Sounds like some sort of blockage in the carby jets or fuel line.

Take the carby apart and clean out all the water and crap that has found
it's way in there. Have you parked it outside when it rained and had water
get in via the fuel cap air bleed hole? Count how many turns out the needle
valve(s) are and they can be set back to original after cleaning and they
don't need much adjustment. A bit of fuel in an old ice cream container and
a toothbrush, and a bit of fishtank line or a plastic drinking straw so you
can blow through the holes that fuel should go through. If you have a
compressor and a blow gun you can do it with that, but the mouth and a deep
breath can do it. Don't suck up the petrol, or breath in enough fumes to get
you high!

Hope this helps,
Peter