The Stovebolt Site Forums The Shop Area Engine Shop vacumm source

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?

Switching from ported vacuum will affect idle settings. As soon as you open the throttle, it will quickly switch to full vacuum, same as a manifold connection. You would not want to change the static distributor setting either way. Do you have a manifold vacuum port on your engine? If so connect the advance to it, and see if you can still set a good idle. (Or a have a friend with good lungs keep sucking on the vacuum hose....or use a pump.)

I now have a 2-barrel Holley AA-1 carburetor on my 261 (over-bored 060"). I think this carburetor was used on GMC 302s. A very similar carburetor was used on many other engines (as a Holley 94, 2100, 2110 ?). I found an adapter to use the AA-1 on the stock intake manifold.

The carburetor was drilled/tapped to provide appropriate vacuum to the vacuum advance. It also has readily available adjustable jets.

This carburetor has improved the engine's performance through the full range of low to high RPM. I am still adjusting the jets to provide the optimal mixture at high RPM.

If you want to use a Zenith, try one off a 270 GMC. One off a 228 or 248 might not be enough for your 261.