I suspect I have a leak at the rear intake port.
When I short out 1 & 2 cylinders there's a noticeable
change, but when I short 3 & 4 there is virtually no change. I've changed coils, no difference.
The plugs are black, not brown and not wet.
Compression is even in all 4 cylinders, about 40+
Is there something I can spray around that port that would indicate a leak?
Have you tried shorting both 1 & 2 at the same time? If it still runs it is not ignition to 3 & 4 unless for some reason, the timer is off center or the contacts in the timer have moved (anderson) which would change the timing to those two cylinders.
Someone has suggested spraying WD 40 around the intake manifold to see if it makes a difference.
By the way, how is your exhaust manifold? If it sags enough in back it could cause more backpressure in the last two cylinders. Especially if you have the 3 in one gaskets without the glands.
to your question:
you can flow some oil over the intake and hope it will temporarily seal the leak and the car will run better, or you can open a small tip propane torch (not burning) near the suspect intake and see if the engine picks up.
Or carb cleaner with the little straw on it around the intake. It will stall the engine if it gets sucked in, or at least bog it down.
Use A small propane torch not lit pass it over any part you think is leaking on the intake------the engine will speed up if its leaking. Learned that trick from a factory rep used it for years.
We always used carb cleaner sprayed at the suspected mating surfaces.
I always used water spray, if leaking it will stumble. Also spray around the spark plugs as they frequently leak.
If it is a leak at the manifold and you have to replace the gasket, check the manifold for pits where it seals. If it is pitted, file it flat and smooth to remove them or it may leak again.
And use these copper gasket rings...bit more $, but well worth it. Couldn't find where Lang's has 'em, but Snyders does.
Agree with Paul Vitko.... Propane gas...no residue to smoke or catch fire outside engine.
I'll go with David Silver. If you use flammable carburetor cleaner, when you spray the stuff at the point of leakage, the engine speed will increase. Just be sure that you spray so that the carburetor cannot inhale it's vapors.
My symptoms were identical to Dave Loving's, ie,
"When I short out 1 & 2 cylinders there's a noticeable
change, but when I short 3 & 4 there is virtually no change. I've changed coils, no difference."
My suspected intake leak was a bad carburetor-to -intake manifold gasket, verified by an unlit acetelyne torch while idling. A new homemade gasket cured the problem.