I don't know if you are a 'newbie' or not, but the choke lever on the carb should be all the way down. There are mornings, when the weather is cool (and when I first got my bike), I put the lever to half way to get her started and warming up, get the bike loaded with my lunch and such, start riding, and hit some nasty dead spots, and realize I had not put the choke back down. The ironic thing is, I wasn't even a newbie to my bike, a TMEC 200, since I had just owned an older model and should have known better!!
If that is not the case, it sounds like your bike could use some carb timing. As well, these bikes often come with awfully big rear sprockets which should be changed out, pronto. Most likely, you have a wheel which is a clone of a '80 Honda XR185 which makes sprocket shopping quite easy (and low cost!!).
Moderator 'Weldandgrind' has links to a continuous ebay seller who has Chinese Mikuni carbs...the advantage of changing out your carb to a Mikuni is the fact that the jets can be purchased most anywhere, whereas your stock carb is probably the decent (decent most of the time) Kiehen carb (which states PZ30 on it, and 'Standard of Japan') These are decent as stock carbs (I have not yet upgraded mine in the 4200 miles I have commuted on mine since the end of May) but their jets cannot be changed.
Stock, the bike should be able to hit 60 mph, and with a rear sprocket change, it should be able to maintain (cruise at) 60 or up to 65 without sounding like the engine is screaming for mercy.