If you check out the MSD and everything is ok.

I would think your losing prime, take the gas cap off the tank while it is running after the engine is warm, shut down and leave cap off 20 min, then restart, If the engine fires right up than you need a vented gas cap.
Where are the floats in the carb set? you can raise them a little to keep more fuel in the carb bowls.
I'm thinking you may be causing a vacuum in the tank, when you shut it down this vacuum sucks the fuel back into the tank.

this was the first thing to come to mind, both easy to check and rule out fuel delivery.
Godspeed
MrC.