Have you checked your gap in the points? Are your leads on the right way? Firing order is 1342 1 being closest to the radiator and then going anti-clockwise

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1964 smooth roof panel van
1967 Mk 1 X-Police cooper s
1970 mini k
1975 leyland

If the points gap is too close it will be very hard to start and fire occasionally but won't run. The leads are worth checking. 1 should be closest to the radiator then goin anti clockwise on he dizzy the next lead should go to spark plug 3 then 4 and 2. Number 1 spark plug is closest to radiator

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1964 smooth roof panel van
1967 Mk 1 X-Police cooper s
1970 mini k
1975 leyland

questions...

1/ did it 'suddenly' start this behaviour? IE was it OK one day, and then stalling like it is overnight, or did it gradually get worse? over days, weeks, months?

2/ if you raise the idle speed (or press the accelerator a little to hold higher RPM) will it keep running?

3/ does it have the 'usual' amount of power when driving around?

4/ does it stall with the air cleaner removed?

5/ what work have you done to the car recently?

6/ what have you tried to remedy the problem?

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did I tell you that I won a trophy?

Sounds kinda like it's running very lean, do you have choke connected?

I guess you have an 1.25" or 1.5" carb, though could you please post a photo of your engine bay, and maybe a video of it running?

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did I tell you that I won a trophy?

May be a stupid question but have you tried idling it up?

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1964 smooth roof panel van
1967 Mk 1 X-Police cooper s
1970 mini k
1975 leyland

Have you checked the oil level in the damper?

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DID I MENTION THAT I AM THE BEST SUPPLIER FOR MINI ELECTRICAL PARTS AND REPAIRS?
"Let us remove your "SHORTS"

Different thickness flanges on the new exhaust and inlet manifold? That can lead to leaks and lean running if air is coming in at the flange.

Maybe you need stepped washers?