Vacuum Leak: How to find it?

[Keith W.]

Recently, I had my carb repaired for leaks. It came back to me with a new gasket setup going from 1 to 3 gaskets between the carb and intake manifold. Ever since putting the carb on the car hasn't run correctly. (Rough idle, hesitates) I have adjusted the air/fuel mixture a few times hoping that was the issue. Floats are at the correct level. I am only getting 5 -6 in-Hg on the vacuum gauge where it should be around 15-18. It has been suggested to look for a hose that may be disconnected that is causing the low readings. Apparently, I have a vacuum leak somewhere. Any suggestions on how to find it? I looked for disconnects but didn't find any. In researching how to find vacuum leaks there are options that include pumping smoke thru the system, or using carb cleaner spray, or propane. Has anyone tried these or have different searching methods?

[Frank D.]

Re: Vacuum Leak: How to find it?

An unlit propnae hand torch passed around all gaskets and rubber lines is one of my favorites; once you hear the engine rev up you've found your leak - it's called propnae enrhichment. There are several Youtube videos on the technique. Not a fan of various carb cleaners or starting fluid as they are hard on rubber parts.

[Tom M.]

Re: Vacuum Leak: How to find it?

Download a vacuum system diagram. On a ‘69 there is one vacuum line on the intake manifold that powers lights, wiper door, venting etc. finding and plugging that line takes all of the accessories out of the question and you are left with the carb and distributor.
If the carb and dist are good, borrow or buy a vacuum pump so the engine can stay off for troubleshooting. Listen around for the leaks.
Your mileage may vary depending upon the year of car but you get the idea - isolate systems as much as possible and troubleshoot one at a time. I did this last winter and found a bunch of connection leaks and two vacuum switch failures. The fixes got lights and wipers working. Your gauge reading shows a big leak so it will probably show itself quickly.

[Patrick H.]

Re: Vacuum Leak: How to find it?

Which engine and carburetor?

You went from 1 gasket to now 3 between the carb and the intake?

[Brian M.]

Re: Vacuum Leak: How to find it?

Vaccum.html

Hope this helps.

[John F.]

Re: Vacuum Leak: How to find it?

Have a can of WD40? Insert red straw. Start engine. Shoot a little around base of carb. If there is a leak, the engine rpms will increase. That is what we do with our 90 year-old Model-A.

[Brian M.]

Re: Vacuum Leak: How to find it?

I misread what the OP requested and posted the wrong information. Sorry about that. But if one needs information on the Vacuum system, there it is. :-)

[Robert P.]

Re: Vacuum Leak: How to find it?

If you are friendly with your auto repair guy a smoke machine is your best friend when it comes to finding vacuum leaks , and some of the machines are portable.
Bob

[Keith W.]

Re: Vacuum Leak: How to find it?

'70 LT-1; Holley 4bb
Where do I download the vacuum system diagram? I thought I found one but it doesn't look right.
My headlights and wiper door are slow and the wiper door doesn't go up all the way on one side. Just doused it WD-40 to clean up the pivot points. I think I need to replace the inner gasket of the headlight actuators and the wiper door control.
Thanks gents for all of the advice.

[Patrick H.]

Re: Vacuum Leak: How to find it?

Clarification on number of gaskets?
Picture of your setup?

[Keith W.]

Re: Vacuum Leak: How to find it?

In order from the intake to carb is 1 black thin gasket, then 1 thicker (cork like) gasket, then another black thin gasket. I will add a picture of how it looks in a minute.

Gaskets.jpg

[Bill B.]

Re: Vacuum Leak: How to find it?

Keith,

You may want to check your PCV valve and hose for proper ops. That much vac loss almost sounds like a PCV valve that is free-flowing (stuck in an intermediate position) Been there, done that!

[David M.]

Re: Vacuum Leak: How to find it?

Looks like you built your own spacer using just a stack of standard throttle base gaskets? If yes there's your problem.
You cant simply stack standard base gaskets. Not enough mechanical support. The gaskets will get sucked in from engine vacuum and you run the risk of warping the carb as the stack does not provide proper support to torque the carb to spec.

A simple test would be to remove all but one gasket and see how the car runs.

Here is an example of a proper spacer for a Holley:
https://www.holley.com/products/fuel...ers/parts/3406

[Chris H.]

[Keith W.]

Re: Vacuum Leak: How to find it?

Bil,

I just checked the PCV valve and it appears to be fine as well as the hose. I blew air thru the hose without any obstructions and shot the pcv valve with some carb cleaner. The valve rattles and moves I believe as it should.
Thanks,
Keith