As I recall you had a 1959 235 out of a passenger vehicle. Here's what I can tell you from GM data for that year/model: Carburetor would have been a Rochester BC, model 7013003 for std transmission and model 7013000 for auto transmission...both types auto choke. Timing...5 degrees btc at idle. Spark advance at 450 rpm = 0 Centrifugal advance begins at/after 450 rpm and the maximum advance is 24 to 28 degrees at 3500 rpm. Vacuum advance begins at 6 inches Hg (which will provide 4 degrees of timing advance) and the max is 22 degrees of advance at 15.5 inches Hg. (I think with a vacuum gauge you can determine at what rpm you will have 6 inches Hg just for point of reference)
The holes in the B model throttle body (from Rochester data I was able to find...dated 1964) are identified below. I was not able to find the scheme for the BC model, but it seems clear GM did not intend for the 1959 235 to operate with any vacuum supplied to the advance canister at idle. Jon H (CarbKing) may have this info. I would be inclined to give him a call and see. Good luck. Hope this is helpful. If full idle manifold vacuum is connected to the vacuum canister, you're going to get a large spark advance (maximum, in fact) and it won't increase much beyond that unless you're going downhill.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
