1983LU5TA

Should the EGR valve on an 83 LU5 305 Trans Am be a positive or a negative style? My GM FSM doesn’t specify. I have been battling a light throttle misfire and have tracked it down to the EGR valve. The one on the car is an aftermarket positive style. Vacuum is applied a few seconds off idle. With the EGR disconnected and the line capped, its smooth as can be. I have owned 15 thirdgen Trans Ams and have never seen a positive style EGR. I blocked it off and took it for a drive, and it runs beautifully, however an occasional ping can be heard.