Inconsistent Speed



Ok so a while back, I finally got my schwinn skyliner with Hua Sheng / Grubee gearbox with a 48T rear sprocket going. Soon after, the key that holds the output sprocket from the gearbox on the shaft sheared in half. I have fixed that problem and changed from using lithium wheel bearing grease inside the gearbox to using half synthetic gearlube and half motorhoney engine stopleak. Sometimes, I can get up to about 32 mph. Sometimes the top speed is 21mph. Sometimes it oscillates between the 2 speeds in one trip. I am having difficulty figuring out exactly what the problem is. With the way it's geared, the top speed should be MUCH higher. There are those on this forum that claim to be able to get up to 40 mph with a similar setup. Top speed aside, the fact that my speed is so inconsistent really bothers me. IS this a problem with the gearbox? Is the clutch just not engaging properly all the time? Or is this an engine problem? I have about 100 miles on this engine; is it possible that it just needs more breaking in? Please help.
Is your clutch slipping? I have had slippage with a gear oil mix. Have you checked your fuel filter or installed and in line filter? Some times the tanks have gunk in them. Drilled a hole in your exhaust pipe? Does the engine feel like it is surging? Could be carburetor, fuel filter or vacuum lock. Make sure the engager engages fully. I can do 31/32 mph on the flat with a 56th rear and have hit 38mph on a slight down hill and thats at 6,000 ft up.