VM 26/28 air jet confusion

[Agemax]

ok, finally finished cleaning/dipping/rebuilding the VM28's i bought for my GS.
during final inspection before fitting i have noticed a peculiarity!
on the intake (air filter) end of the carbs there are 3 holes, 1 at 2 oclock, 1 at 4 and 1 at 8.
the hole set back at 2 oclock i know is to do with the choke. on the other 2 there appears to be a jet in 1 of them.
carbs 1 &2 the jet is in the 4 oclock hole and carbs 3 & 4 the jet is in the 8 oclock hole. is this correct and if so what is its purpose?
the 26's i removed have the same 3 holes but no jets in any holes.are they supposed to be there or is it a mod specific to the 28's?

[MisterCinders]

Pics would help, but if I follow your clock hand directions, those ports sound like the air pilot passages on my VM26s.

From the fiche:



The air screw is actually un-numbered, but appears on the right most carb in the diagram and goes in at about 2 o'clock.

[Agemax]

these are the 3 holes i am referring to. this is no.3 carb and you can just make out the jet in the 8 oclock hole

[OldVet66]

I don't know much about them except they look just like my friend's 28's from the UK.

[MisterCinders]

Does the VM28 have a pilot fuel screw like the VM26? Looking at that pic and your description of the jetted hole positions, it sounds like each ties into the fuel line entry point. They are on the inside face of each carb, right near the spot where the fuel line enters the carb body. The hole on the opposite side ties into the pilot air screw, as you can seen in the pic.

Perhaps they are part of the pilot system and serve as the pilot fuel jet?

[Agemax]

the 28's have seperate air and fuel screws as well, same as the 26's

[MisterCinders]

Have you shot some carb spray into the jetted hole to see where it goes?

[Agemax]

just tried that, its coming out the main jet?!

[MisterCinders]

Still thinking it's part of the pilot system somehow. The passage feeds air (and maybe fuel) into the main jet and bypasses the throttle slide, just like the air passage that runs by the pilot air screw. Of course, the main jet is isolated within the throttle slide, so I don't really fathom how adding fuel/air into that part of the carb does anything when the throttle slide is low/closed.

[Agemax]

there must be a tiny passage that connects into the emulsion tube. that would explain how it comes out the main jet. just checked the 26's and they dont have any threads in the holes so obviously not meant to have these jets.
maybe its some kind of boost jet to allow a bit more air/fuel mixture into the carb if you open the throttle quickly??????

[Agemax]

stripped out 1 carb again and there is indeed a small orifice that feeds the emulsion tube from the tiny air jet. whether it is just air or air and fuel i cannot tell

[Agemax]

anyone know if you can alter these little jets? cant find part numbers for them anywhere. there is no number on them to state what size they are.
just wondered if they are "tunable" same as mains and pilot jets etc, to compensate for a tuned set up