CBR250rr Not running smooth, think needs a tune

So I've got my cbr250rr up and running after a year of not running. I can start the bike but it is sometime a little bit tricky to start but nothing too bad. the problem is that even once the engine is warm, i cannot heavily rev the bike without it stalling, if I simply open the throttle quickly, it will bog down and stall. i can get the revs up high but i have to do it slowly and once it's up beyond 5 000rpm, it runs smooth without bogging. The old owner told me that the mechanic told him it needs the valve clearances done but im not entirely sure that this is true.
I think that the carby just needs a good clean out after having fuel sit in it for about a year and would have gone stail. I think the carby must be gummed up or might even need a tune to adjust the low speed fuel mix adjust screw. I have decided that I will buy and run some fuel additive carby and injector cleaner throught the tank with the petrol and see if that fixes the problem.
Any help would be great.
Thanks, Tom