Old Suffolk Punch mower keeps stalling?

I have a (very) old Suffolk Punch, with the engine that uses a Zenith carburettor. It was running very well. Then, suddenly, it stalled on me and wouldn't restart.

I cleaned & regapped the spark plug, checked the carburettor float, etc. It would start and run for a few seconds (but at low power, however much throttle I used), then stop.

Today, I fitted a new spark plug, new float & new float chamber gasket. I checked the ignition points.

The beast started beautifully and ran at full power for a minute or so. I stopped it, because I wanted to adjust the clutch take-up. When I restarted it, it was running at low power for about 30 seconds, then stalled (ie, back to square one). I suspect the carburettor may be flooding, but not sure.

Any ideas?