Well, it might help to know the reason you had for wanting to change out the regulator.
As far as how installing a new regulator might be the cause of this, I can think of 3 possible things you need to check.
First, check the condition of the fuel vapor hose at the regulator nipple. The OEM hose is a stiff wire reinforced type and these hoses sometimes tend to "unravel" and split open, especially when they are tugged on when removing and reinstalling.
A split open hose here will prevent the engine induction vacuum from drawing sufficient fuel for high RPM and will lean out the slight amount of fuel that is being drawn.
Second, the regulator has a drain port for periodic draining of tar oils from the secondary chamber. Usually there is a hex head drain plug in that port when you unbox the new regulator. Some applications will feature the optional ball valve draincock (which must be swapped to the new regulator). If the regulator drain port does not have either the hex plug or the draincock in place (and the draincock must be CLOSED) the secondary fuel valve will not open sufficiently for the engine to draw enough fuel to operate at high RPM or under any load.
Third, the secondary chamber has an atmospheric vent. On some early models this is just an open hole in the bottom of the secondary cover. On later models there is a 90 degree hose nipple that connects to a vent hose. If the vent port is blocked with the plastic shipping cap........the secondary fuel valve will not open wide enough for the engine to draw sufficient fuel due to the secondary diaphragm being unable to move (because of the blocked vent port).