Is it flooding out when you try to start it cold if it is have the needle and seats pressure checked in the carbs if they don't hold 5 to 7 psi indefinatly they are bad and should be replaced this is more than likly your problem as when the motor is cold it will be tougher to start because it is getting toomuch gas and when the motor is warm the gas will burn quicker.
Check the compression in all cylinders. They should be within 5% to 15%(max) of each other. Any more than this the topend should be rebuilt(at least ring replacement).
Make sure your floats are adjusted properly. A good rule of thumb is to adjust them so they are parallel to the gasket surface when the carb is held in the vertical position.
Make sure chokes are working properly. Pull the plungers from the carbs and check to make sure they are all working.
Also, try replacing the fuel pump if none of the above work.
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