Make sure you have about 16-22* initial timing on that thing to get the throttle plates closed down.

If you can't kill the engine running the idle mix screws in, the throttle plates are open too far.

My 340 has a 528 cam in it and I've got 22* initial and it idles well with a 13.7 AFR and VERY little stink out the pipes.

In my experience 90-95% of "carb" problems at idle are rooted in the ignition system. Usually, not enough initial timing.

Wanna find out if the PV is the culprit... install a PV plug and tune the idle, then, install the PV and see if it remains the same. I used do that all the time on tough to tune deals. Removes a variable so I can get a clean idle.