This afternoon I was helping a friend get his '26 Touring running properly. The car would start and run at a fast idle and seemed to have power when accelerating, But when the throttle was returned to minimum, the engine would die. If you would slowly approach a slow idle, the engine would surge and run rough. I suspected an intake manifold leak, either at the engine or the carb.
What is a good way of determining if there is a leak like this. I seen on youtube methods where you spray carb cleaner on the intake and if the engine speeds up there is a leak, but I'm concerned that with the hot exhaust manifold right there the carb cleaner will ignite and I'll have an even worse problem.
Anyway I removed the intake manifold and found that it was not sealing. After new gaskets etc. and a different carb, the car is running like a champ. The only reason for changing the carb is that I had one that was all set up and I knew it worked.
How do you test for an intake manifold leak?
Bernzomatic torch, UNLIT... the propane will cause a racing condition. Liquid stuff can ignite. Do it from the bottom side.
Wait until the exhaust manifold has cooled down so you can touch it, start the engine and try the spray test before it's getting real hot again
(Usually I test for a leaking intake gasket with starting fluid, but I suppose trooper's propane is safer)
Langs sells an adapter and gauge which can be fit between the carb and intake. It will detect vacuum leaks at the manifold, leaking valves, poor rings, and other maladies.
Yes, Scott's product is the T-Vac! Just don't google that as it brings up some, uh, medical equipment? LOL that you hopefully don't really need. ANYWAY . . . the Model T-Vac is a great diagnostic tool, everyone should have one! I see Lang's carries it now (as they should =) but I bought mine straight from Scott.
I was taught to squirt a small amount of engine oil around the intake manifold where it mates to the block while running. If the engine smooths out then you have found your leak. Since the small amount of oil applied to the manifold has a much higher flash point than solvent, fire danger is virtually non-existent.
Thanks for the help.
I use the unlit propane torch or this is a good place for WD-40, sure smells nice! Ether case, I would check a engine that has been just started, not brought up to temp. first.