Thanks gents. Verified / corrected the timing today (full advance was spot on, idle was at 5BTDC instead of 5ATDC), plus had the carb off again, float height checked, main & pilot jets checked, all galleries blown through with isopropyl. Reed valves checked ok with Mk 1 Fingers & eyeballs.
The engine a bit smoother, but basically the same. The missing / popping as you back the choke off is so pronounced (engine shakes (250cc)) that I began to question the ignition. I put a strobe into each of the 2 HT lines, and noted the strobing failing iaw the missing. Seems odd that BOTH coils might be failing, so I disconnected the kill switch in case of some rogue grounding - no change.
‘Charge coil’ (ie magneto sys) resistance checked ok, though the 2 ignition coils (primaries) are out of spec. Will order new ones.
CDI unit... have no idea how to check.
Q - on magneto type ignition circuits, if the engine is running badly due mixture / crankcase leaks whatever, can you expect the spark to continue reliably all the way down to engine sputter & stall? (would have assumed so, but just verifying)
There’s a moral here - don’t bloody buy stuff off teenagers 😂😂😂