Ch Ignitions made a lot of ignitions that were not automatic advance. In later years, they sold what they called a "Synchro Spark" that was a module that plugged in between the sensor and the ignition box, and provided automatic advance. Then they made ignitions with the "Synchro Spark" automatic advance built right into the ignition box. There are a lot of the older systems out there that did not have automatic advance. Look carefully on your ignition box, and see if it says "Synchro Spark'. If not, it does not have automatic spark advance, and is running on whatever you set the timing at, all the time. This could cause a kick back problem, especially on a larger engine and/or if the engine gets slightly flooded on a start.

Depending on your setup, a good backfire or kickback can make the timing slip too.

AV8TOR