Checking for vacuum leaks

[benji808]

Hey- I'm looking for step by step direction on how to check for vacuum leaks. I'm chasing a mild rough idle that has persisted through engine rebuild, carb rebuild, and bunch of other work.

I've tried the usual visual inspections, listening, etc.
I've read about people hooking up a gauge to get more scientific about it...how do I do that? Do I need a gauge with a pump (like for brake bleeding? Where do I hook it up?

Thanks in advance!

[fishnjim]

I'd start by putting in all new vacuum hoses.   Don't reuse any hose over about 8 years old.   You'll literally tear your hair out to find, if it's an old cracked hose.   
Use pipe dope, non-teflon type, on any tapered thread joints for the barbs fittings.
You can use "snoop" like you do for pressure leaks, but it'll disappear not bubble. 
You assume, but I don't and aren't sure if the idle issue is vacuum or not.   Disconnect/plug off everything vacuum, like the advance, booster, and manifold/carb, so there's no possible leaks and see if it level out.   That's what you do for tune up to get it timed right, so it may not be in tune.
You can use a brake bleeder pump which has a gauge on hose sections and isolate for leaks too.   You can test the advance that way for a bad diaphragm.  See if the rod moves.  If it doesn't hold steady vacuum, it leaks.

[Alan Harris CLC#1513]

I have used WD-40 or similar to spray along the intake manifold to cylinder head joints and other possible suction areas with the engine running. When you hit a leaky spot, the idle speeds up and smooths out momentarily. This works!

[James Landi]

Alan's approach is simple and some wd on the leak will immediate cause a very brief change in the idle. Spray along the intake manifold...  another issue that many folks miss is the vacuum brake on passenger  side of the carburetor.  It's made of plastic, it has a very short hose, and they usually develop cracks in the old casing, so spray that.  If your car has one of those repurposed V8 juice cans that is used as a vacuum reservior, check that out---spray the bottom and the top... those cans develop cracks there.   Good look, and be careful spraying the WD around a hot engine!  Have a fire extinguisher handy and don't let the engine heat all the way up...    Hope this helps,  James