top speed around 50mph?

[rockand0rroll]

I've had my GZ250 for about 2 weeks, and so far so good in general. I bought it used, and had a friend who is an excellent mechanic give it a full overhaul before even taking it out. Generally the bike is running great and it's working great as a commuter in the city. Tonight I tried to push it a bit when I went riding with a friend (who was on a much faster bike), and i found that even in 5th gear it was kinda slowly approaching 50mph. I am about 6' and just under 200lbs. From what I've read, it should do another 10-20mph faster, but does it just accelerate to those speeds a bit sluggishly? Does this point to any apparent problems? Is this typical? Does the bike need to warm up for a bit to get to it's higher speeds? Any help would be greatly appreciated... Thanks.