its an AED 750 HO 4150 and need to adjust secondary throttle stop.
Now on the older carbs the used to be a straight slot set screw but this one has an allen set screw.
any one know the size that it is?
I tried a .050 allen wrench and it still seems too big.

They are standard sized, not metric usually. It should be fractional.

I knew that?
but what size is it?

They vary. I can't remember the sizes off hand. Get one of the small allen wrench kits around that size. It is possible your is stripped.

I doubt it its a new carb.