That is strange. A GM HEI will not power the stock hood tach properly. I used to mount a Cal Custom Hawk tach to my steering column on the '68 hard top. It would not work with an HEI nor an MSD unit. I eventually had to buy the MSD converter unit so the tach would work. I wonder what is different with the Pertronix? Do you have the original GM tach filter in place? When I look up the hood tach on Firebird Cental they list the tachs as being for points type ignition only. The filter is also recommended to eliminate spikes and distortion of the signal. It converts the spiked wave to a semi-square wave form. This smooths out the signal so the pointer doesn't bounce around.
