Stock 1600TP.
Backfired starting hot & misfired at low speed going home but OK full bore.
@ home, pulling HT leads off during a fast but rough idle showed removing 1 & 2 slowed down the eng but 3 AND 4 had no effect. Easy! Dirty plugs? No! New plugs no effect. Both leads give a bit fat spark at the connector. Spark arcing inside the cap? New dist cap - no effect.
Oh shyte!! Cracked head bet 3 & 4?? Compression test show 180 & 175psi!
Anything I've missed? Any other dianostics I can do?

check the timing, both the distributor, and the valves

Condenser playing up? If intermittent.
Someone swapped two HT leads for a joke?
Idle circuit partly blocked in carb so running OK on main but lean at idle.
Damaged rotor button, or spring right inside it's base has fallen out? Makes for 'funny' running.
Points damaged/worn?
Broken/high resistance in HT leads. Or insulation failing somewhere.
Loose connexions to coil.
etc. etc.!

Rotor replaced- no effect.
Valve timing : #3 & 4 have both valves shut at TDC
Def running on 1 & 2 only when idling but it seems perf normal at operating revs???
Replacing leads: no effect.
Ign timing? Cant see how it will only kill 2 pots but I'll check with a strobe anyway!

Found it I think!
I should have thought about what happened.
Backfire = explosion in intake manifold.
I got one while testing ign timing & noticed a wisp of smoke coming from the LH rubber boot on the manifold.
Looks like the backfire blew the boot off & is now sucking air. Prob would be worse at high vacuum situations ie idle & less impt at low vacuum ie full bore!
Stay tuned!

All better now!
Memo: keep ur eye on those boots & keep the hose clips tight!