Vacuum Questions... - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Christiansok · 10-25-2010, 10:43 AM

Last night my Dad was showing me a little about timing and we ran into some weird (maybe not weird?) vacuum issues. I disconnected the vacuum line to the vacuum advance and plugged it while he was showing me the timing light and stuff. But with the truck idling we unplugged the vacuum line and it made no difference in how it ran/idled vs having it hooked up. My dad thought this was strange (idk maybe this is normal?) so I put my finger on the vaccum line, no suction at idle. Took the line off and cleaned a little bit of crud out of the end that was connected to the carb. Putting my finger where it was connected to the carb at idle, still no suction. I was wondering if this is normal or if my vacuum lines are hooked up wrong or perhaps something worse.

There are 4 spots on the front of my carb (4 barrel) where vacuum lines are connected. The lowest one near the base goes to the vacuum advance. Another line comes off and is plugged (assume it went to charcoal canister before). Then there is another line that comes out and hooks right back up to the carb about an inch away. Sorry I don't have any more specifics being at work but I can check stuff out when I get home later. Thanks.

rbagley · 10-25-2010, 10:56 AM

I think the plugged port would have vacuum if you pulled the plug off. I'm running mine on the no vacuum at idle port because it was knocking a bunch on the other one.

**Zeke's Garage** · 10-25-2010, 10:58 AM

Sounds like you've just connected it to a vacuum port located above the throttle plates. If you open the throttle just a bit, you should get vacuum on that port.

Most folks will state that you need to have the distributor connected to a ported- or always on vacuum port (Located below the throttle plates) for best results.

ChevLoRay · 10-25-2010, 10:58 AM

A pic of the carburetor would help someone to pinpoint where the vacuum needs to be connected.

geezer#99 · 10-25-2010, 10:59 AM

It's called ported or manifold vacuum. Ported is taken from above the throttle blades and shows no vacuum until the throttle is opened. Manifold is full time vacuum which is from below the throttle blades.

Christiansok · 10-25-2010, 11:31 AM

I'll get some pictures of the carb on my lunch break and post them to help.

Is there a preferred method (ported/manifold) of vacuum? Assuming switching mine doesn't cause any pinging or other problems like rbagley mentioned. The way it is hooked up now it seems to be running ok.

danleaman · 10-25-2010, 11:46 AM

Ported vacuum for your advance, if you connect your vacuum advance to manifold vacuum, you will have have full advance at idle. Ported is above the throttle plates and only becomes active when the throttle opes.

Dan

Christiansok · 10-25-2010, 12:44 PM

Here are some phone pics of my carb setup

The line on the lower left is the one to the vacuum advance. And you can also see the line that comes out and loops right back to the carb.

10-25-2010, 10:43 AM	#1
Christiansok Registered User Join Date: Aug 2010 Location: Wausau, WI Posts: 69	Vacuum Questions... Last night my Dad was showing me a little about timing and we ran into some weird (maybe not weird?) vacuum issues. I disconnected the vacuum line to the vacuum advance and plugged it while he was showing me the timing light and stuff. But with the truck idling we unplugged the vacuum line and it made no difference in how it ran/idled vs having it hooked up. My dad thought this was strange (idk maybe this is normal?) so I put my finger on the vaccum line, no suction at idle. Took the line off and cleaned a little bit of crud out of the end that was connected to the carb. Putting my finger where it was connected to the carb at idle, still no suction. I was wondering if this is normal or if my vacuum lines are hooked up wrong or perhaps something worse. There are 4 spots on the front of my carb (4 barrel) where vacuum lines are connected. The lowest one near the base goes to the vacuum advance. Another line comes off and is plugged (assume it went to charcoal canister before). Then there is another line that comes out and hooks right back up to the carb about an inch away. Sorry I don't have any more specifics being at work but I can check stuff out when I get home later. Thanks.

10-25-2010, 10:56 AM	#2
rbagley Registered User Join Date: May 2009 Location: Walla Walla, WA Posts: 327	Re: Vacuum Questions... I think the plugged port would have vacuum if you pulled the plug off. I'm running mine on the no vacuum at idle port because it was knocking a bunch on the other one.

10-25-2010, 10:58 AM	#3
Zeke's Garage Senior Member Join Date: Feb 2010 Location: Kennesaw, GA Posts: 288	Re: Vacuum Questions... Sounds like you've just connected it to a vacuum port located above the throttle plates. If you open the throttle just a bit, you should get vacuum on that port. Most folks will state that you need to have the distributor connected to a ported- or always on vacuum port (Located below the throttle plates) for best results.

10-25-2010, 10:58 AM	#4
ChevLoRay Old Skool Club Join Date: Jun 2001 Location: Benton, AR "The Heart of Arkansas" Posts: 10,880	Re: Vacuum Questions... A pic of the carburetor would help someone to pinpoint where the vacuum needs to be connected. __________________ Member Nr. 2770 '96 GMC Sportside; 4.3/SLT - Daily driven....constantly needs washed. '69 C-10 SWB; 350/TH400 - in limbo The older I get, the better I was.

10-25-2010, 10:59 AM	#5
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,734	Re: Vacuum Questions... It's called ported or manifold vacuum. Ported is taken from above the throttle blades and shows no vacuum until the throttle is opened. Manifold is full time vacuum which is from below the throttle blades.

10-25-2010, 11:31 AM	#6
Christiansok Registered User Join Date: Aug 2010 Location: Wausau, WI Posts: 69	Re: Vacuum Questions... I'll get some pictures of the carb on my lunch break and post them to help. Is there a preferred method (ported/manifold) of vacuum? Assuming switching mine doesn't cause any pinging or other problems like rbagley mentioned. The way it is hooked up now it seems to be running ok.

10-25-2010, 11:46 AM	#7
danleaman Registered User Join Date: Jul 2010 Location: Las Vegas, Nevada Posts: 43	Re: Vacuum Questions... Ported vacuum for your advance, if you connect your vacuum advance to manifold vacuum, you will have have full advance at idle. Ported is above the throttle plates and only becomes active when the throttle opes. Dan Last edited by danleaman; 10-25-2010 at 11:48 AM.

10-25-2010, 12:44 PM	#8
Christiansok Registered User Join Date: Aug 2010 Location: Wausau, WI Posts: 69	Re: Vacuum Questions... Here are some phone pics of my carb setup The line on the lower left is the one to the vacuum advance. And you can also see the line that comes out and loops right back to the carb.



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