1971 750 Commando backfires

I took my Commando out yesterday, mild weather, city traffic , never got it into 4th gear, everything running as it should. Mikuni VM34 carb, fuel filter in the line, TriSpark ignition. About 40km in I felt a small hesitation while accelerating. A short while later another, then another. The frequency and severity increased until the bike was shuddering and backfiring constantly. I aborted the ride and turned for home. It seemed to idle okay but under power was jerking and banging. I made it home safely and observed it idling in the garage. I changed the spark plugs, NGK B6ES out, B7ES in.  No change . Will be  installing BP7ES later today. still would idle but not rev. I waited a couple of hours, Checked exhaust tightness and inlet manifold, appeared good. it was difficult to start but did after 10 or so kicks, idling okay but still wouldn't rev, backfiring, erratic and orange flames out the mufflers when backfiring, I persisted and it seemed to rev out after a few tries.  1200 trouble fre miles to this point. Fuel or ignition?

Thanks for the replies.
2 problems.
First, see the attached photo. The yellow/black wire from the ignition module to the coil pack was hanging on by a thread. I started it up after repairing the connection and it revved better then came the second problem.
The carb fell off.
As the plugs were sooty I lowered the needle one notch to see if that improves things while I was at it.
Starts first kick and sounds strong again.
Cheers.