Possibly a quick thing you can try is to remove the air filter and the housing if poss. this will weaken the mixture. Or block part of the inlet off which should enrichen the mixture. I don’t expect it to sort your problem but I would expect to see/feel a change if it is the carb.
I don’t know 251s but I presume the carb fitted is essentially the same as fitted throughout the two stroke era. If it has it I would inspect the cold start rubber as if that perishes then it enrichens the mixture. A bit like running with the choke on.