Posted by Alan Bowes on May 11, 1998 at 23:49:08:
In Reply to: Re: Carburetor Confusion - Carb/Governor Articles Now Online posted by Alan Bowes on May 11, 1998 at 23:14:17:
Forgot to mention, the screw you were talking about is the "main vent tube," otherwise known as an air emulsion tube, air bleed tube, etc. etc. It is designed to allow a metered amount of air to be drawn into the fuel flow going to the discharge nozzle in the venturi. Besides making a bubbly froth of air and fuel for better atomization, the air bleed also is essential to controlling air-fuel ratio, and it allows some mixture compensation to take place when under different venturi vacuum levels, as the fuel level in the vent tube well changes. Make sure you use a fiber washer and tighten this tube snugly (but don't strip it). There are something like five air discharge holes in the side of the thin tube. One of these holes is very tiny. All must be clear. Don't use a wire to clean the holes, as this can damage them and destroy the calibration between the air vent tube and the main and enrichment jets.
By the way, here's a picture of the Carter ETW1 B&B carb: