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air jet size and what it does?

  • Thread starter Thread starter johnbb71
  • Start date Start date
J

johnbb71

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so if a BS36 comes with a 125 air jet stock would a larger jet make it run more rich or more lean?i am theoreticaly thinking a larger jet would let more air pressure into the carb pushing more fuel through the pilot?:confused:
 
johnbb71 said:
so if a BS36 comes with a 125 air jet stock would a larger jet make it run more rich or more lean?i am theoreticaly thinking a larger jet would let more air pressure into the carb pushing more fuel through the pilot?:confused:
Click to expand...

You got it right. ;)
Actually, it lets MORE air PULL more fuel. Close enough.

Eric
 
cool, ty very much, i have studied all i can find about these carbs but never found that particular info yet, i just narrowed it down to that by diagrams and my mechanical aptitude.
tricky they can be with how everything acts together and you can accomplich similiar things by changing more than 1 thing.
 
You are one smart cookie. ;)
This is the best pic that I have that shows how they work.
It was intended just to show where the well plugs are located but I am certain that you can figure it out.
The larger the hole, the more air that passes over the pilot port, the greater the pull into the carb throat.

CarbRubberplug-1.jpg


Eric
 
so there are 4 ways then to enrich the pilot circuit or should i say the 1/4 throttle range? yah 4 i would guess,the fuel mixture screw(to an extent) the air jet going larger, the fuel level being raised or increasing the fuel pilot jet.
intricate lil buggers these cv carbs are hm.
 
Actually, enlarging the air jet on the pilot circuit will have a leaning effect. It allows more air to mix with the fuel being pulled through the idle circuit and it influences performance up to 1/4 throttle position.;)
 
49er said:
Actually, enlarging the air jet on the pilot circuit will have a leaning effect. It allows more air to mix with the fuel being pulled through the idle circuit and it influences performance up to 1/4 throttle position.;)
Click to expand...
49er is correct.
A smaller air jet ENRICHES the pilot circuit,
A larger air jet LEANS the circuit.

Just to be sure, I doublechecked one of my Mikuni manuals. This is for the HS40 carb (a very advanced high-performance smoothbore, not a CV carb), but this pilot circuit operation circuit is identical to the CV carbs on our old GS's. (emphasis added)
IDLE CIRCUIT (PILOT SYSTEM)
The idle circuit supplies fuel at idle speeds and has
a major influence on fuel flow up to 1/4 throttle. There are
three tunable parts in the idle circuit:

1) PILOT JET --- controls maximum fuel flow through the
idle circuit.

2) PILOT AIR JET--- controls the maximum amount of fuel
that will flow through the pilot jet by allowing a higher
(smaller air jet) or lower (large air jet) vacuum signal at the
pilot jet.

3) PILOT SCREW --- controls how much fuel is allowed to
enter the carburetor venturi.

The pilot screw is used to control idle mixture. Turn
the screw out to richen the idle mixture. Turn it in to lean
the mixture. The engine should have a smooth, steady idle
with the screw between 1/4 and 3-1/2 turns out from fully
bottomed (gently!). If the engine requires more than three
turns out, the pilot or pilot air jet may be too lean. If it
requires less than 1/4 turn, it may be too rich.

As the throttle is opened the pilot screw’s position
becomes less important than the sizes of the pilot and pilot
air jets. A larger pilot jet richens the mixture from just off-idle
to 1/4 throttle. A smaller one leans it. A change in pilot air jet
has the reverse effect. A larger pilot air jet leans the mixture
and a smaller one richens it. The pilot jet and pilot air jet
have slightly different effects on mixture strengths. These
effects are discussed in the “General Tuning Procedure”
portion of this manual.

The idle circuit can be adjusted by changing either
the pilot or the pilot air jet. A one-size larger pilot jet will
have nearly the same effect as a one-size smaller pilot air
jet. It is generally easier to change the air jet since it is
more accessible.

After changing either jet, it is necessary to re-adjust
the pilot screw for best idle.
Click to expand...
 
Last edited:
So that explains why the carbs on my GSX1100G have a 210 air jet and a tiny little 32.5 pilot jet yet my GS1000's have a 160 air jet and a rich 40 pilot jet.
If I increase the pilot jet size in the GSX1100G to a 35, your eyes will hurt as it is running incredibly RICH.
And the same thing happens when changing the air jet on the 1000's to a 180.

:confused:


Eric
 
7981GS said:
So that explains why the carbs on my GSX1100G have a 210 air jet and a tiny little 32.5 pilot jet yet my GS1000's have a 160 air jet and a rich 40 pilot jet.
If I increase the pilot jet size in the GSX1100G to a 35, your eyes will hurt as it is running incredibly RICH.
And the same thing happens when changing the air jet on the 1000's to a 180.

:confused:


Eric
Click to expand...
You raise an interesting question, Eric. There is a big table on Bass Cliff's website listing all the jets and other carb settings for the GS series bikes.
http://members.dslextreme.com/users/bikecliff/images/carbspec_float_height.html

Lets see if I can figure this out.... Ok, I made a table below. The jet specs for your GS1000 appear to match right up, but the jets for your GSX1100 do not. It appears your GSX1100 has a pilot jet significantly larger than stock (making the pilot circuit richer). Unfortunately, they don't list the air jet. The specs for the GS850 kind of match your GSX1100 with a 32.5/200 combo. So my guess is you have a rich pilot jet and a lean air jet to balance it out, like the GS850. Since you don't tell me anything else about your bike's setup (pipes,air filters, and possible non-stock engine goodies), it's hard to understand why it's set up the way it is. If she runs, and runs well, that's all that matters!
 
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