You probably have introduced an air leak somewhere... loosly mouynted carb... bad seal on the needle valve, or bad backplate gasket are the 3 primary culprits.

Its also possible that you got the head on slightly crooked, which would cause you to see bbubbles comming out around the head...

It is also easy to put the cyl liner in offset, and fouol up the port timing. (the engine will barely run at all if the cyl liner is 180 deg out...) The hole furthest fromt he cyl head should point nearly straight at the exhaust. Then there is a key-slot in the cyl liner which on some engines points directly forward, on others directly away from the exhaust, some directly at the exhaust.... (always straght in line with the crankshaft or 90 deg off... never an odd angle)

The burn pattern on the piston top should point the cleanest portion toward the intake port.

If it was running reasonably well before dissasembly... it should run well again after proper reassembly.