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VM Air Jet - how's this work?

dardoonk

dardoonk

Forum Mentor
On VM carbs (29 smoothies to be exact) how does the air jet that screws into the outside lip function? Z1 sells a 0.9 air jet for the VM29 carb, if you install a smaller jet, which I assume would restrict air, what part of the carb would that effect? pilot system? idle?

Conversely, if you enlarged the hole to 1.0, what would be required?
It is #12 on the lower carb in the schematic below

VM29 Sudoc drawing

DSC05451.jpg
 
dardoonk said:
On VM carbs (29 smoothies to be exact) how does the air jet that screws into the outside lip function? Z1 sells a 0.9 air jet for the VM29 carb, if you install a smaller jet, which I assume would restrict air, what part of the carb would that effect? pilot system? idle?

Conversely, if you enlarged the hole to 1.0, what would be required?
It is #12 on the lower carb in the schematic below

VM29 Sudoc drawing

DSC05451.jpg
Click to expand...

I thought others would have chimed in sooner, i have cv carbs but the function is similiar.

1-On mine although they look different than your pic are the Main Pilot Air jets.

2-The Main pilot air jets on mine allow a certain amount of air into the pilot circuit, fuel from the pilot fuel jet mixes with this and goes to the mixture screws which on mine are actually called pre mix as there is fuel and air in this circuit.

3- The other air jet on mine is a pressed in air jet, this allows air to go to the emulsion tube which is where the needle jet is. This air " emulsifies " the fuel with air and this is sent to the venturi.

4-Bigger air jets will allow more air into the idle circuit and a smaller number air jet will allow less air. As such the bike may idle rich or lean depending on how much air is present to mix with the amount of fuel available form the pilot jets.

Not a scientific explanation but hope it helps. Your factory manual should have diagrams of this to fully illustrate the functions.
 
Those little blighters are in the pilot circuit and thus mainly effect idle, though there will be some effect on very low throttle openings as well. As you guessed, bigger air jet equals more air, smaller air jet equals less air.

Have a look at below - you'll see how they fit in the circuit. Basically, you can set your airscrews between just above 0 (theoretically though I would say minimum 3/4) and 3 turns out - any more and the screw could undo itself as the spring loses functionality. (Out equals more air, in equals less air). If you get out of this range when setting your pilot circuit you need to swap your air jet.

http://www.mikuni.com/pdf/vmmanual.pdf
 
Thanks, guys... Just what I was figuring.. I just ordered new ones from Z1 since two of the originals seemed kinda bunged up.
Hopefully, this clears up some of the issues I am seeing in the pilot circuil
 
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