Took the old 26 T out for Ice Cream with the family tonight...discovered a slight problem.
T run great on Battery, but when I switch it to mag it starts to backfire and just doesn't run as it should.
Is something out of adjustment, or is mag going bad?

Mike
First, you need to verify if your magneto has sufficient power (health) to operate properly adjusted coils.
If you satisfactorily pass that test you will need to trouble the cause of the engine missing and focus on your coils and possibly the timer.
You need to select someone to help you off line. Troubleshooting on-line will only lead to you being buffeted around like a ping pong ball.
If you would like my help give me a call tomorrow after 10am EST when I am in the shop.
Ron the Coilman
859-881-1677

Thanks Ron...Timer is brand new..so that shouldn't be the issue. I ran it for about 5 miles on battery no problem...when I switched it to mag..that's when problem started.

Mike
Do not jump to premature conclusions. A correctly operating timer on battery is no assurance it is working correctly on magneto. At this point do not rule ANYTHING out regardless of how "new".
The original Model T ignition system is compromised of complex and sometimes trouble prone subsystems. Each subsystem must be working correctly at the same time for the entire system to work correctly.
Ron the Coilman

Not to be argumentative Ron but if it was me the easyest thing I can think of is a dirty or worn mag post connection.