[Roadsters] Vacuum advance?

[Tom PRATHER]

The List,
   
       Can some one please enlighten me to the facts on vacuum advance? Before the conversion to Mikuni flatslides  I had my timing set to 16 degrees as U20's should be with SU's (presmog). The car never pinged or at least it wasn't audible to me and ran good. I am now tuning the flatslides to that sweet zone where jetting is almost on but I have this off idle ping and stumble that doesn't allow me to get a great launch. I have even backed off on the timing some more to about 10-12 dgrees but no go. I know on the U20 w/Solexes are set to 20 degrees w/advance off. What is the reason for not having the advance with these hi performance carbs? Wouldn't one want it to overcome that big hole on launches where velocity is slow? I am presently running without advance. Once I baby it past 2500rpm it goes like a bat out of hell but I don't need holes in my piston either. Do I need to have my Boone EI reworked? Thanks  guys, 
                                                        Tom, 69 U20 w/Mikuni Flat Slide RS-40