Disconnecting vac a big no no?
Thread Starter
Burning Brakes
Joined: Jul 2013
Posts: 963
Likes: 57
From: Vancouver, BC
Disconnecting vac a big no no?
Hey guys,
So I finally got my 68 back from the shop and after test driving it I had a question. My mechanic disconnected the vac system from the headlights and wipers and said that the vac is solely connected to the power brakes (my car has no ac). One of the diaphrams on the headlights is leaking and since I wont be needing headlights or wipers, those two systems were disconnected.
Now what I have noticed is that off the line the car still feels somewhat sluggish and then you sort of feel a surge of power as it gets going. Initially it would stall in gear when coming to a stop or while waiting at a traffic light.
My mechanic readjusted the Holley carb and after that the car didn't stall or turn off at all. It did feel more responsive but still not what I was expecting. I also noticed that if I revved the engine high while driving it would sort of hesitate and sounded like it was kind of dying out.
Does that have anything to do with the vac system or is it just trail and error with the new engine until timing and advance and all that has been sorted out?
So I finally got my 68 back from the shop and after test driving it I had a question. My mechanic disconnected the vac system from the headlights and wipers and said that the vac is solely connected to the power brakes (my car has no ac). One of the diaphrams on the headlights is leaking and since I wont be needing headlights or wipers, those two systems were disconnected.
Now what I have noticed is that off the line the car still feels somewhat sluggish and then you sort of feel a surge of power as it gets going. Initially it would stall in gear when coming to a stop or while waiting at a traffic light.
My mechanic readjusted the Holley carb and after that the car didn't stall or turn off at all. It did feel more responsive but still not what I was expecting. I also noticed that if I revved the engine high while driving it would sort of hesitate and sounded like it was kind of dying out.
Does that have anything to do with the vac system or is it just trail and error with the new engine until timing and advance and all that has been sorted out?
Le Mans Master
Joined: Oct 2000
Posts: 6,362
Likes: 30
From: Oak Harbor WA
You need to know the specs on your cam, the specs on your gear ratio as well.
The drive train has to be thought of as an entire package built/tailored for a purpose. A street car will not be perfect for drag racing, nor will a drag racing car be good for the street.
It could be your cam is designed for high RPM use.
Has the timing been dialed in at all?
What is the size and model of the Holley carb?
What exhaust do you have?
Is the car a manual transmission?
What is the rear gear ratio?
The drive train has to be thought of as an entire package built/tailored for a purpose. A street car will not be perfect for drag racing, nor will a drag racing car be good for the street.
It could be your cam is designed for high RPM use.
Has the timing been dialed in at all?
What is the size and model of the Holley carb?
What exhaust do you have?
Is the car a manual transmission?
What is the rear gear ratio?
Thread Starter
Burning Brakes
Joined: Jul 2013
Posts: 963
Likes: 57
From: Vancouver, BC
Thread Starter
Burning Brakes
Joined: Jul 2013
Posts: 963
Likes: 57
From: Vancouver, BC
You need to know the specs on your cam, the specs on your gear ratio as well.
The drive train has to be thought of as an entire package built/tailored for a purpose. A street car will not be perfect for drag racing, nor will a drag racing car be good for the street.
It could be your cam is designed for high RPM use.
Has the timing been dialed in at all?
What is the size and model of the Holley carb?
What exhaust do you have?
Is the car a manual transmission?
What is the rear gear ratio?
The drive train has to be thought of as an entire package built/tailored for a purpose. A street car will not be perfect for drag racing, nor will a drag racing car be good for the street.
It could be your cam is designed for high RPM use.
Has the timing been dialed in at all?
What is the size and model of the Holley carb?
What exhaust do you have?
Is the car a manual transmission?
What is the rear gear ratio?
Safety Car
Joined: Apr 2006
Posts: 4,057
Likes: 421
From: BC
The first thing I would figure out before playing with the carb is what they set your timing at and how much mechanical advance you have. Not sure about 68 but my 69 has stock initial timing from the factory at 4 degrees. It will hardly run like that so if your guy set it to factory specs.....? Make sure your timing is set correctly before looking at the carb and find out what they set it up at.
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Is my vette stock?? HAHA
Joined: Oct 2015
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Likes: 9,401
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
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It sounds like your timing isnt advanced enough and your not getting any timing from either mechanical or vac. I would verify your vacuum advance is connected. Check your timing and see what it is at idle and if it gets to 36* before 3000 rpm. Email Lars and get his timing papers, he only sends them to Emails and if you search for them onlline you may not get a full copy or an up to date copy.
Basically the intial timing of 4-8* is for emissions standards. Good performance comes about 14* initial and 36* total mechanical advance with the weights and springs) before 3000 RPM and not increasing after that and 48*with vacuum and mechanical. Once you get the 36* before 3000RPM, the initial may not matter if your dizzy is working properly. Yo can buy a kit that has the springs and weights f yo need to change them.
FYI i had a distributer advance system seize years ago and it acted exactly like yo describe, so make sure your timing is changing when you blip the throttle by hand from idle to 1200 or so RPM
Basically the intial timing of 4-8* is for emissions standards. Good performance comes about 14* initial and 36* total mechanical advance with the weights and springs) before 3000 RPM and not increasing after that and 48*with vacuum and mechanical. Once you get the 36* before 3000RPM, the initial may not matter if your dizzy is working properly. Yo can buy a kit that has the springs and weights f yo need to change them.
FYI i had a distributer advance system seize years ago and it acted exactly like yo describe, so make sure your timing is changing when you blip the throttle by hand from idle to 1200 or so RPM
Last edited by Rescue Rogers; Jun 16, 2019 at 03:53 PM.
Thread Starter
Burning Brakes
Joined: Jul 2013
Posts: 963
Likes: 57
From: Vancouver, BC
It sounds like your timing isnt advanced enough and your not getting any timing from either mechanical or vac. I would verify your vacuum advance is connected. Check your timing and see what it is at idle and if it gets to 36* before 3000 rpm. Email Lars and get his timing papers, he only sends them to Emails and if you search for them onlline you may not get a full copy or an up to date copy.
Basically the intial timing of 4-8* is for emissions standards. Good performance comes about 14* initial and 36* total mechanical advance with the weights and springs) before 3000 RPM and not increasing after that and 48*with vacuum and mechanical. Once you get the 36* before 3000RPM, the initial may not matter if your dizzy is working properly. Yo can buy a kit that has the springs and weights f yo need to change them.
FYI i had a distributer advance system seize years ago and it acted exactly like yo describe, so make sure your timing is changing when you blip the throttle by hand from idle to 1200 or so RPM
Basically the intial timing of 4-8* is for emissions standards. Good performance comes about 14* initial and 36* total mechanical advance with the weights and springs) before 3000 RPM and not increasing after that and 48*with vacuum and mechanical. Once you get the 36* before 3000RPM, the initial may not matter if your dizzy is working properly. Yo can buy a kit that has the springs and weights f yo need to change them.
FYI i had a distributer advance system seize years ago and it acted exactly like yo describe, so make sure your timing is changing when you blip the throttle by hand from idle to 1200 or so RPM
Thread Starter
Burning Brakes
Joined: Jul 2013
Posts: 963
Likes: 57
From: Vancouver, BC
Race Director
Joined: Mar 2017
Posts: 11,971
Likes: 4,522
From: Virginia
This is not a job for the mechanic. There is a slight chance they are tuning it correctly, but at least an equal chance that they are reading the 4 degrees BTDC on your emission sticker and calling it a day.
Email Lars for his timing papers (and HEI distributor paper). You can absolutely do this yourself with a digital timing light. Anything you will need to fix will be 10 times cheaper if you DIY.
As examples, I've followed Lars' instructions on two cars:
1979 L48 auto, with stock torque converter: Replaced distributor with ZZ4 takeoff. Set timing to 36 degrees total mechanical advance, added adjustable vac advance, dist shims, and counterweight springs. HUGE difference in performance and responsiveness. With an Innova timing light, dist wrench plugs and wires (unrelated, but a while I was in there item), less than $300-.
1980 L48 4-speed: Confirmed that everything else was correct (22 degrees of mechanical advance, all in at 3000 rpm), so my car had already been recurved. I changed the timing from 10 initial / 32 mech to 14 initial / 36 mech, a change of only 4 degrees, and it made a noticable difference. With new wires, plugs, and over the valve cover/headers wire guides(again, unrelated, but nice), total cost was less than $100-.
Email Lars for his timing papers (and HEI distributor paper). You can absolutely do this yourself with a digital timing light. Anything you will need to fix will be 10 times cheaper if you DIY.
As examples, I've followed Lars' instructions on two cars:
1979 L48 auto, with stock torque converter: Replaced distributor with ZZ4 takeoff. Set timing to 36 degrees total mechanical advance, added adjustable vac advance, dist shims, and counterweight springs. HUGE difference in performance and responsiveness. With an Innova timing light, dist wrench plugs and wires (unrelated, but a while I was in there item), less than $300-.
1980 L48 4-speed: Confirmed that everything else was correct (22 degrees of mechanical advance, all in at 3000 rpm), so my car had already been recurved. I changed the timing from 10 initial / 32 mech to 14 initial / 36 mech, a change of only 4 degrees, and it made a noticable difference. With new wires, plugs, and over the valve cover/headers wire guides(again, unrelated, but nice), total cost was less than $100-.
Last edited by Bikespace; Jun 17, 2019 at 06:48 PM.
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,259
Likes: 9,401
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
It looks like the mech has the pcv line going to the distributor vacuum advance can. That line is too large for where its connected, thats why it has zip ties on it. I would find a diagram of what the vacuum ports are and see where the lines actually go. https://www.bing.com/search?q=holley...ZI&form=MOZLBR
Last edited by Rescue Rogers; Jun 19, 2019 at 02:11 PM.
Thread Starter
Burning Brakes
Joined: Jul 2013
Posts: 963
Likes: 57
From: Vancouver, BC
It looks like the mech has the pcv line going to the distributor vacuum advance can. That line is too large for where its connected, thats why it has zip ties on it. I would find a diagram of what the vacuum ports are and see where the lines actually go. https://www.bing.com/search?q=holley...ZI&form=MOZLBR
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,259
Likes: 9,401
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
the link I added has a bunch of diagrams but I dont know the model number of your carb. Yes you want to figure out what port on your carb is supposed to be connected too
Thread Starter
Burning Brakes
Joined: Jul 2013
Posts: 963
Likes: 57
From: Vancouver, BC