1. it may need a rebuild
2. sounds like the secondarys are opening too fast, what about a hard roll does it do it?
3. 750 is also too much carb for a semi stock 350. 600 may be a lil better. i ran a 670 street avenger on a 11:1 357 with 250 shot a few years back and was deep into the 11's. so it may be too much carb