zzr600 lumpy below 3000rpm and also smells of petrol

[ddtwelve]

Hi all I know its not a fazer etc but my brother brought a old zzr600 only done 6500miles,when he first got it it wouldn't run proper felt like it was a cylinder down so went back to stealer who sort of sorted it said there was sand in air filter, so carbs were clean etc etc weve got it back it will start and run but its  lumpy below 3000rpm and also smells of petrol went out for a ride to squires and still not any better any ideas on what to do as don't want to take it back to dealers

cheers