This sounds like a possible air leak around
the carb intake boot or rubber.
When the intake leaks it reduces the vacuum
you need for the carb to draw in the fuel.
Do you have some carb cleaner or starting fluid
you could spray around the carb intake boot
on the engine side? watch for any change
in engine speed indicating a leak in the gaskets
or rubber boot.
The other possibility is cracks in the
carb slide rubber diaphragm in the top
of the carb.
Also on the side of the carb
is a coasting enriching diaphragm
under a little cover, check that rubber too.
Then check the main and idle jets are clear
inside the carb. Picture here:
Make sure the float and float needle valve inside
the carb are moving freely and are clean as well.
A carb clean might help if the other items I mentioned
are also okay.
Get back to me with any new info.