runs great til attach vac advance

[arter]

Greetings. Running great with no vacuum advance connected, hook up vac advance it sputters, backfires, and wants to quit. 401 .30 over, RV cam, Performer intake, 770cfm Truck Avenger, electric fuel pump @ 5 psi, TFI upgrade w/ new plugs, wires, rotor, cap, 3" exhaust. Steady 21" manifold vac at idle. Static timing 10 BTDC. Vac advance holds vac, tang stamped A12 for 12 deg total advance, adjusts to a stop if turn clockwise, will turn 18-20 times counterclockwise then sounds like end of spring scraping on side. Running ported vac. Fine at idle (no vac) but problem starts when revved (and vac created). Been trying to ballpark but suspect vac unit is too sensitive, any idea how many ccw turns from stop it should be? Other factors? Thanks.

[carrotman]

Maybe the vac advance is set up for manifold vacuum, which decreases as you press on the gas, but is high at cruise. Put it on manifold vacuum, and see how it runs. May have to decrease initial timing though.