Cause of misfire?

UPDATE

Because of the age I decided to replace the whole ignition system with a new Boyer micro power system. Idling and running is now fine but strangely starting is more difficult! I’ve used a strobe to set the timing correctly. With the mk 3 it would start within a couple of kicks, now it’s taking typically 4 or 5. I’m wondering whether the dip in the micro digital unit’s initial advance curve actually makes starting more difficult?

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I bought a 1973 750 Roadster recently and have been servicing it (it’s only done 500 miles in past 20 years, since an engine rebuild!).

I noticed random absence of strobe flashing when connected for checking ignition timing, which coincided with misfiring that I’d previously assumed was due to carb adjustment.

I’ve gone through all ignition wiring and no faults found (have fitted new ignition switch and bypassed the kill switch, also new plugs).

Bike starts fine, 1 or 2 kicks.

Runs/idles ok when cold but misfires when hot and the idle slows to a stall.

I’m suspecting faulty ignition coil, ignition unit or pickups (mark 3 Boyer with 12V Boyer double output coil, all around 15 years old).

The only anomaly is that if I put my hands near the exhaust outlets, the misfire seems worse on the left side, also the down pipe is deep blue on that side. I tried swapping the plugs and the misfire persisted, if anything the right side got worse but misfiring on both sides. I’ve not had the bike long so the bluing might have been due to an old problem eg lean carb (I’ve overhauled the Amal’s including clearing the idle jets) or due to an exhaust joint leak.

I’m inclined to change at least the coil and perhaps the whole Boyer system. Any thoughts?