gilera runner vx 125 problem losing power

hi my gilera runner vx 125 02 plate its 4 valve was running perfect fast at seting off and reached 70 with ease. my mate whos bike it is also went the garage and filled it up and wen he set off from the garage it was slow at seting off it like bogs out and loses power it still reaches 70 but takes a lot longer to get there very slow at seting off . i have drained the fuel an put somin in from esso but still the same

 

i have the same problem i havnt been able to fix this now for around 3 months tryed everything