Mikuni Air-Jet.... is this where it goes?

jdcat · Post by **jdcat** » Sat Mar 29, 2008 4:46 pm

Airjets - Mike Sayers
Air going into air jets is NOT intake air... The tiny amount of air going into the air jets is completely insignificant compared to the air going through the carb throat even at idle. Very few people seem to understand the purpose of air jets in a Japanese carburetor. The air going into these jets is mixed with the fuel (at more or less equal amounts) to produce a bubbly "emulsion" of fuel and air. This does two things: by replacing some of the fuel volume, it allows finer control of mixture because they can now use a larger fuel jet to pass the same amount of fuel. See? This makes it easier to adjust the carb, because each change in fuel jet size or screw needle has less effect on the mixture. The second thing it does is start the fuel breaking up & evaporating sooner so it burns more thoroughly.

I believe the Vmax carbs have 3 air jets. the biggest one feeds the idle and off-idle jets. The smallest one feeds air into the main jet & needle. The medium size jet (#190 exc maybe in CA) mixes air into the idle & off-idle circuit as well. This medium air jet CLOSES completely when the vacuum rises above a certain point due to letting off the throttle (coasting). In other words, the "coasting enricher" works by closing off this jet.

So the effect of reducing the size of this jet is to make the idle & off-idle mixture a little richer by reducing the amount of air mixing into the fuel emulsion when the throttle is opened & brings the mixture a little closer to where it is when coasting. The effect on idle can be cancelled out by readjusting the idle mixture screw. If you have an off-idle mixture problem (like when cruising at low speed) this is the way to fix it. Now, if running with the choke partially on gives you more power (only a theory not proven I know), changing this air jet down will have the SAME effect because it affects the SAME fuel circuits the SAME way. It will NOT restrict the breathing of the engine in any way, the engine does NOT breathe through the air jets.

The air that comes in the air jet is displacing fuel in the carburetor circuits, not just going into the engine. Air jets are not just vacuum leaks. Actually the smallest air jet will have the most effect at high speed!

Joliet Jake · Post by **Joliet Jake** » Sat Mar 29, 2008 6:13 pm

Yes 400 brain your right he is talking about the tiny air jet.

Sorry!

I always thought that the air jets were not installed in the primary type VM carbs because so many were just missing.
But I have been informed that the truth!....... There is ton of them lying in the bottom of the belly pans and air box?s. of this nations snowmobiles
I think that?s why most of the new VM carbs don?t have them ?.just a drilled passage

The air jet acts to assist atomization at high speed (3/4 to full) by and to help in high speed fuel metering.
If you look at the hole you have shown in your picture you can see that it goes to the needle jet. The needle jet on a primary type carburetor has a hole that connects to the passage and feeds air around the fuel stream so it gets a so called jump start to the atomization process. The fuel will be broken down further once it enters the main air stream.

JDX-8 · Post by **JDX-8** » Sun Mar 30, 2008 12:05 am

uhh..... <_< I had to read that 4 times!...lol... I have been in school for a bit, but...... im thinking you have either taught at a college... or should!!!... Nice info, and it does make since!.....

So air that is coming through the Air-jet, at near full throttle, mixes with the fuel IN the bowl and assist in atomizing the fuel when it is sucked between the needle jet and jet needle????? Then being intoduced into the intake then the motor.....

So am i on track here????

Now... as for the effect at idle and part throttle..... does the Air-Jet have much effect then?.... or is it pretty much for the full throttle proformance?

With not having an Air-Jet installed on these carbs... as they are not threaded for them.... would i be looking to use larger or smaller needles to begin tuning with?.... as i was using the parts # for a LF 440 motor.... but now i dont have Air-Jets..... uncertain what to do?????

Thanks guys!- Kelly

harleysportster · Post by **harleysportster** » Sun Mar 30, 2008 8:35 am

Guys,
Some of the info that has been given is incorrect on the purpose of the Air Jet. Here is a scan from a service manual explaining it. In the scan, letter E represents the air passage. Some carbs have air jets installed in passage E
Mikuni carb

Joliet Jake · Post by **Joliet Jake** » Sun Mar 30, 2008 12:26 pm

JDX As far as does the air jet help with low speed metering the answer is yes somewhat remember that the secret to carburetor tuning is to have the transition points flow smoothly from one to another.
It?s the transition from low speed metering to the main metering is somewhat helped by the air that comes thru the air jet.
But as JD CAT said the amount of air coming thru there is insignificant compared to the bowl vents and other metering devices that run the pilot system.

Mikuni makes two different main metering systems Primary and Bleed systems.
Primary systems were made for piston ported two stroke snowmobiles and the reason was: piston port engines with their blow back and low intake velocity did not have a good of a signal as four strokes.
Because of the low velocity the transitions were difficult to tune smoothly and they did not atomize the fuel as efficiently.
This was not a problem on four strokes BUT because of mid range leanness IT WAS DEATH TO TWO STROKES.
Mikuni answer was to have air enter the fuel as it was ?pushed? up the needle jet and since that mixture of air and fuel was lighter then just fuel, it would be ready to enter the air intake once the slide was opened and the pilot system signal was waning.

But the real benefit was that this air injected frothy fuel charge was atomized better at high speeds even with the lower velocity intake speeds

jdcat · Post by **jdcat** » Sun Mar 30, 2008 4:17 pm

I could be wrong but ive seen mikunis with a brass plug factory pressed into the air jet passage to plug them off. If a carb didnt use the air jet on a certain sled ,Shouldnt it be plugged?

JDX-8 · Post by **JDX-8** » Sun Mar 30, 2008 4:47 pm

The hole on my carbs that the Air-Jet would normally go in..... it does taper down very slightly... not very much... maybe 1-1.5mm at most near the deepest part...

so,..... with all the info that i have gotten on this subject.... i still am not sure what to do about where to start on jetting these carbs for this KEC-440

I however do understand that some of the newer Mikuni carbs do not have these Air-Jets in them... but when im using the specs for a LF motor with dual carbs.... should i be starting with a larger needle or a smaller needle since MINE dont have the Air-Jet.......

Have a good one guys, - Kelly

harleysportster · Post by **harleysportster** » Sun Mar 30, 2008 6:21 pm

Kelly,
If you used the heads from a '77 cyclone, you don't need an Air Jet. The '76 or earlier with a higher compression engine used a .7 air jet.

JDX-8 · Post by **JDX-8** » Sun Mar 30, 2008 6:52 pm

Cool thank you guys for all the help.. i think i have what i need to get it all underway!...

Thanks for all the help and reading all my rants..... hope all is well in your parts of the world and catch you all later,.. have a good one!

-Kelly