Our Langdon's HEI has a vacuum canister and the dizzy does not move and this fools a lot of old timers. The original 235 dizzy for this truck did move through advance. We have set our advance at 10 degrees but we still get a bit of a ping so we are going to try 6 degrees of advance and see what happens. BTDC -CW -before top dead will give you a retard on your spark and ATDC-CCW- after top dead center will give you advance. These guys know what they are talking about so follow their steps and you should find what is going on with your dizzy. T/C Steve