My '26 is a pain to start and barely runs, I have to pull the throttle way down to keep it going and it sounds like its running on one cylinder. The second I flip it to mag I can pull the throttle back all the way to a low idle and it purrs like a kitten. Going down the road on mag it has plenty of power and runs smooth. Put it on battery and it shakes like crazy. The engine was totally rebuilt, new wiring, good coils (tried others), new battery, cleaned the timer and tried everything I can think of. I'm out of ideas.
Check the upper contacts in the coil box
Well, if it "purrs like a kitten" on the mag, then it's probably not the fuel,air, timer or coils. I think I'd be looking for a loose connection somewhere between the battery and the coil box, maybe in the switch or the terminal block.
It could be the switch, the wire from the terminal block to the switch. But most likely the adjustment of the coils. If you can find someone who has a car which runs good on battery, (same volt battery as yours), try swapping your coils. If yours runs good on battery and the other one doesn't, the problem is in the coils. They usually run better on mag than battery, but they can be adjusted to run good on both.
Try starting on Mag, if it starts well then the problem may be your switch, wire, or wire connection at the switch. To start the engine on Mag you need to place the spark lever about 4 notches vs fully retarded as should be done when starting on Bat.
Have you tried fiddling with the key when it's on battery?
Been there.........switches don't last QUITE forever....... ;)
How about checking out the coil box.
Sometimes those 'good original' coil boxes can be a pain.
Changing out all the coil box bolts along with checking to see if the coil contacts are still springy to make a good contact can make all the difference.
I did the above to the coil box in my 1919 roadster and it made a big difference.
The sometimes sputtering, intermittent missing pretty much went away.
The bolts are copper coated and after all the years of dampness and etc. take their toll on a coil box.
Model T's need all the continuity they can get to run well.
My vote is a cruddy ground from the battery to the frame. On my '27 I cleaned up the rust from the frame where I fastened the ground strap. I also run a heavy ground wire direct from the battery to one starter mounting bolt.
Also check your cables, a 6 V system needs big fat cables, not the little delicate things you put onto a 12V battery.
On Wednesday I had the T wizard Benton over to my place. We replaced the coils with new ones from RV Anderson and Benton fiddled with a few things. Car runs great now. Actually had a free start today after it warmed up.