Motorcycle Questions? Ask a Mechanic.
It sounds like it might be the electric choke not closing properly.
They have a plunger that closes electrically but maybe the wire
is off or the choke unit is faulty on the carburetor.
It is choking out the engine with too much fuel.
If this is the problem the spark plug will be very black and maybe
It sounds like it is lacking fuel probably due to plugged
main jet in the carbs center. Cleaning the carb and checking
the fuel supply to the carb will probably fix it.
When you block the air intake it gets more fuel through the increased vacuum created. Check the carb jet for obstructions and get back to me if necessary. Tank has to be vented to allow fuel to flow.
This vent is usually in the fuel cap.
Let me know what you find.