excess fuel 550 grizzly

My grizzly tank only lasts about 30kms (15miles) then definitly need fuel and thats cruising 40 to 60kms/hr. Fuel guage now flashs although has worked for most of a trip. Full throttle it dies and gently ease up throttle to move and won’t idle for long.
Dealer replaced throttle injector and serviced now wants to do fuel pump but I am loathe to spend this money as I think they are guessing.
Maybe I am wrong and am under fuelling and its putting more in at wrong time? Wrong spark plug? (Maybe they put wrong 1 in) Ambiant air from 20c to 40c (100f) in the shade and chase cows slowly so done a plug? (or ideas reliable plug high temps) Not high elevation. Doesn’t boil fuel. Cleaned air filter.
Anyone had similar symtoms?