Low vacuum diagnostics

[BigBlock]

From BillH on another topic: Well, I have to say I have never been able to pull any more than 16 on the vacuum gauge and the wipers have never worked like I think they should.

16 is not super low, but... the mite should show 17-20 at idle. You can check on either sides at the center fitting on the valve covers.

In my experience, the most common cause of low vacuum is hose or fitting problems. That's the first thing to check.
Second is a carb or intake gasket leak. (Could be a crack in the manifold)
It's kind of messy, but you can spray WD40 around all the gaskets wile the engine is running. If the idle changes, you found a leak...
I've seen some mechanics use propane from an un-lit torch, but I prefer to clean up the mess from the WD40. (The engine probably needed a bath anyway!)

The other common cause is worn piston rings. A compression check will let you know for sure. A wet compression check will help narrow it down to rings/piston or head/valves.

[Bill H.]

Thanks Steve. Carburetor was restored, engine completely overhauled, (new everything), checked timing, valves, all fittings and gaskets were new with rebuild. Now, when I first lit it up it wouldn't run for poop. I found the manifold was cracked under all that insulation. I installed an NOS manifold and it purred like a kitten. That was 15 years ago. You heard it, drove it and it hasn't changed. I suspect my gauge may be bad. It has been bouncing around in my took box for 30 years I have a buddy with a righteous set of gauges. I'll check it with his.

[BigBlock]

Bill, your mite runs great. I wouldn't worry about the vacuum reading unless it drops a lot after you re-route the vent hoses.

[Bill H.]

I rerouted the vent line and my vacuum reading didn't change a bit. Still runs like a champ. Thanks Steve for all the cool information and thanks to Gunny for bringing it up.

[Gunny1]

Well all of my hoses are routed correctly now, with the exception of being cosmeticly(sp?) right. It seems to run fine. I will look for the elbows and tubing to get her looking good.

Thanks for the knowledge guys!