Paul, I have checked all the carburettor/hoses and they all seem good, can the symptoms I have described not be explained by a dirty carburettor alone?
Could be, although the surging thing has me puzzled. Could be debris floating around, then being sucked up into a jet or passageway, & then being released again? Before taking the carb off, try dropping the float bowl, turn the gas on, & work the float up & down to flow fuel through the float valve. If there is any debris in the bowl, I would suggest cleaning the carb.
I have no experience with your particular engine/carb but suggest you bring the carb home, spread out a large towel, take it apart in sequence, from left to right & re-assemble in reverse order. Be careful of possible check valves, balls with springs behind them, or other loose parts when blowing things out with compressed air. Taking pictures while dis-assembling may help later.
Go slow, take your time, & tread lightly. If you tear a gasket or it looks bad, Yamaha no doubt sells a re-build kit, with new gaskets & o rings, probably a good thing to have before starting.
Or, just take the carb to a shop.