On a Barry Grant carb what is the correct way to adjust idle mixture screws.
Idle Mixture Screws
The engine should be at optimum operating temperatures. Lightly seat the idle mixture screws, then back them out
1-1/2 turns to establish a starting point. With the engine running, slowly turn the mixture screws in or out as needed to establish the best idle quality. Do this twice. The first time is a coarse adjustment; the second one is a final fine adjustment. If idle quality can’t be adjusted properly, or if they can’t be backed out far enough to obtain proper adjustment, it may be necessary to have the idle circuits reworked (the cam may not be pulling enough idle vacuum). A common reason for lack of adjustability in the idle mixture screws is having the primary butterflies adjusted too far open.
The secondary throttles should be just slightly cracked open at idle. Even on carburetors with nonadjustable secondary idle mixture, secondary throttle position at idle may be adjusted. It may be necessary to open or close the throttle adjustment due to the cam design. It’s a trial-and-error proposition to find the setting that gives you the best idle. When setting idle speed, the butterfly opening should be adjusted equally both primary and secondary, but not so much as to expose the transfer slot more than .040". Secondary adjustment is set with a screw accessible from the underside of the carburetor.