Land Rover Series III engine not running right

EXPERTISE PLEASE: Land Rover Series III 2.25 5MB petrol engine was completely rebuilt. Since installing the right accelerator cross shaft lever, I have full throttle (due to wrong lever my rebuilt engine was run in on ½ throttle only). As this lever issue wasn't spotted initially, a lack of power was deemed to be due to the carburettor. George Jeffcoate gave the kiss of life to an old Zenith 36 IV. This works much better than the brand new repro from Bearmach that we started with, but the engine didn't run right yet. Landy will not cold start with any amount of choke (suggesting mixture might be rich already?). It will cold start without choke after a good few starter engine turns. Running stationary, you can cut off the cold engine by depressing the throttle just half way. Because you can immediately restart afterwards, I don’t think stalling is due to flooding. Landy with cold engine on the road drives hesitantly (kangarooing) especially on acceleration. Obviously a cold engine can proceed under a very light right foot only. A warm engine drives fine but it doesn’t feel to have full power yet. On the flat with a Fairey O/D and 7.50 R16C Goodyear tyres it struggles to reach 60mph which I think a 2.25P should achieve effortlessly. The fuel pump’s been checked to work fine, new Accuspark coil, new Accuspark HT leads, timing 6 degrees BTDC as recommended for 95 octane petrol, changing multi-electrode spark plugs back to NGK Iridiums made no difference. I suspected that the Zenith's compensating jet might be the culprit and bought a concours Zenith36IV from Fuggle's Fettling and fitted this today. Sadly it doesn't seem to make any difference. If you have Series Land Rover expertise then your input would be very much appreciated!