@rene4406 posted:It is a brand new carburetor.
I have another one that was running when I ran the engine outside the car, a Holley 4160 600 CFM with vacuum primaries, if the same backfires occur I will blow the membrane and gas supply is different as well as the accelerator cable. I'll see if I can adapt it easily.
Just get it on the engine. Worry about the details later. If it runs with no problems and will idle then the only component defective has to be the new carburetor. If it is new, it has a warranty on it.
Hopefully you can return it locally and not from someone like Summit and need to deal with shipping and Customs again.
In a way, I hope that it is the carb, so that you stop pulling everything else apart.
Again, a generic 3310 is probably what you want. Originally it was a Chevy Z28 carb but is used in many generic forms now.
Rocky's 4609 is actually a version of the 3310. The original application for it was a '69 Ford 428. It was intended to have good street manners and reasonable fuel economy (for a 7L). Rocky it is worth a few dollars stock to the Mustang group. I wouldn't go drilling on it any more if I were you.
I've run that set up before and frankly don't know why you drilled the throttle plates. That's a modification for a "radical cam" to get it to idle the engine on a street driven car?
The Pantera throttle adapter will work on many old and current Holley throttle levers. It is not rocket science to adapt them.
The 3310 is a 780cfm, vacuum secondary. Originally was a manual choke on the Chevy but is easily converted to an automatic one and it is the "old" type of idle screw adjustment. Not a newer type reverse idle screw adjustment.
There is also a Ford version of that carb on the 69-70 Boss 302 so neither one is too big for a 5.7L.
It is not a terrible idea to get everything running on an 1850. It is a little small and will lack upper mid range and high rpm power but it will be very responsive up to about 4500 rpm or so and is on the lean side so it is good for running in the engine and not worrying about the piston rings not sealing because of too much fuel.
I would actually expect George's cam to produce 16-17 inches of vacuum at idle. That was part of the intention of his design.
I get almost 14 with my Webers on an individual runner manifold and a .606" lift, 244 @.050 so 11-12 means there is a leak in the system somewhere OR the ignition system does not have enough initial advance.
The Boss 302 from the factory used 16 degrees initial advance so that is really a factory engineering admission that the Cleveland heads need some unique timing. That initial timing is really a separate factor from the camshaft timing. In my view any performance "Cleveland" head engine should use at least 14 degrees initial.
I hope some of this helps.