Found my dads old sears timing light and decided to do a comparison between the D cell, sears, Innova dial back and an inexpensive no name light. All of the 12v lights were connected to an external 12v battery. They all gave the exact same readings at idle and at full total timing. Decided to try the sears connected to the car on board voltage. What i found was that as i increase the RPM timing would retard a total of 5 degrees at 3000-3500rpm. Connected the sears to the external battery and retard with RPM is only 1 degree. Same results with the D cell, 1 degree retard total. I did not check the other 2 lights this way. I would suggest always connecting to an external battery when setting timing or get a D cell light. My connection on the car was the battery side of the start relay and ground. Dont know why this happens, but it does.