Make sure you disconnect your vac advance when setting intial timing. Are you using HEI ignition? Also, you may need to adjust your mixture screws. You have 3 types of timing: intial, vac, and mechanical. Total timing should be around 36-39 depending on setup. Total timing is mechanical+intial. Vac only comes in during cruising and goes to zero at full throttle. Check your spring weights as well in your distributor