14 hp Vanguard V-Twin surges and runs badly...

If choking makes it run, then it is starved for fuel. I would check fuel flow from the tank at the carb inlet, and if that flow proved adequate, I'd drop the float bowl and check the float, needle & seat for contamination, gunk, or binding.
If there is an anti-backfire solenoid on the bottom of the float bowl, you can remove it and inspect to see that it moves freely. A 9-volt transistor battery touched to the connector terminals should operate the solenoid. It should open with power, and spring closed when power is removed. Most can be cleaned with carburetor cleaner, or replaced with a bolt(good luck..) or new, and they SURE are proud of that little gem, asking above $50, closer to $80 from what I've seen.
tom

How does applying the choke, which would make the lack of air problem worse, help the engine run if it is indeed starving for air? Maybe I missed something in understanding the problem. If the filter is dirty or clogged, then I'll file this away mentally as a new phenomenon. I have heard of engines running better without the filter, when the filter is dirty, but never using the choke to make it run better for a bit.
I must have missed something.
Does the engine seem to produce normal power when it is running? Does it slow down, or just start surging? Will it keep running by playing with the choke? I am now lost.
tom

I was just stating what mine does surge if the filter is clogged but he stated filter is new. Tom knows what hes saying but I'd add, make sure the gas cap is venting and not plugged as you say it takes a minute to react. I'd guess you already cleaned the carb by soaking for a few hours then blowgun out the passages... Guess I shouldn't assume:dunno:

Petcock screen?

Please post when you find the problem as I have a 13hp single piston on an older tractor that acts about the same, not using it though till cutter & rear axle bearings get replaced