got a 1977 honda express. was running great for about two weeks after i got it (with a carb i believe was from a 1978). it gradually required a coupld more kicks to get it to turn over, then started occassionally stalling after running at top speed for half or mile or so - i would let off the throttle and it would wind down, and i could not prevent it from stalling. for a couple days i could ride it under 20 mph and it would be fine. now it takes a long time to start it, and it will stall after even moderate speed. basically, it's stalling after throttle with increasing frequency. any adivce?