Update for anyone interested...
The Chinese BBD clone was running rich at idle (even with screws all the way in) and going very lean with the throttle open. I spent a lot of time adjusting the stepup piston, trying to get the fuel/air mixture rich enough at high speed/load while also lean enough at low speed/idle. I never could get it right. Finally I bought a remanufactured BBD labeled for 1978-1979 Slant use. It runs like a dream. I can adjust the idle mixture to 13 or 14 AFR, and then at WOT it runs around 13. I got the reman carb from United Automotive Remanfacturing.
