Bass1 ,A list of a few things I have found to cause simptoms similar to yours on Brison and other engines over the years.
Low ignition voltage - bad or discharged battery or switch.
Ignition timing incorrect .Its not difficult to get a Brison's timing linkage out of wack.
Cracked carb isolator block . Goes lean as it warms up .
Very rarely - an ignition sensor , but have seen them cause weird problems intermittently . Usualy a wiggle test or heat applied will cause problem to appear.
If there is an ignition kill switch or optical isolator - bypass them and test run
Carb diaghram gasket in wrong position? .gasket goes between diaghram and carb body