acme,
I don't know how intense you want to get on this issue, but to 'prove' the vacuum advance unit, you could disconnect the vacuum line and plug it at the carburetor. Use some good old duct tape or a rubber band and restrain the distributor housing from movement. This would keep the distributor in the static timed position and bypass the advance unit.
Take the rig out on the road and see what happens. You will notice some reduced low speed response because the vacuum unit is disabled.
Wouldn't it be great if the backfiring quit!
Stuart
