HI my sbc 350/400 hp has a pinging problem. I currently have a mallory unilite distributor with vacum advance. I tried setting my total timing at 33,34,35 degrees and it did not help. I went back to base timing of 6,8, and 10 degrees and no change. The fuel has been changed to fresh 93 octane tried octane boost no good, I am thinking distributor I am currently trying the vacum advance adjustment by turning the allan key in the vacum advance canister counter clockwise to reduce the advance curve so far ng. does anyone have any suggestions? can the distributor be defective? should I try another maybe msd or pertronix? thanks for any help..