I don't have direct experience of the exact carbs used on Rotax engines, but aren't these "Bing" motorcycle type or similar, i.e. Constant velocity? Unless they are of a very unusual design, they don't need fuel "pressure" to work, only a fuel supply sufficient to keep the float chamber filled to the designed level. So a lower maximum fuel pressure probably wouldn't cause a problem.
The engine driven pumps I've dealt with over many years simply consist of a diaphragm and two simple non-return valves, which work in opposite directions, one opening to "suck in, close out" and the other vice-versa. The operating arm on the rotating cam pushes a circular metal plate to move the diaphragm against a return spring. There isn't a way of regulating fuel pressure in that design, so it would probably vary depending on engine rpm and fuel demand. At full power and max revs, the pump would move the maximum amount of fuel, but the pressure might be lower in the supply line to the carb. The size of the pump would be chosen to provide slightly more fuel flow than the engine could consume at maximum output. With the throttle closed but with high engine rpm, the fuel flow would be lowest whereas the pressure would be highest. The output pressure must not be sufficient to overcome the float valve in the carburettor, for obvious reasons.
Sometimes a separate fuel pressure regulator is fitted after a mechanical pump. This may use a return pipe to bleed off excess pressure.
Last edited by ShyTorque; 5th Nov 2016 at 09:05.