Vacuum connection question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Toddmac · 06-01-2008, 07:22 AM

Hi Gang, Hopefully someone here has the same combo or close enough to tell me what the heck I need to do.
I pulled the engine from my 67..it has a 77 305, Edelbrock Performer and now a Holley 600 vac secondary carb. We rebuilt the engine and added some goodies but the carb has a vacuum port on the front metering block sticking out the right side....anyone know what I can use this for? I still need to hook up the vacuum advance for the dist. and I'm not sure where to go with it so I wondered about this port. The carb didn't come with the motor,it's a good ol Craig's list special ya know.
I have looked all over Holley's site and can't find it, even in the pdf files for the carb....ugg.
I would be glad to cap it, if someone knows the best place on a Holley to get vacuum for the distro. It has a decent little cam now, but I don't think vacuum will be an issue. Hopefully she fires up tomorrow for the first time in months.
Thanks for any help!!
Todd

**El Jay** · 06-01-2008, 07:34 AM

Personally, I'd cap the vacuum on the carburetor (it's a smog deal, that really didn't help much) and pull the distributor vacuum right off the intake manifold, or the carburetor below the throttle plate(s).

copywritten · 06-01-2008, 07:36 AM

I have a Holly Street Avenger 670 CFM secondary advance. I hooked my trans modulator to one of the side ports and hooked up the HEI to full manifold vacuum which on my carb is the one in the front passenger side of the carb. My cam I guess is a race cam because even with hooking these up to two different vacuum ports my secondaries barley open up.

Toddmac · 06-01-2008, 07:45 AM

Thanks for the info guys! I will cap off the one on the metering block-really didn't want a line running up there anyway and maybe use the T on the manifold where the trans modulator is connected for now. I am a little worried about the cam but it's not huge....bout as big as I dared for my kid to drive the truck still and be reliable within reason.
Thanks again! I will post some pics of the thing when we get some more, I haven't taken any since we rebuit it and got rid of the grease covered chev blue.

70GMCer · 06-01-2008, 01:47 PM

The one on the side of that block is for your vacuum advance for the dizzy. There should be a small vacuum port located in front, under the fuel bowl that is for constant vacuum, like for using a vacuum gage. That's how I have mine set up anyway.

LONGHAIR · 06-01-2008, 04:04 PM

Quote:

The one on the side of that block is for your vacuum advance for the dizzy. There should be a small vacuum port located in front, under the fuel bowl that is for constant vacuum, like for using a vacuum gage. That's how I have mine set up anyway.

You will be surprized at how much better it would be if you hooked it up to full vacuum. You will probably have to back the idle down though.

Unless you have one of the "Factory" HEI distributors from the late 70's, everyone should be running full vacuum to the distributor. Those late 70's units have an odd advance curve to them. Simply switching to full vac may cause a pinging problem. Even if you do have one of those, it will be to your benfit to fix this issue. Changing to a better advance curve (or simply swapping to a distributor with a better curve) and running full vacuum to the advance unit will improve drivability and fuel economy.
If you have an aftermarket HEI, this should not be a problem. The advance curve may not be optimum, but it will be set-up for full vacuum use.

That whole "ported vacuum" thing was a "band-aid" in the early days of emissions reduction. There is nothing good about it...even in the cars/trucks that came that way from the factory.

Quote:

My cam I guess is a race cam because even with hooking these up to two different vacuum ports my secondaries barley open up.

This has absolutely nothing to do with opening the secondaries. That is a function of the carburetor itself. Different brands of carburetors go about this in different ways. With a Holley, it is the cannister on the outside of the carburetor body. You need a tuning kit to change springs.

copywritten · 06-01-2008, 05:09 PM

so a different spring kit will help my secondaries open?

LONGHAIR · 06-01-2008, 05:54 PM

Yes, there is a kit with complete tuning instructions just for that. Jegs, Summit, local speed shop, etc.

70GMCer · 06-01-2008, 06:08 PM

Thanks for that info longhair.....I'll give that a try on my engine....

Toddmac · 06-02-2008, 07:17 AM

Thanks for all the info, we fired her up today. I got myself a huge vacuum leak to deal with but other than that no big issues. Now to begin the fine tuning...thing has a hole in the exhaust and the Cam break in sucked having to listen to that for a long time. My son is a happy camper, I decided to give him the duty of starting it for the first time in months and the first time with the rebuild. It was awesome watchin him grinning from ear to ear but nervous as a cat waiting for something to go wrong.

06-01-2008, 07:22 AM	#1
Toddmac Registered User Join Date: Dec 2007 Location: Independence MO Posts: 19	Vacuum connection question Hi Gang, Hopefully someone here has the same combo or close enough to tell me what the heck I need to do. I pulled the engine from my 67..it has a 77 305, Edelbrock Performer and now a Holley 600 vac secondary carb. We rebuilt the engine and added some goodies but the carb has a vacuum port on the front metering block sticking out the right side....anyone know what I can use this for? I still need to hook up the vacuum advance for the dist. and I'm not sure where to go with it so I wondered about this port. The carb didn't come with the motor,it's a good ol Craig's list special ya know. I have looked all over Holley's site and can't find it, even in the pdf files for the carb....ugg. I would be glad to cap it, if someone knows the best place on a Holley to get vacuum for the distro. It has a decent little cam now, but I don't think vacuum will be an issue. Hopefully she fires up tomorrow for the first time in months. Thanks for any help!! Todd

06-01-2008, 07:34 AM	#2
El Jay Gone to greener pastures Join Date: May 2007 Location: Gateway to the Delta Posts: 7,354	Re: Vacuum connection question Personally, I'd cap the vacuum on the carburetor (it's a smog deal, that really didn't help much) and pull the distributor vacuum right off the intake manifold, or the carburetor below the throttle plate(s). __________________ '69 Chevy 1/2 T LWB Stepper: Daily Beater '72 GMC 3/4 T Fleet: Another Daily Beater '72 Plymouth Gran Coupe: ? "Ah women. They make the highs higher and the lows more frequent." Friedrich Nietzsche "Never kick a fresh turd on a hot day." Harry S. Truman GUN CONTROL: Never having to say, "I missed you." Always fire two warning shots into your attacker's chest area before putting a bullet between his eyes. Paraphrased from Louis Awerbuck

06-01-2008, 07:36 AM	#3
copywritten CopyWritten Join Date: Dec 2007 Location: Cypress TX Posts: 173	Re: Vacuum connection question I have a Holly Street Avenger 670 CFM secondary advance. I hooked my trans modulator to one of the side ports and hooked up the HEI to full manifold vacuum which on my carb is the one in the front passenger side of the carb. My cam I guess is a race cam because even with hooking these up to two different vacuum ports my secondaries barley open up.

06-01-2008, 07:45 AM	#4
Toddmac Registered User Join Date: Dec 2007 Location: Independence MO Posts: 19	Re: Vacuum connection question Thanks for the info guys! I will cap off the one on the metering block-really didn't want a line running up there anyway and maybe use the T on the manifold where the trans modulator is connected for now. I am a little worried about the cam but it's not huge....bout as big as I dared for my kid to drive the truck still and be reliable within reason. Thanks again! I will post some pics of the thing when we get some more, I haven't taken any since we rebuit it and got rid of the grease covered chev blue.

06-01-2008, 01:47 PM	#5
70GMCer Senior Member Join Date: Apr 2005 Location: Joppa, Maryland Posts: 4,408	Re: Vacuum connection question The one on the side of that block is for your vacuum advance for the dizzy. There should be a small vacuum port located in front, under the fuel bowl that is for constant vacuum, like for using a vacuum gage. That's how I have mine set up anyway. __________________ 1997 GMC Sierra Ext 1965 Mustang

06-01-2008, 05:09 PM	#7
copywritten CopyWritten Join Date: Dec 2007 Location: Cypress TX Posts: 173	Re: Vacuum connection question so a different spring kit will help my secondaries open? Last edited by copywritten; 06-01-2008 at 05:12 PM.

06-01-2008, 05:54 PM	#8
LONGHAIR just can't cover up my redneck Join Date: May 2004 Location: Columbus OH Posts: 11,414	Re: Vacuum connection question Yes, there is a kit with complete tuning instructions just for that. Jegs, Summit, local speed shop, etc.

06-01-2008, 06:08 PM	#9
70GMCer Senior Member Join Date: Apr 2005 Location: Joppa, Maryland Posts: 4,408	Re: Vacuum connection question Thanks for that info longhair.....I'll give that a try on my engine.... __________________ 1997 GMC Sierra Ext 1965 Mustang

06-02-2008, 07:17 AM	#10
Toddmac Registered User Join Date: Dec 2007 Location: Independence MO Posts: 19	Re: Vacuum connection question Thanks for all the info, we fired her up today. I got myself a huge vacuum leak to deal with but other than that no big issues. Now to begin the fine tuning...thing has a hole in the exhaust and the Cam break in sucked having to listen to that for a long time. My son is a happy camper, I decided to give him the duty of starting it for the first time in months and the first time with the rebuild. It was awesome watchin him grinning from ear to ear but nervous as a cat waiting for something to go wrong.



Register or Log In To remove these advertisements.

Register or Log In To remove these advertisements.