670 Street Avenger and Flat Cam Lobe

Fisher said

Start by replacing the power valves and then play with the idle mixture screws. They are supposed to have power valve blow out protection, but you never know. A 670 Is a pretty large carb for a 318 even with a big cam. I have one on my 355 sbc that makes 350 hp, and i feel its a bit too big. It does work better now that i swapped the gears from 3.08 to 3.55 that made a world of difference, and i had to re-jet and tune that carb again. I found i was too lean.

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The 670 SA's have the same venturi and throttle bore sizes as a 600 1850. Hardly too big for a 318. The problem is they're designed for street cars with stock-ish type builds- not quite the application the OP has. It'll work with some serious tuning, but the right carb for what he's got IMO. I'd put a 650 DP on the thing and call it good.

A blown power valve shouldn't affect the idle because they pull fuel from the main circuit.

OP- Do the regular checks- float levels, vacuum leaks, see if the boosters are dripping at idle, etc. and see what you find.