet passage for the #3 carb is enlarged. I'm not sure if some idiot drilled it out or got crazy trying to clean it or what, but it is bigger than the other three. The question is, will this affect the performance of the carbs? On one hand, I think that the air jet limits the air flow so if the passage is a little bigger it might not matter. On the other hand, a spray of carb cleaner acts very differently on that carb, so the air would too, right?

The following video was taken spraying directly into the air jet passage from the back of the carbs with the air jet removed. The main jet and the needle jet were also removed. As you will see, the fluid flow is very different for the #3 carb (same result when main jet is installed). Using multiple sizes of rod as a measuring device I was able to determine that the #3 passage is larger than the others. That seems to be the cause of the flow issues.

Do you guys think this will this cause problems with how they run?? Thanks

Link to youtube video:

If #3 is larger then that cylinder will run leaner. You can get air jets from jetsrus.com/a_jets_by_carburetor_type/a_...mikuni_selection.htm
Steve

swest wrote: If #3 is larger then that cylinder will run leaner. You can get air jets from jetsrus.com/a_jets_by_carburetor_type/a_...mikuni_selection.htm
Steve

Unfortunately, it's the passage in the carb body, not the jet. The passage leads from the air jet to the needle jet/main jet. The opening at the needle jet end is larger. The video was taken with the air jets removed.

Do the emulsion tubes have threads on the end?
The passage going to the emulsion tubes doesn't matter. The air jet should meter the volume. The openings need to be the same on the tubes.
Steve

Looking at the video it seems the passageway is clogged. But you measured the passageway and it's bigger than the others. If that's the case maybe the velocity is reduced because the passageway is larger, resulting in the carb cleaner making a smaller squirt into the air.

I would send the carbs to Wired George - get it done right and pay for it. Alternatively find a 29mm smootbore single carb on the bay. That may take awhile and then we can't be sure the single carb will perform the same as the other three carbs on your rack.

Is it just the alloy opening in the body or the fixed brass jet that is further in ?
If it's just the alloy then it won't affect the fueling.
If the brass air jet has been enlarged on 3 you can thread the opening at the mouth of the carb and fit a restrictor bored to match the other carbs.
Those passageways will usually take an M5 thread.
A simple brass grub screw drilled to the appropriate size will do the job.
I would do all four and then you can vary the size equally across all four to fine tune any fueling flat spot when coming off the pilot circuit
Google LEDAR air restrictor jets to see how this was a common mod when using velocity stacks or pods.
The spray through 3 carb also appears restricted and that will cause that cylinder run rich regardless of whether the passage is larger on the inlet side so that needs addressing first.
I sounds like there is a partial blockage that the PO has tried and failed to clear.

I do not believe that spray stuff in the carbs like the video shows is indicative of flow through the carbs while running. The air jet will regulate air through that passage.

Had to sleep on it. I'd like to see pics of the one carb opening. The 33 and 29's are similar internally. Some parts are interchangeable. #3 on my 33's has a fuel port at the bottom of the bowl. The linkage has linkage that can be used for an accelerator pump. I suspect yours has an option of some kind and that's why it is acting differently. As you said, they do come out. Looking at the jetsRus needle jets, I don't see mine.


Can you take a pic of yours to compare?


You can get the air jets if the ones you have are munged up.


Syeve

Kapahulu wrote: Looking at the video it seems the passageway is clogged. But you measured the passageway and it's bigger than the others. If that's the case maybe the velocity is reduced because the passageway is larger, resulting in the carb cleaner making a smaller squirt into the air.

I would send the carbs to Wired George - get it done right and pay for it. Alternatively find a 29mm smootbore single carb on the bay. That may take awhile and then we can't be sure the single carb will perform the same as the other three carbs on your rack.

I was so hoping it was clogged...that was my initial thought too...but no such luck. I'm thinking the larger passageway = less velocity too, but I just don't know if that will ultimately affect the quantity of fuel flowing through the main jet or maybe the fuel/air ratio.

zed1015 wrote: Is it just the alloy opening in the body or the fixed brass jet that is further in ?
If it's just the alloy then it won't affect the fueling.
If the brass air jet has been enlarged on 3 you can thread the opening at the mouth of the carb and fit a restrictor bored to match the other carbs.
Those passageways will usually take an M5 thread.
A simple brass grub screw drilled to the appropriate size will do the job.
I would do all four and then you can vary the size equally across all four to fine tune any fueling flat spot when coming off the pilot circuit
Google LEDAR air restrictor jets to see how this was a common mod when using velocity stacks or pods.
The spray through 3 carb also appears restricted and that will cause that cylinder run rich regardless of whether the passage is larger on the inlet side so that needs addressing first.
I sounds like there is a partial blockage that the PO has tried and failed to clear.

I can't get a look at the opening into the main jet passage...it's just too small. I can barely see a wire poke through when run through it. Looking from the air jet side it doesn't look like it goes straight through. I wish I could find a cutaway view of the passage. It doesn't look like there's anything pressed into the other end but I just can't see....and the air jet is currently removed.

A wire can be run all the way through the air jet passage, up into the main jet passage and out by the bottom of the needle so I'm pretty sure there's no blockage.

Oh, and to make sure we're all on the same page...it's the hole on the inside end of the passage that is enlarged....right near the main jet. It would be the outlet of the air passage, not the inlet where the air jet screws in.

Nessism wrote: I do not believe that spray stuff in the carbs like the video shows is indicative of flow through the carbs while running. The air jet will regulate air through that passage.

That sure is what I'm hoping for but air flows like water and that stream is extremely different than the others. I believe the air would be "sucked" through that passage and would mix in with the main jet flow, so it seems like it could have an affect on the quantity of fuel and or air or maybe just the type of stream flowing. If it's more of a spray and not a proper stream compared to the others, would that affect performance? This is deep stuff here!

swest wrote: Had to sleep on it. I'd like to see pics of the one carb opening. The 33 and 29's are similar internally. Some parts are interchangeable. #3 on my 33's has a fuel port at the bottom of the bowl. The linkage has linkage that can be used for an accelerator pump. I suspect yours has an option of some kind and that's why it is acting differently. As you said, they do come out. Looking at the jetsRus needle jets, I don't see mine.

Can you take a pic of yours to compare?

You can get the air jets if the ones you have are munged up.

Syeve

Mine are like the needle jet and main jets shown in your first picture. The needle jet threads into the carb (into the jet block, actually). Then the main jet into the needle jet. The air jet is like the one pictured last and comes right in and out no problem. The problem is not at the air inlet side (by the air filter) but at the other end, where the passage opens up into the main jet passage (where the needle jet and jet needle are)