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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?
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?
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 shop did some basic timing and tuning before the car left the shop and then my mechanic did some additional tuning to the carb since it kept stalling when idling in gear. The carb is a 750 I belive but I will check the build sheet for exact specs of everything. Exhaust is obx long tubes with side pipes, straight through. Car is auto, the tag says 4.11 gears but I don't know for sure, nor did the previous owner. FWIW the car has a big block hood and the high rpm redline tach.
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.
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
Last edited by Rescue Rogers; 06-16-2019 at 03:53 PM.
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
The car is going back to the shop for a few minor things and some fender trimming. Mechanic already confirmed that he needs to fine tune the timing and advance. Hopefully it runs like normal after that. The cam that was put in does not make much vac so the vac line from the intake was disconnected.
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-.
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; 06-19-2019 at 02:11 PM.
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
you mean a diagram of the vacuum ports on the carb or the distributor? I do see the zip ties that go from the can on the distributor to the port on the front of the carb. But I don't have any knowledge of how theyre supposed to be hooked up.
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
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
Okay thanks. I will go look at the carb again when I get home and find the model % on it.
06-13-2019, 10:31 AM
Disconnecting vac a big no no?
