Sucking to much air as in through the choke. Remember its just brass gauze , not a filter like later mowers. If its fully open and you hit full throttle it will bog or flat spot, then rev, then possibly die until you close the choke to suit or decrease the throttle.
The carby has no seal, it only relies on the clamp as you noted, it is rarely if at all a problem, as it seals well.
Basically your trying to find the best air fuel ratio by adjusting the choke, not just by the throttle alone. When new, you would have started it on choke, warmed it then set to run. Then you would increase the throttle to suit....But its old and tired and needs fine tuning to get it to run well.
