Model Engine Company of America
How to adjust a K&B Mixture Disk Carb

First you need to know how the carb works.

Metering the fuel mixture at idle is done with the actual rotation of the barrel. This is the way the mixture disk works on the K&B 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.

As the K&B mixture disk carbs do not have an idle stop screw so you must be sure the barrel is positioned correctly. Both slots must be pointed down into the engine. The barrel must close in the clockwise direction when looking at the throttle arm side.

If you can not obtain a consistent mixture or a rich enough high speed mixture check inside the spray bar. Many times something is plugging up the metering slot that controls the fuel flow for idle and midrange. Symptoms are not being able to obtain idle settings, not getting a rich enough high speed setting, and erratic settings.


If you have trouble with a rich idle and are unable to adjust it properly and you have checked all of the above, The inner tube of the mixture disk (where the slot and hole are in the picture to the left) may have excessive wear with the mating tube inside the barrel. This is a very close and precise fit. If it is loose it will not meter the fuel properly at idle. Kinda like a leaky faucet. At this point the carb should probably be replaced or at minimum the mixture disk and barrel replaced.


A good fit must also be maintained between the barrel and body. If this is worn, you will have air leakage and not be ably to get consistent idle or have the ability to shut the engine down when carb is closed.

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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