1974 and up bbd solid fuel design main metering system. 1973 and back air bled main metering system. Two completely different systems. Theory of operation and part style and numbering systems. In the solid fuel type carburetor circuit timing is affected by the actual level of fuel in the bowl. High fuel level main system activates early, low fuel level main system activates late. In Air bleed carburetors actual operating air fuel ratio is sensitive to the level of fuel in the bowl
