How does the engine sound when you twist the throttle and it loses power as you say, does it put out a lot more smoke from the pipe or does it seem to be running out of gas?
about 10mph is usually about minimum speed of these motors and then they start to put on up to 25+ depending on your rear sprocket size, if you can only get 10mph you got some issues possibly with the fuel mixture being way to rich are you arent getting enough fuel, or possibly you have something in the drive train that is in a bind.?????
Lock your clutch lever into position so that it is disengaged and then lift rear wheel off ground and make sure the rear wheel can be rotated easily, if it does then I say start the engine and run it up to the "10mph" point and then reach down and lift the choke lever up on the side of the carb about 1/2 to 3/4 of the way and see what happens, if it starts to run better then you are starving for fuel , but if it dies or acts like it starts running worse and bogging down then I say you are running way to rich and are in need of main jet modification and/or raising the clip up a notch or 2 on the metering needle in carb. try these couple of things and see what happens and then maybe some of us who have been down this road can help you get it resolved.
Good Luck and let us know what you find out.