Mountfield SP465R major problems?

In desperate need of some help, have a mount field SP465R mower that’s just had a new gearbox, carb cleaned out, new spark plug with gap set, new blade belt and now it occasionally starts but doesn’t last long or just cuts out straight away. Does anyone have any ideas or answers, I’ve stripped the carb several times now and still no joy
Richard Bolton, April 2022


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