power problem

[Andreas40]

Hellos guys
I just bought a 2002 GZ250 , when I test it in parking lot and seemed ok, however now I have a power problem… first , second . and even third gear seemed ok accelerate ok and the engine seems to have enough power and I can reach up to 40mph , after that in forth and fifth gear the bike has no power it remains to 40mph or even go down… it seems like there is no power after third gear , it seems to me like no more gas flow but it confuse me the fact that until third gear is ok , is it any adjustment in curb for high speed? If yes where I can find it ? is that something I can do by myself ?
It also seems like timing problem, does this bike have timing adjustment ? how I can find it ? the bike have only 4500miles … did anybody have similar problem ? I will appreciated any advice and suggestions

Thanks

Andreas