I can't get a carby for a Honda gv400 motor

If there are threads on the mover and the dipstick then yes. If there are no threads then no. That being said most dipsticks do not thread into the tube as this makes the readings less accurate by a small amount

check for a blocked main jet in the carby
the principle is that when the trigger is released there is no pressure in the pump so the engine is idling
when the trigger is pulled the pressure control valve closes the bypass back to the pump and so the motor is loaded up if there is a problem in the carby ,then the motor will stall

Replace cable.

check for blockage in carby bowl. sediment etc blocking the main jet and restricting flow under full load causing the engine to fluctuate in revs

have a look at the motor carefully and try and see if the motor is a licensed copy of any other bike engine, e.g the chinese bikes copy honda's CG125 and CBR 125 engines, i know in japan and other places around the world they use CBR 150's which is just a bored out 125 motor.

if it helps find the manual of the bike in question and look at the set up for the carb and try it, but based on what your saying depending on the carb type

i'd suggest moving the carb needle down a notch as its seems to be getting too much fuel.

1 inch diameter keyed shaft

honda dealer and eBay come to mind

if you have good blue spark and it fires when fuel is added externally , then the problem is fuel flow through the carby
have the carby professionally overhauled or get a new carby
make sure that the gasket to the manifold / head is in good condition

firstly you are the only person i know of with a mr710 other than myself mine still has serial no on it 00054.To me this says not many made. i could not find a blade for mine so done my own mods with cox swivel blades but do not know of any others in australia. I can not even find any info on net. honda motors with vertical shaft are not common but never give up like all hondas. try the mower shops as an alternative. hope this may help