Another Vacuum Question
Thread Starter
Instructor
Joined: Jan 2012
Posts: 215
Likes: 6
From: Mebane, NC
Another Vacuum Question
I just purchased a car with an '81 crate motor with a slightly modified cam in '73 c3. It is just reaching the end of break in, went over 1000 miles today. Distribiter , carb and other accessories are stock '73. Hooked up all the tools I have been saving for too many years. Dwell is good, timing was adjusted to 8°btdc, car seems to be running ok. On a whim I hooked up a vacuum guage to the vacuum advance line to the distributer and it read 0. If I goose the throttle a bit I get a reading, but at 800 rpm is doesn't move off the bottom stop. As far as I know it is a stock rear end with a 4 speed. Getting about 13mpg on the first tank. I'm thinking I should be seeing some vacuum at idle. Bad carb/manifold gasket? Vacuum leak from another source? I haven't gotten that far into how things work yet. Tried to get cold air into the car today. Nothing out of the vents, only heat. Is the ventilations system controlled by vacuum? Could a broken/disconnected line behind the instrument panel be the cause? Not sure where to look next.
Thanks for the help. Books have not yet arrived.
Thanks for the help. Books have not yet arrived.
Le Mans Master
Joined: Aug 2006
Posts: 7,417
Likes: 1,554
From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2019 C3 of Year Finalist (appearance mods)
I just purchased a car with an '81 crate motor with a slightly modified cam in '73 c3. It is just reaching the end of break in, went over 1000 miles today. Distribiter , carb and other accessories are stock '73. Hooked up all the tools I have been saving for too many years. Dwell is good, timing was adjusted to 8°btdc, car seems to be running ok. On a whim I hooked up a vacuum guage to the vacuum advance line to the distributer and it read 0. If I goose the throttle a bit I get a reading, but at 800 rpm is doesn't move off the bottom stop. As far as I know it is a stock rear end with a 4 speed. Getting about 13mpg on the first tank. I'm thinking I should be seeing some vacuum at idle. Bad carb/manifold gasket? Vacuum leak from another source? I haven't gotten that far into how things work yet. Tried to get cold air into the car today. Nothing out of the vents, only heat. Is the ventilations system controlled by vacuum? Could a broken/disconnected line behind the instrument panel be the cause? Not sure where to look next.
Thanks for the help. Books have not yet arrived.
Thanks for the help. Books have not yet arrived.
#1Vacuum is produced at the carb end, so you checked for vacuum at the distributor end of the hose right?
#2 There are two types of vacuum, ported vacuum sounds like what you have-no vacuum at idle and vacuum above idle.
Manifold vacuum is produced at idle and steady cruise.
Read BarryK's sticky on timing.
For the heat problem check the source line coming into the cockpit.
Regards,
Pete.
