Check for adequate fuel flow.
Shut off fuel tap, pull right hand chain cover, with a long skinney screwdriver reach thru the web of the chain sprocket and remove the float bowl drain plug, turn fuel tap on, see if you get a decent flow. If not, shut off tap, pull hose from tap, turn tap on. If no flow, screen in tap is plugged up. Pull tap and clean out.
If fuel flow is OK then I would agree with joew that it is probably a problem in the carb. When you had the carb apart did you remove the emulsion tube and the main jet?
Is the engine revving? Remove left side variator cover and see if it is working.