Vacuum Leak Trouble shooting
Thread Starter
Heel & Toe
Joined: Aug 2013
Posts: 17
Likes: 0
From: romoland california
Vacuum Leak Trouble shooting
Hello All,
I am not sure where to start. I have been trying to restore my 1969 corvette 350 engine back to stock replaced the stock air intake manifold and carb. got it running OK.
Got some issues with headlights and windshield wiper door. so I presumed I had a vacuum leak in the lines controlling those.
I put a vacuum gage at the intake manifold, and when idling at about 500-600 rmp am only getting about 5 in Hg moving wildly. Should be getting 18-20 per the book.
I had a leaks between the heads and intake manifold when I had a mechanic put it on. I redid it and dont think it is leaking there (I sprayed with carb cleaner, and did not get a change in engine noise).
compression tested all cylinders and they are all in the 160 range.
any recommended steps in trouble shooting?
any help appreciated.
69 Vet
I am not sure where to start. I have been trying to restore my 1969 corvette 350 engine back to stock replaced the stock air intake manifold and carb. got it running OK.
Got some issues with headlights and windshield wiper door. so I presumed I had a vacuum leak in the lines controlling those.
I put a vacuum gage at the intake manifold, and when idling at about 500-600 rmp am only getting about 5 in Hg moving wildly. Should be getting 18-20 per the book.
I had a leaks between the heads and intake manifold when I had a mechanic put it on. I redid it and dont think it is leaking there (I sprayed with carb cleaner, and did not get a change in engine noise).
compression tested all cylinders and they are all in the 160 range.
any recommended steps in trouble shooting?
any help appreciated.
69 Vet
Last edited by vet69; Oct 28, 2013 at 04:52 PM.
Drifting
Joined: Aug 2005
Posts: 1,443
Likes: 65
From: Wauconda IL
Are you sure you have the gauge hooked to manifold vacuum?
I find it hard to believe you could get that motor to idle at 500-600 RPM with only 5" of vacuum?
What is your timing set at?
Are you running vacuum advance?
What is the timing at idle without vacuum advance (if you are running it)?
I would start with the basics- Something doesn't seem right.
Make sure you have a known, working vacuum gauge and that you are truly hooked up to manifold vacuum.
If you are really running with 5" of vacuum at idle, I'm surprised the engine runs at all.
I find it hard to believe you could get that motor to idle at 500-600 RPM with only 5" of vacuum?
What is your timing set at?
Are you running vacuum advance?
What is the timing at idle without vacuum advance (if you are running it)?
I would start with the basics- Something doesn't seem right.
Make sure you have a known, working vacuum gauge and that you are truly hooked up to manifold vacuum.
If you are really running with 5" of vacuum at idle, I'm surprised the engine runs at all.
Thread Starter
Heel & Toe
Joined: Aug 2013
Posts: 17
Likes: 0
From: romoland california
Are you sure you have the gauge hooked to manifold vacuum?
I find it hard to believe you could get that motor to idle at 500-600 RPM with only 5" of vacuum?
What is your timing set at?
Are you running vacuum advance?
What is the timing at idle without vacuum advance (if you are running it)?
I would start with the basics- Something doesn't seem right.
Make sure you have a known, working vacuum gauge and that you are truly hooked up to manifold vacuum.
If you are really running with 5" of vacuum at idle, I'm surprised the engine runs at all.
I find it hard to believe you could get that motor to idle at 500-600 RPM with only 5" of vacuum?
What is your timing set at?
Are you running vacuum advance?
What is the timing at idle without vacuum advance (if you are running it)?
I would start with the basics- Something doesn't seem right.
Make sure you have a known, working vacuum gauge and that you are truly hooked up to manifold vacuum.
If you are really running with 5" of vacuum at idle, I'm surprised the engine runs at all.
timing is about 12 degrees BTDC. vacuum advance is hooked up. engine and vacuum are better at higher RPM.
here is a youtube video of what the gage is doing at idle
https://www.dropbox.com/s/zyrynszrfk...2014.51.39.mp4
hope this makes more sense but I can clarify if needed.
69 Vet
Melting Slicks
Joined: Dec 2009
Posts: 3,233
Likes: 657
From: Thunder Bay
I spent most of last summer chasing down vacuum leaks so I guess that makes me a bit of an authority. Wish I wasn't.
I started by disconnecting all the vacuum lines and plugging them at the carb. That would eliminate any vacuum leaks in the disconnected lines. Then I could reconnect each line one at a time to determine the source of the leak.
FWIW mine turned out to be a combination of intake manifold, carb base gaskets, and the carb itself.
I started by disconnecting all the vacuum lines and plugging them at the carb. That would eliminate any vacuum leaks in the disconnected lines. Then I could reconnect each line one at a time to determine the source of the leak.
FWIW mine turned out to be a combination of intake manifold, carb base gaskets, and the carb itself.
Drifting
Joined: Oct 2005
Posts: 1,615
Likes: 150
From: Forth Worth TX
To start with your engine should probably be idling around 750rpm. Disconnect the vacuum source for the headlights/wipers from the manifold and plug it. That way if you have a vacuum leak you know it's in the engine.
I can't see your attachment, so not sure what carb you are using.
Some quadrajets have two ports on the front side. The port closest to the driver side is ported vacuum, little to no vacuum at idle. The port closest to the passenger side is manifold vacuum, 12-20 at idle (use this side for the distributor).
With your engine idling at 750, vacuum disconnected from the distributor, you should be able to get 12-20" vacuum.
I can't see your attachment, so not sure what carb you are using.
Some quadrajets have two ports on the front side. The port closest to the driver side is ported vacuum, little to no vacuum at idle. The port closest to the passenger side is manifold vacuum, 12-20 at idle (use this side for the distributor).
With your engine idling at 750, vacuum disconnected from the distributor, you should be able to get 12-20" vacuum.
Last edited by Revi; Oct 28, 2013 at 07:49 PM.
Thread Starter
Heel & Toe
Joined: Aug 2013
Posts: 17
Likes: 0
From: romoland california
To start with your engine should probably be idling around 750rpm. Disconnect the vacuum source for the headlights/wipers from the manifold and plug it. That way if you have a vacuum leak you know it's in the engine.
I can't see your attachment, so not sure what carb you are using.
Some quadrajets have two ports on the front side. The port closest to the driver side is ported vacuum, little to no vacuum at idle. The port closest to the passenger side is manifold vacuum, 12-20 at idle (use this side for the distributor).
With your engine idling at 750, vacuum disconnected from the distributor, you should be able to get 12-20" vacuum.
I can't see your attachment, so not sure what carb you are using.
Some quadrajets have two ports on the front side. The port closest to the driver side is ported vacuum, little to no vacuum at idle. The port closest to the passenger side is manifold vacuum, 12-20 at idle (use this side for the distributor).
With your engine idling at 750, vacuum disconnected from the distributor, you should be able to get 12-20" vacuum.
I have the vacuum system for lights / wiper door disconnected, I used that port on the manifold to hook up the gage (picture). I tested the transmission and break vacuum, and they don't appear to be leaking (they are connected in the picture).
69 vet
