idle screw does not change carb air flow
 

I was adjusting the carbs for a neighbor.s 914-6 and had 2 problems:

1. Spit back which might be ignition timing, or carb wearing out.
2. Cyl number 2 mixture screw (one with the spring) does not affect the air flow meter set on top of the carb throat at all, all the way in or all the way out does not change a thing.

Spit back I think I can deal with. But the non adjustable air flow stumped me.

What are the causes?

Coming at this from BritBike experience with Amals, so YMMV...

First, ensure the meter is well plugged in (so to speak) and has no issue itself that might be causing the non-reading. For example, swapping with another carb to ensure it is reading at all. Seems like a classic case of plugged air passage preventing air from metering into the idle mix (assuming this is what you are reading/setting).

From here I can tell you of the E-string, the syringe with the carb cleaner, the use of the electrosonic cleaning to remove deposits, etc. Many ways to skin a cat. But I must say a thorough cleansing using a quality injector cleaner (SeaFoam? Techron?) may be a good way to start on this.

No fuel screw on a carb will affect air flow.

What kind of carbs are you working on? If the throttle shaft is one piece per carb, the only way to adjust separate cyl air flow is with air bleeds on the side of the carb (if it has them)

idle jet on that cylinder is clogged ? The idle jet is the smallest orifice in the carb so that's where any dirt or crud will end up. Just pull the idle jet and blow it out.

1. The meter is good and is well plugged in.
2. The carbs are 2 triple webers on the 914-6
3. The same adjustment on the other 5 screws affect the amount of air flow sucked into the carbs, except for number 2
4. Idle jet was pulled and cleaned out with no effects.
5. The air balance screw also has no effects

if the idle is too low, you won't get much response from anything. try it at 1000. the mixture screw should slightly lower, raise and lower the idle as you go from too lean to just right to too rich respectively. so that cylinder may have issues, an intake leak, fouled plug, a plugged idle jet or idle circuit. the balance screws with the 8mm locknut are used to match the airflow as well as possible between cylinders. i start with them closed, balance both carbs close as possible with the speed screw on the ends, with one vertical linkage rod disconnected. then even things out with the balance screws, and then the mixture screws.

common problem - remove all the idle screws and "blast" the ports with NOT MORE THAN 15 psi air pressure

- usually cleans 'em out - if not - rebuild 'em

high psi blast will collapse your floats