Some BB-1 updraft carburetors have a main jet adjustment. This is so the carburetor can be adjust to different engines. Generally this would be a replacement carburetor.
To adjust – 1st adjust the idle mixture. The main jet adjustment is done basically the same way except instead adjusting at idle bring the RPM up to about 1/2 way of maximum RPM. Start out by turning the adjuster out one or more turns unless it loads up, then turn in 1/4 turn, wait a second for the engine to catch up. Do this until the RPM starts to drop, then turn back out 1/4 turn.