Low speed surge

7" of idle vac becomes a can of worms. If this carb has a choke, try this: engine idling, slowly close the choke blade until the engine note changes. Back off the blade a tad & tie/secure in place for a test drive. If surge has gone, it is too lean. But why?

With only 7" of vac, I would expect waaaay too much T slot exposed, which upsets the idle/cruise transition. Also, assuming the low vac is just due to a big cam, the engine is going to need a TON of idle timing for best idle & off idle response. Maybe over 40*. 12-15* is certainly not going to cut it.