Model Engine Company of America
This is generally how Model Engine Carburetors work

A model carburetor usually has two mixture controls. High speed and low speed or idle. These mixture controls alter the fuel/air mixture at the various carburetor openings. Usually the needle valves will meter the amount of fuel allowed to mix with the air. A needle valve is a tapered needle that is moved into a hole to provide a very precise adjustment.

Less expensive carburetors have an air bleed for the idle adjustment. This is just that, it bleeds in extra air at idle to obtain the correct mixture. The only issue with air bleed is you usually won't get a good crisp transition to high speed or a good mixture just above idle or mid range.

A very popular design uses two needles, one on each side of the carburetor. The rotating barrel moves in and out as it is rotated. As the barrel is closed to an idle the idle needle moved into the spray bar as shown below. This will then restrict the fuel to set the idle mixture. As you can see the idle needle is tapered and it is going into the end of the spray bar.
It is very important with this design to never bottom out or force the idle needle into the end of the spray bar. If you force the idle needle into the end of the spray bar you can bell mouth or flair the end of the spray bar. If this happens you will never be able to get the proper idle adjustment, mid range or transition. This is why we do not suggest "turns out" for idle needles. Because to find turns out you will usually bottom the idle needle and screw up the spray bar. See How to adjust aero carbs or marine carbs

Shown below is a variation of the same principle as described above. As the barrel closes and moves inward the slot is covered thus restricting the fuel flow at idle. With this design the possibility of ruining the spraybar is eliminated.

Another way of metering the fuel mixture at idle is done without the in and out motion of the barrel but with the actual rotation of the barrel. This is the way the mixture disk works on the K&B carb or a Perry carb. As the barrel is rotated the outer sleeve rotates with the barrel and the inner tube remains stationary as it is attached to the carb body. This type of design requires very little movement to the idle adjustment. The adjustment is made by rotating the inner tube only a few degrees in either direction so the slot is open or closed more at idle. See How to adjust a K&B carb with the brass mixture disk

The above information is provided as a guide. Since MECOA/K&B has no way of determining the ability of the individual using and understanding this information, we assume absolutely NO RESPONSIBILITY for any damage to person or property from the use of this information.

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