Mixture adjustments

[J-T]

Many riders and wrenchers have asked about mixture adjustments. Many have misunderstood the purpose and function of the mixture adjustments. Screws located on the engine side of the carb (between the engine and the throttle plate) are generally fuel screws. Turning them in (clockwise) makes the mixture leaner, out makes the mixture richer. Screws located near the carb inlet/choke plates are generally air screws. Turning them in (clockwise) makes the mixture richer, out makes them leaner. While there are exceptions, as a general rule, the adjustment screws we find on our carburetors are used to adjust the mixture at idle and their effect diminishes rapidly as the throttle is opened and rpm increases. In pretty much all cases the settings specified in the service manuals are a place to start, nothing more. Proper adjustment requires that all the other stuff is correct. Valves properly adjusted, plugs are good,points adjusted (if you’ve got them), timing correct (if adjustable), no air leaks, carbs are clean and float levels correct,carbs are at least bench sync’d.

Start by going for a ride long enough to get the engine stabilized at operating temperature. Park on the center stand and direct a fan at the engine to provide cooling air. Adjust the idle no higher than the middle of the specified idle rpm range. Adjust one mixture screw to where the engine runs at the highest rpm. If the rpm exceeds the maximum specified idle rpm, re-set the idle and start over. Repeat the procedure on the opposite carb.


Some folks do find that issues like hesitation coming off idle can be improved by setting the mixtures a bit on the rich side. Nothing wrong with that, if it works for you, do it. Problems like running rich or lean at riding speeds are better addressed by correcting whatever is actually causing the problem.


If the carbs have not already been dynamically sync’d this is the time to do it, but that’s for a different page.