I have got an 03 softail twin cam. Its carbureted.
It runs very smooth when I first start riding. After about 5 minutes I get a "bum ba bum pa pa" thing going on. Its not real bad but noticeable.
It was really bad until I changed out-the Ignition Module, speed sensor and Maf sensor.. & plugs. I used an OHMS meter and checked the coil & wires,"they are ok".
I cleaned and rejetted the carb and changed the intake gaskets and carb boot.
There's nothing left to try ! why does it stumble when warm. I'm beaten & lost.


Update-The mechanic said I should have tested the coil Hot, since the problem only shows up after the bike is warmed up.
I put a special "firing light tool " on the plug so I could actually see the Spark in action. Boy did I get a surprise !....front cylinder spark would DIM as the motor stumbled. The rear cylinder light was very intermittent at low speed. seeing is believing . 1 coil coming up LOL.

 

Let us know how it works out when you swap out coils. Tell me more about this "firing light tool". YD