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| | | Joined: Nov 2010 Posts: 56 'Bolter | | 'Bolter Joined: Nov 2010 Posts: 56 | I have a question about Weber Carb Vacuum ports. My set us a 56 Chevy with an I6 250 motor. It has new offenhauser intake and 2 Weber 32 DFT carbs that I bought from Tom Langdon. Each carb has 3 vacuum ports on the side. I don't know where each gets connected. Any information, diagrams or pictures on how to connect these would greatly be appreciated. Thanks to all for the help. | | | | | Joined: Mar 2014 Posts: 4,482 Moderator, Electrical Bay | | Moderator, Electrical Bay Joined: Mar 2014 Posts: 4,482 | Well...as I recall some 250 engines used ported vacuum and some used manifold vacuum for the distributor advance. And I don't know which year/model of 250 was which, but I do seem to remember ported vacuum was used on the ones which had smog pumps. Do you know the year of your engine? Has the distributor been changed? That would help here. In your carburetor, I believe you will find one of those vacuum ports will be at or slightly above where the butterfly rests at idle. That will be ported vacuum. And you will find one where the vacuum connection is below the butterfly (and this will be manifold vacuum).
The difference is important and it all has to do with how your distributor was designed. If you connect manifold vacuum to a distributor designed for ported vacuum, you get full advance at idle and then lowering advance (lowering vacuum) as you step on the gas. Your distributor probably wanted to see the opposite. If you have a distributor that is looking for manifold vacuum, you have higher vacuum at idle and then as you drive, your rpms increase and the centrifugal advance (weights) in your distributor sling outward and more or less take up the slack and increase the advance for you. So you have to match the two. As for the third connection, I don't know. Didn't Tom send you any info with the carburetors?
~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
| | | | | Joined: Feb 2000 Posts: 5,201 'Bolter | | 'Bolter Joined: Feb 2000 Posts: 5,201 | Hook up which ever port makes it idle the best. Ported or manifold vacuum act just the same once off idle, so it doesn't matter all that much. Manifold vacuum will have the port below the throttle shaft, ported vacuum will be above the throttle shaft. Vacuum advance add timing on top of what the mechanical adds. The design of the distributor is exactly the same no matter what vacuum source is used. Read this, https://www.crankshaftcoalition.com/wiki/images/e/e4/Vacuum_Advance_Specs.pdf | | |
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