Re: stumped????
A couple of things you might check.
While it is 'stalling', bump the choke, does it make it better, or worse?
If better, then the carbs may be to lean on the idle mixture adjustment or possibly the float levels are a bit too low.
If worse, then the idle could be too fat, or the floats a bit too high.
Have you choked for a leaking float valve?
Trim the engine level, and while it is turned OFF, look down the carb throats while you pump and hold pressure on the primer bulb. Do you see ANY fuel leaking into any carb? If so then it is time to clean all carbs and replace the needle/seats.
When it has stalled out, have you pulled all the spark plugs and inspected them to see if they are wet/dry/sooted up?