My 1914 roadster was not running smooth especially on the mag. I contacted Steve Strom, Vice President and tour director for the Western Idaho Model T Ford Club, and he loaned me some new rebuilt coils to try, What a difference! Steve tested the coils I had and 2 out of four were not working properly. He has the Fun Projects coil tester and it pin points problems. Now I need to get my coils rebuilt since I really like the smooth running my Roadster is capable of. If things are not running smooth and your T jerks when accelerating it could be coils. A model T really purrs when it is right.
Fun Projects also sells new coils for just a little more than the cost that you can pay to ship and have those old coils rebuilt.
I have a StroboSpark tester and tested a set yesterday for a friend. They all passed with flying colors and his TT runs much better now. He had never heard of a compression start and the coils demonstrated a few of those for him.
Old wiring can cause lots of trouble too.