I was told one reason the 1916 is running bad is that the 40 dcn doesnt have a grove between the chambers. Im not sure if thats on the manifold or the base of the carby. but if this grove is put in, it evens the flow. Now she runs rich, then sought of normal as the revs pick up and then she leans out the harder you push it. has anyone hear of this grove?

Its like a balance tube. Should help with the reversion that you can experience with twin single throats (basically what you are running). I would try grooving the manifold, worst case scenario is it doesnt help and you get it welded up !!!

Don't think you could make enough of a difference with a groove on the manifold you likely have... The best would be if you could make up a plate about .25-.5" thick with a gap in the middle between the two pipes to provide the plenum (that's what such a chamber is called.)

How deep should the grove/gap be? and why is it needed? so it will never run properly without it?

bill fischers book says by slotting the base of the carby or creating a cavity by using a plate.spacer to create a plenum chamber you help eliminate fuel standoff created by engine pulsations. the bigger the chamber the more the softening effect. it will mean you run a little leaner at high rpm as the softer pulsations cause the airstream to draw out less fuel from the discharge nozzle.
for performance he recomends 32mm venturi1.30mains,1.85airs,f33emulsion
24mm venturi1.30mains,2.00airs,f38emulsion