[C2] Vacuum leak
Drifting
Joined: Aug 2021
Posts: 1,403
Likes: 999
From: Sacramento, Ca
I usually use WD40 and spray it where the intake manifold mates to the head. Also around the base of the carburetor. If it's leaking, you'll see it suck the WD40 in. Also, you should check all of your rubber vacuum hoses. May have one that is broken or cracked.
Ted
Ted
Le Mans Master
Joined: Aug 2005
Posts: 6,545
Likes: 1,977
From: Temple Terrace Florida
The best way to find a leak is with a smoke machine. A good mechanic will have one. You can also use a propane torch around the usual spots. If the idle goes up, you have found the leak. Did the leak come on quickly? Do you have AC, PB, Auto transmission? Disconnect them all and plug the vacuum lines to the manifold fitting. Is it possible some of your cam lobes went bad? Jerry
Team Owner
Joined: May 2006
Posts: 37,671
Likes: 1,115
From: CA
The best way to find a leak is with a smoke machine. A good mechanic will have one. You can also use a propane torch around the usual spots. If the idle goes up, you have found the leak. Did the leak come on quickly? Do you have AC, PB, Auto transmission? Disconnect them all and plug the vacuum lines to the manifold fitting. Is it possible some of your cam lobes went bad? Jerry
Le Mans Master
Joined: Feb 2014
Posts: 8,956
Likes: 5,506
From: Richardson Texas
1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,147
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From: US-PNW
Le Mans Master
Joined: Mar 1999
Posts: 5,401
Likes: 986
From: Itasca IL
I don't know what you are working with, but if it's a base engine car then 17-21" is the range, others lower as cam performance level increases. I just went through this with my 63 L75 that had a L-79 cam added at some point, as well as the heads swapped. Turned out to be very subtle interference with the oil separator in the valley, preventing a seal at the bottom of the intake ports. Was sucking oil vapors at 6,8,5, and 7 and oiling up the plugs. Had only 13" of manifold vacuum. There is no way to diagnose a leak at the bottom of the runners except for disassembly and visual inspection. External (atmosphere-side) leaks are easy to find.
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Michael S. Palmer
1967 Pedal Car Champion
Joined: Mar 2020
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From: US-PNW
Former Vendor
Joined: Aug 2011
Posts: 1,307
Likes: 115
From: Tampa Fl
St. Jude Donor '12
Corvette Tuner in Tampa
To save a lot of time, Id get a smoke machine off Ebay or make one off a Youtube video. Given the age of the car, you may have more than one cracked hose, and the smoker will help you find it.
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Racer
Joined: Mar 2015
Posts: 470
Likes: 156
In the past I've used Carb Cleaner....with a snorkel.... directly along the edges of the intake while idling & monitoring the vacuum gauge looking for a temporary spike. I keep a rag nearby and cleanup is easy. I imagine WD 40 is also an easy wipe clean. But first......check all your intake bolts especially the rear ones.