Misfiring engine

[PeterW]

Help please before I go mad !. After a full rebuild of my 1933 Austin 7 RP , when starting up the engine misfires . It will run with misfires , and run better at higher revs with the occasional misfire . However , when in gear and under load , it constantly misfires and hardly moves without cutting out . I've replaced the coil , replaced the condenser , replaced the leads , and replaced the plugs . The points , rotor arm , and distrbutor cap are in good condition and the spark acrooss the points is excellent....blue and fat ! . The carburettor has been overhauled , jets are clear and the choke and float operating correctly . I'm using premium petrol for temporary start up  before adding an additive . Before the restoration started ( mainly body and interior ), the engine was working perfectly without problems . The rebuild has taken three years .
Any suggestions would be very welcome as I'm nearly demented . Thanks to all in advance