Help with Timing, Vacuum and Backfire - 289c.i.

[agazzo]

FOR Ryan.
Thanks for you collaboration, at the beginning, I also think that it was a vacuum leak problem (I am still in doubt), so I use a clinical stethoscope, after closing all vacuum hoses, to hear if there is a “his” around the carburetor and intake manifold, but there is nothing that hears like that
A vacuum leak (or I can’t recognize it), I also, try by covering the carburetor, and the engines begins to die. I have hear about testing methods that uses carburetor cleaner, and propane (?) to do this leakage test, I tried with the cleaner and noting happens.
Which do you think is a good way to test for leakage?
Many thanks in advance.

[agazzo]

One thing that I forgot to tell. My Mustang is also equiped with an Edelbrock Performer Nitrous Plate Systems, but just plate and solenoids are installed.