Originally Posted by Jim C
If the motor is running and revs it is not getting power to the rear wheel. Gas to the motor has nothing to do with it. The motor is getting gas or it would not rev up. It may be the clutch, drive sprocket, chain or somewhere in between. I will assume the drive chain is connected and on the small sprocket. Check to make sure the driven small sprocket has a key in it to drive the sprocket. The shaft may be spinning inside the drive sprocket.
I concur, but I'd also take a close look at your clutch cable adjustment. Did you change anything when you were repairing the flat? You had to remove the rear wheel. Did you touch the clutch cable?
I'm curious how you're starting the engine if the clutch isn't engaged. Are you actually positive the engine is running? I'm not sure what you mean when you say "the engine revs". With the bicycle stopped after you've "started" it, is the engine sitting there idling until you twist the throttle?