McCoullach 18 inch with BandS 450series 8 years old. Has always started easily cold or warm. Recently started to hunt running too rich. Changed diaphragm and gasket, now starts 1 or 2 pulls from cold runs perfectly until stopped to empty box. Afterwards when warm will not start at all. Tried no choke half and full choke and primed. Leave for app 15 mins starts and runs perfectly. Any suggestions please .