air screws?? some q's

What do the do?? they should make fa ine spray/atomize the feul.
When do they start working??
what if they are bigger? do you richen the mixture??
Does the idle adjustscrew have an affect on all throttlepositions??

thanks martijn

The terms air screw and pilot mixture screw often get confused.

A true air screw adjust the amount of air being fed or mixed with a fixed amount of fuel downstream which comes out the small hole in the venturi. Turning this type screw out allows more air into the mix and leans the mixture.

A pilot mixture screw works in a two-hole system. The first hole provides mixture from the air jet and pilot jet in a fixed amount and the pilot mixture screw a secondary mixture to supplement where the mixture (both air and fuel) are added to the primary pilot mixture in the amount determined by how far out you back the screw. The pilot mixture screw tends to richen the pilot mixture when turned out.

For instance, on a BS series carb, there is a hole on the engine side on the bottom of the venturi (in front of the needle jet opening) that has the primary pilot mixture coming out. This hole is joined DIRECTLY and is pretty much directly over the pilot jet. The air jet on intake side venturi edge feeds some air to the pilot jet and gas and air from the pilot jet well come out this hole. The pilot mixture screw, located on the engine side ON TOP is also joined to this circuit and gets both air and gas from the same source BUT uses a sharp, tapered needle to meter gas coming out this hole which is located on the top of the venturi and several inches in front of the primary air/fuel hole. Gas from both pilot sources is atomized and enters the combustion chamber.

To adjust the amount of primary mixture, you can change the air jet size on some BS carbs and can change the pilot jet but there is no non-static way to change the mixture once you have installed a certain air jet and pilot jet. On most Kaw BS carb versions, the air jet is not changeable. On Suz BS versions, it is. The pilot fuel mixture is changeable using the pilot screw for the secondary opening.

It is very important to seal the pilot mixture screw using the correct oring because if air leaks in that location, idle is effected... it will be erratic due to air leaking in.

The pilot circuit works when the throttle is closed mostly as it is supplying ALL the air/fuel mix to the combustion chamber. When you open the throttle and the slide raises, gas and air via the needle jet/jet needle (and main jet) start becoming the primary source for gas and while the pilot circuit is still doing its job; that is, they add gas/air at about the same rate, the percentage of the total air and fuel becomes less significant as the throttle is opened and the other circuits in the carb start to contribute air and fuel. To get a sense of this proportion, note the opening UNDER the slide and the opening on the air jet... this is the amount of air that gets through the carb to the combustion chamber when the slide is shut. Compare that to when the slide is all the way open... MANY times the air... the fuel is about in the same proportion.

Hope this answered your questions... by the way, the idle screw opens the slides a bit and allows more air through which draws more gas as well and thus, increases the rpms... obviously if the slides are open, the throttle position must be altered some (I guess you are talking about the bellcrank which is the thing that pivots on the throttle shaft where the cables are connected). I didn't understand the question about "what if they are bigger"... what if WHAT is bigger? The AIR SCREWS? If they are bigger, they wouldn't fit, would they?

The terms air screw and pilot mixture screw often get confused.

A true air screw adjust the amount of air being fed or mixed with a fixed amount of fuel downstream which comes out the small hole in the venturi. Turning this type screw out allows more air into the mix and leans the mixture.

A pilot mixture screw works in a two-hole system. The first hole provides mixture from the air jet and pilot jet in a fixed amount and the pilot mixture screw a secondary mixture to supplement where the mixture (both air and fuel) are added to the primary pilot mixture in the amount determined by how far out you back the screw. The pilot mixture screw tends to richen the pilot mixture when turned out.

For instance, on a BS series carb, there is a hole on the engine side on the bottom of the venturi (in front of the needle jet opening) that has the primary pilot mixture coming out. This hole is joined DIRECTLY and is pretty much directly over the pilot jet. The air jet on intake side venturi edge feeds some air to the pilot jet and gas and air from the pilot jet well come out this hole. The pilot mixture screw, located on the engine side ON TOP is also joined to this circuit and gets both air and gas from the same source BUT uses a sharp, tapered needle to meter gas coming out this hole which is located on the top of the venturi and several inches in front of the primary air/fuel hole. Gas from both pilot sources is atomized and enters the combustion chamber.

To adjust the amount of primary mixture, you can change the air jet size on some BS carbs and can change the pilot jet but there is no non-static way to change the mixture once you have installed a certain air jet and pilot jet. On most Kaw BS carb versions, the air jet is not changeable. On Suz BS versions, it is. The pilot fuel mixture is changeable using the pilot screw for the secondary opening.

It is very important to seal the pilot mixture screw using the correct oring because if air leaks in that location, idle is effected... it will be erratic due to air leaking in.

The pilot circuit works when the throttle is closed mostly as it is supplying ALL the air/fuel mix to the combustion chamber. When you open the throttle and the slide raises, gas and air via the needle jet/jet needle (and main jet) start becoming the primary source for gas and while the pilot circuit is still doing its job; that is, they add gas/air at about the same rate, the percentage of the total air and fuel becomes less significant as the throttle is opened and the other circuits in the carb start to contribute air and fuel. To get a sense of this proportion, note the opening UNDER the slide and the opening on the air jet... this is the amount of air that gets through the carb to the combustion chamber when the slide is shut. Compare that to when the slide is all the way open... MANY times the air... the fuel is about in the same proportion.

Hope this answered your questions... by the way, the idle screw opens the slides a bit and allows more air through which draws more gas as well and thus, increases the rpms... obviously if the slides are open, the throttle position must be altered some (I guess you are talking about the bellcrank which is the thing that pivots on the throttle shaft where the cables are connected). I didn't understand the question about "what if they are bigger"... what if WHAT is bigger? The AIR SCREWS? If they are bigger, they wouldn't fit, would they?

That a very claer story George!! Thanks a lot. What I meant with bigger air srews are/is the air jet.
I once saw a Dynojet kit with apparently drills to drill the air jets??
What is the result when drilling the air jets?? will the mixture get richer or leaner??
hope you can answer this question.

It's because my 28mm stock vm carbs bogs at 6000 up to 8500 rpm.
It pulls very hard up to 5k, makes then a huge drop and comes back at 9k
It seems to get to much feul...

Martijn

Drill bits in a Dynajet kit are for constant velocity carburetors. They provide a 1/8" drill bit. You first drill out the plug over the mixture screw using this drill bit. They also provide a sheet metal screw to pry the plug out bwhahaha (they really do). The have you fiddle with the mixture screw as they don't provide a pilot jet.

The other use for the 1/8" drill bit is that there is a pressure relief hole in the bottom of the slide. It is about 1.5mm and they have you use the 1/8" drill bit to open it up some. They claim the slide returns quicker; thus giving you better throttle response. My experience with this mod has been that it is pretty much a waste of time.

I have NEVER seen a drill bit for an air screw. I have found that when you have a wide flat spot, it is because airspeed is slowed because the throttle is open and a SMALLER airjet helps. You can't mod the size of airjets on a VM28 anyway so I would look elsewhere to resolve this problem. I would also never drill any jet because it is hard to drill using a TINY drill bit in a precise manner.