Author Topic: No Vacuum at Carb Ports  (Read 113 times)

Offline Chevynation

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No Vacuum at Carb Ports
« on: April 17, 2021, 08:03:11 AM »
Hello, I had a carb rebuilt (7042220) and I don’t have vacuum at any of the ports. The choke pull off does have vacuum there but I’m trying to get full manifold vacuum for my distributor vacuum advance. So I can’t adjust the idle mixture screws since my gauge doesn’t read anything. I’ve tried seeing if my gauge was malfunctioning but the ports are clearly not sucking when I uncap them. The gasket is on correctly but what seems to be the issue? Could they have been blocked during rebuild?

Offline Cliff Ruggles

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Re: No Vacuum at Carb Ports
« Reply #1 on: April 18, 2021, 03:38:07 AM »
The vacuum port just to the right of the fuel filter housing in the main casting is the distributor port.  It is ported vacuum and will work fine for the vacuum advance.

What is the application and engine specs?  Most engines are fine with a good ported vacuum source designed for the distributor.......Cliff

Offline Chevynation

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Re: No Vacuum at Carb Ports
« Reply #2 on: April 18, 2021, 08:21:34 AM »
The large port in the front for the pcv has vacuum and the choke pull off has vacuum. There is not a large port in the rear for power brakes. I use a fitting on the manifold for power brakes. Numbers 1 and 3 do not have vacuum.

https://cimg9.ibsrv.net/gimg/www.corvetteforum.com-vbulletin/1600x1200/carbs_l1600_83f94708a78c1e0d7a24b60152618a279819e0fe.jpg

Offline Kenth

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Re: No Vacuum at Carb Ports
« Reply #3 on: April 18, 2021, 10:26:31 AM »
#1 port is "timed"/"ported" and as such it will provide vacuum with throttle blades opened some.
#3 cause of no vacuum here is most likely due to incorrect gasket between throtlle plate and float bowl.

Offline Chevynation

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Re: No Vacuum at Carb Ports
« Reply #4 on: April 18, 2021, 11:18:38 AM »
#1 port should be manifold vacuum and #3 should be ported. I don’t have vacuum to either. I do have vacuum at the port for the choke pull off and the pcv.

Offline Kenth

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Re: No Vacuum at Carb Ports
« Reply #5 on: April 18, 2021, 01:16:37 PM »
Sorry for my mixup on port-#´s, but still the ported source should provide vacuum at slightly opened throttle blades IF the throttle plate is correct for the application.

Looking closer to your pic it seem like you have a "frankencarb" put together with parts from several different units.
F.e. Carter © air horn with Rochester Parts RP choke blade never happened from factory, two completely different plants.

Offline Cliff Ruggles

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Re: No Vacuum at Carb Ports
« Reply #6 on: April 19, 2021, 03:04:37 AM »
I see the WRONG gasket used between the main casting and baseplate on those divorced Chevy all the time.  There was a change made in the early 1970's when EGR started showing up so they changed the baseplate some and the gasket is different.  Since "generic" rebuild kits come with a bunch of gaskets not needed for your carb number it's pretty common to see the wrong one used. 

Of course when mixing/matching main castings and baseplates you have to know which gasket to use for the parts you have selected or there will not be any vacuum at the port coming out on the choke side........