Motorcycle Repair: 1987 honda xr 80, honda xr 80, piston movement

Question
idk if you will be able to answer my question but i have a 1987 honda xr 80 and i just rebuilt the top end and when i got to start it...it back fires out the carb...could you help me what might be wrong?

Answer
Either the timing is off a little or the intake valve lash is to tight. Remove the spark plug and place the piston at TDC. You may have to feel the piston movement with a small screwdriver or a stiff piece of wire. The timing mark for TDC should be visible in the timing hole. Now check the camshaft to see if lobes are the farthest away from the rocker pads and the mark on the cam sprocket is aligned with the mark on the cylinder head or in a straight line from the crank to the cam then to the mark.