Vacuum Leak Diagnosis and Repair

Vacuum Leak Diagnosis and Repair

Finding and Fixing a Suspected Vacuum Leak

One of the niggling problems I’d been having was a loud whistle from the engine compartment.  The noise drove me nuts because, it being high-pitched and all, it was impossible to find it.  It sounded like it was coming from everywhere!  Also it seemed to diminish once the engine was warm.  I put my mechanic’s stethoscope on everything I could think of, initially suspecting it was yet another poorly remanufactured accessory.  No dice.  Eventually I stumbled upon the only place I could hear it…through the throttle linkage on the primary throttle shaft.  Immediately I blamed a vacuum leak at the throttle shaft because the carburetor had been rebuilt in Winnipeg…and you all know how I feel about the Winnipeople.  I was wrong…kindof.  I had a vacuum leak alright, but it wasn’t where I thought it was.

Jeff Ford from Autorestomod happened to see my Facebook post and suggested I take a look at the manifold.  He pointed out, for those of you who don’t know, that the Edelbrock manifold doesn’t mate well with the Autolite carb.  As a regular viewer of his show, I’d already learned that the fix for this is a Boss 302 carb spacer, but it was a good catch.  Jeff strongly suspected my manifold to carb/spacer gaskets were to blame for my vacuum leak.  In his words, “the Felpro stuff is made out of bible paper now.”  And doggone-it!  He was right.  I changed out the gaskets with some thicker ones I had, after I cut them to fit, of course, and the noise was gone!  The sweet taste of victory was mine!

Troubleshooting steps…

Normally I’m a big proponent of testing and confirming which part is at fault before making a repair.  Scientific diagnosis saves money and time.  In this case, though, I just went ahead and replaced the manifold-to-spacer and carb-to-spacer gaskets.  Why?  Because they’re cheap and I have a ton of extras.  Also it’s a really easy repair, the symptoms suggested fault with the existing gaskets, and I didn’t want to go through testing to confirm what I already suspected.  The diagnosis process for finding a vacuum leak can be messy, dangerous, or both.

Each guy will have his own preference about this, but generally the best way to find a vacuum leak is to spray something around the suspected leak area to see if the idle changes.  Some guys use water, some use WD-40, still others use carburetor cleaner.  It’s all about personal preference and how much you value your eyebrows.  I didn’t want to go through the process of spraying solvents or water around my freshly detailed engine compartment, so I skipped the diagnostics.

However, had I thought about it a little more I would’ve used propane ingestion, which is mess-free.  Turn on a propane torch without lighting it and run a stream of propane around the suspected problem area.  Simple and effective, I’ve done it on the Packard to find intake manifold leaks.

In closing…

Whatever your method, though, find your vacuum leak if you suspect you have one.  Even a small leak can screw with the idle and fuel mix.  Mine had to have been monumentally gigantic.  After I had the carb back on and the engine fired up, I blipped the throttle to pull off the choke…and the engine died.  As it turns out, my leak was so big that now the idle was set below 100 RPM.  I had to turn the throttle stop screw several turns to get it so it would even idle at all.   Several more turns, and I had the curb idle back where it’s supposed to be.

From there I also had to readjust the fuel mixture, which was way too rich.  Using my vacuum gauge I was able to get a decently smooth pointer and 18″ of vacuum at 750 RPM.  A little low in my opinion, but good enough for now.  I’m hoping the engine will develop a little more vacuum once the rings seat, or maybe I’ve still got a vacuum leak somewhere.  Time will tell!