Ok Here here's what I have. 1977 f150 factory 460 c6 with a new holley 600 cfm square bore and edlebrock performer intake, new gaskets from the exhaust manifold up including head gaskets, otherwise factory stock with 128k miles. Theres a "before" picture of it here somewhere.

The Plugs, cap, rotor and wires have been replaced. I have replaced all the vacuum hoses. Timing is set at 12 deg BTC, idle is 700 rpm, air fuel mixture is set at 1 1/2 turns from bottom. The vacuum gauge reads a steady 20.5 in.-hg. @ 750 ft. El. and the barametric is near 30 in.hg.
Vacuum gauges bounces less than 1 in-hg when the modulator hose is disconnected and plugged This is good, I think. When it is connected however, it rocks the truck fairly hard and the needle bounces almost 2 in.-hg.

Obviously it has a vacuum leak right? Again, I replaced the hoses and later replaced modulator valve. Inccidently, that modulator was strangly American made, odd nowadays.

Alright back to problem. Why is this thang leaking? Do those numbers sound ok? the Idle seems High to me.
Any help is a pre she ated.

 

I just installed a Edelbrock intake and 750 cfm carb on my 460. It sounds like your Idle Mixture may be too lean. I would open them up at least a half of a turn rich, maybe a full turn to see if that fixes the problem. If so, follow the calibration instructions that came with the carb. I had mine tuned too lean and it had a large flat spot when accelerating. Edelbrock tech. told me to open the Idle Mixture Screws until the engines RPM's stopped rising, then go leaner enough to get a 20 RPM drop in speed. Then do the same to the other IMS. He also told me to adjust my timing from 10 degrees to about 14 - 16 degrees. Then reset your desired idle RPM.