Multi Carb Vacuum Issues

multicarbyblock

Q.

I have a 216 in my 37 Chevy with 3 Holley 94 carbs on progressive linkage. The center carb has the vacuum hook up to advance the distributor. The distributor turns with vacuum applied but there is not enough vacuum from carb I tried it by sucking on line and distributor turns. Would it be better to drill and tap spacer under carb?

A.

I have rebuilt my share of Holley 94’s and Stromberg 97’s and can say in all honesty they both have their own merits. If memory serves me right the 94’s do not have any vacuum ports, but perhaps you have a later variation of the 94 that include the AA-1, the 2100 and 2110. These later carbs have a larger venturi and often the size is marked on the side of the carb. This is not a big concern given you are running progressive linkage.

I am not certain I would trust the vacuum port on the Holley 94 as a good source for vacuum and have done precisely as you’ve described, by drilling and tapping a spacer under the main carb. Of course, after I do the main carb, I always feel compelled to buy more spacers so all of my carbs are the same height again, not necessary but I think it looks better. Make sure you plug the vacuum ports on the carb if you go this route.

You can also drill and tap a vacuum port into the intake manifold, although if you have a particularly rare manifold, the prospect of drilling it may give you some pause.

Low vacuum pressure can be an issue on multi-carb setups. Often the culprit is how well the throttle blades fit in the secondary carbs. If your carbs have been rebuilt, you may want to remove them and check the fit. With the carb bases removed from the rest of the carb, hold them up to the light to check the fit between the throttle blades and the bores. The blades do have a little adjustment to them and by slightly loosening the set screws the throttle blades can be moved into their optimal position with very little to no light passing between the bores and blades.

Wrench Safe, Mark

Discussion
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5 Responses to “Multi Carb Vacuum Issues”
  1. JOHN CRUISE

    The vacuum port on 94’s appeared in 1949 and thereafter,8BA’s etc. It is a timed port and only supplies vacuum to the distributor as RPM’s rise. To access vacuum directly off the manifold,would immediately “pull”distributor to full advance at idle and continually until vacuum drops,under acceleration,then advance would drop back. Low vacuum can be a result of a performance camshaft,kind of a trade off,to go to a mechanical advance distributor. Your suggestion about the fit of the butterflies in the secondary carbs is well founded. At idle and until the progressive linkage starts the opening of the secondary carbs,their butterflies should be closed VERY snuggly to prevent unwanted bleeding of air into the system. Just some thoughts. John.

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  2. cHuck Holmes

    Check bushings in the body for the throttle shafts. It is a potential vacuum leak area. I like phynelic material for spacers to reduce heat transfer to carbs.

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  3. John

    Rebuild gasket are made to fit several models.

    Rebuild gaskets are made to fit several carb models. Check the vacuum port. Gasket may be covering port opening.

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  4. Diesel Dan

    Like the others have already said , the vac ports on carbs are really finicky and most arn’t intended to give actual manifold vac, , I just wanted to add two other possible solutions to this problem that I didn’t see listed here that I know from experience do work to cure it ! one is use a aluminum carb spacer that has a vac port tapped into it , this is a win/win too as it helps to keep carbs cooler and gas wont boil off out the vent , but may cause fit issues like hood clearance so my second idea , unless your going for the ” die hard purest” look consider switching over to full electronic ignition, these have an electronic advance and advance at starting automatically and more evenly over the entire rpm range (you have to program it in) , I have saw some really trick set-ups with someone even making a triple throttle body fuel injector on a rat rod , it looked just like the triple carb set-up and the only telltale was the tiny injector wires running down the side of each tower , but that was a total custom job and ive never seen that listed in a parts book (wish I did !!))

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