MOR404, rather than throwing out a series of suggestions I would be interested to know some details... Is this Morgan new to you, if not has the fault just started recently. I note you mention carbs, plural..? Type of carbs. Is the engine tuned. Do you have the correct settings for valve clearances and ignition timing to suit the cam installed. Have you performed a compression test, are the ignition leads in good condition.. Run the engine in the dark, look for sparks leaking around the leads, dizzy cap and coil. You may have noted I mentioned using a length of rubber tube to check for air leaks around the inlet manifold to head joints and carb to inlet manifold I take it all the spark plugs have the same colour and gaps and are correct grade for the state of tune and that the spark is strong. Dependant on your location how is your fuel quality... water in the tank..? Also is the breather hole in the fuel cap clear, does the tank suck air when you take the cap off after a run...As has been pointed out already you have changed the simple stuff like rotor arm points and condenser, if fitted..and checked the advance and retard mechanism is not sticking, can be checked by holding the rotor arm and trying to turn it, it should turn fractionally in one direction and spring back. While you are in there lift off the rotor arm and add a few drops of oil on to the cotton wool if still in there, if not drop some oil on to the head of the screw that will now be visible. Clean the tip of the rotor arm by rubbing it on the tyre sidewall, ideally you do not want to take any metal off at this point as you will increase the air gap between it and the segments in the dizzy cap. The segments foul up in time give them a scrape with the blade of a screwdriver you may find crud flakes off them. Check the carbon brush that makes contact with the rotor arm check it springs or that the spring on the arm (dependant on type), makes good contact, clean the point of contact on the rotor arm.While at the dizzy pull the end off the vacuum pipe near the base of the carb or somewhere on the inlet manifold (if fitted) and suck on it while looking at the dizzy internals and the plate which the points are mounted should move and spring back when you stop sucking, proving that the vacuum advance if fitted does in fact operate...

That should keep you busy for a while.. (-: Good luck, let us know how you get on...