Surging
The original vacuum canister on the car had a hole in the diaphragm but the arm was stamped 9.5, so it would put in 19 crankshaft degrees of timing at full vacuum. I didn't measure the timing advance after the wire modification but figure it is about in line with the original canister now.
This was the problem I had, as my 383 car was surging at highway speeds and light throttle. I disconnected the vacuum advance and the problem went away, so I took the distributor apart and found the replacement vacuum canister I was using was marked 13 (distributor) degrees, so it was putting in 26 (crankshaft) degrees advance at high vacuum. The canister arm is sometimes (usually?) stamped with the degrees of distributor timing it will impose at high vacuum but you need to double that number to get "crankshaft" degrees - mine was stamped 13, thus 26 crankshaft degrees. I wrapped some wire around the canister arm to limit travel and it fixed the surge problem.
The original vacuum canister on the car had a hole in the diaphragm but the arm was stamped 9.5, so it would put in 19 crankshaft degrees of timing at full vacuum. I didn't measure the timing advance after the wire modification but figure it is about in line with the original canister now.
