How much vacuum? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

1972Blaze · 01-12-2016, 04:50 PM

I am getting almost no vacuum at idle where my vacuum advance connects to the carb. I am getting what seems like strong vacuum for the PCV valve and power brake booster. I am getting vacuum at the manifold fitting for the A/C, etc. The truck is a 1972 w 350, auto and original 4 bbl carb, recently rebuilt by a very reputable source. I have not detected any leaks in the vacuum system.

How much vacuum, if any, should I get at idle on the carb for the VAC? How much at the manifold?

If the lack of vacuum is not normal, what are the most likely culprits?

Thanks.

geezer#99 · 01-12-2016, 06:07 PM

That port where you have your vac advance connected is a ported vac source. Shows no vacuum until you open the throttle slightly. It's source is above the throttle plate.
So you should be zero at ported and maybe from 10 to 22 " of vacuum on manifold vac ports. The amount depends on your mods to the motor. Wilder cams and higher compression have less vacuum.
Nothing wrong with your carb. It's all normal.

davepl · 01-12-2016, 06:10 PM

You're all set. There should be NO vacuum at idle on the advance port.

There should be strong vacuum for the PCV and brakes (like 16+ inHg).

The vacuum advance system is for part-throttle loads. It is a measurement of vacuum ABOVE the throttle blades. The only time you get vacuum there at idle is if your throttle blades at idle are set too wide open - usually to crutch something else like a vacuum leak. Then you wind up drilling holes in the throttle blades to let air through while not opening the blades so much.

Once the blades are open far enough to generate vacuum at the ported inlet, it's because the transfer slots are already being uncovered.

So you're golden, at least from what you described.

[Edit: I appear to have posted simultaneously with Mr. Geezer, not copying him! At least we agree though.]

1972Blaze · 01-12-2016, 11:33 PM

Geezer and Dave, good info, thanks. Maybe one of you can help me with one more thing. I am getting too much mechanical advance at low RPM. I think I am supposed to get around 2 degrees at 11-1200 RPM but I am getting 5-6. I assume springs are the issue.

If I need to put new springs in the distributor I would like to get it right the first time. Any advice on what to get or where?

01-12-2016, 04:50 PM	#1
1972Blaze Registered User Join Date: May 2013 Location: Covington, Louisiana Posts: 243	How much vacuum? I am getting almost no vacuum at idle where my vacuum advance connects to the carb. I am getting what seems like strong vacuum for the PCV valve and power brake booster. I am getting vacuum at the manifold fitting for the A/C, etc. The truck is a 1972 w 350, auto and original 4 bbl carb, recently rebuilt by a very reputable source. I have not detected any leaks in the vacuum system. How much vacuum, if any, should I get at idle on the carb for the VAC? How much at the manifold? If the lack of vacuum is not normal, what are the most likely culprits? Thanks. __________________ 1972 K5 Blazer Cheyenne 1966 L72 Corvette Convertible 2016 GMC Yukon Denali

01-12-2016, 06:07 PM	#2
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,512	Re: How much vacuum? That port where you have your vac advance connected is a ported vac source. Shows no vacuum until you open the throttle slightly. It's source is above the throttle plate. So you should be zero at ported and maybe from 10 to 22 " of vacuum on manifold vac ports. The amount depends on your mods to the motor. Wilder cams and higher compression have less vacuum. Nothing wrong with your carb. It's all normal.

01-12-2016, 06:10 PM	#3
davepl Registered User Join Date: Aug 2006 Location: Redmond, WA Posts: 6,332	Re: How much vacuum? You're all set. There should be NO vacuum at idle on the advance port. There should be strong vacuum for the PCV and brakes (like 16+ inHg). The vacuum advance system is for part-throttle loads. It is a measurement of vacuum ABOVE the throttle blades. The only time you get vacuum there at idle is if your throttle blades at idle are set too wide open - usually to crutch something else like a vacuum leak. Then you wind up drilling holes in the throttle blades to let air through while not opening the blades so much. Once the blades are open far enough to generate vacuum at the ported inlet, it's because the transfer slots are already being uncovered. So you're golden, at least from what you described. [Edit: I appear to have posted simultaneously with Mr. Geezer, not copying him! At least we agree though.] __________________ 1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible

01-12-2016, 11:33 PM	#4
1972Blaze Registered User Join Date: May 2013 Location: Covington, Louisiana Posts: 243	Re: How much vacuum? Geezer and Dave, good info, thanks. Maybe one of you can help me with one more thing. I am getting too much mechanical advance at low RPM. I think I am supposed to get around 2 degrees at 11-1200 RPM but I am getting 5-6. I assume springs are the issue. If I need to put new springs in the distributor I would like to get it right the first time. Any advice on what to get or where? __________________ 1972 K5 Blazer Cheyenne 1966 L72 Corvette Convertible 2016 GMC Yukon Denali



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