Magneto Question

[Tim ford]

Hello. I’ve owned a 1927 Model T for 30 years. After driving for about 5 minutes I always switch it over to magneto and it always seemed to run better on magneto than on battery. The other day I took it for a ride and when I switched it to magneto the engine started breaking up and sounding like a misfire. Once I switched it back to battery it ran fine. Does anyone have thoughts what might be the problem? Your help would be appreciated. Thanks.