I am considering switching to a zenith carb on my 261 chevy motor... the zenith does not have a ported vacumm source like the rochester carb.

the question is ... has anyone switched the vacumm advance source for the distributor from ported to full manifold vacumm? The difference would be that vacumm advance would increase with increasing throttle on the ported source and advance would decrease with increasing throttle on manifold source.Initial advance at the distributor would likely need to be changed based on vacumm source.

I always preferred the full vacumm setup and decreasing advance on V8's in the past but have not played with this on a 6.

has anyone switched and if so what impact did it have on driveability, mileage, etc?


Old Truck addict
1950 chevy 1 ton napco
1956 gmc 1 ton dually napco
1954 chevy 3/4 ton napco
1957 gmc civil defense napco panel