There is an adjustment for the pump, two holes on the pump arm, you might try a different hole. The further away the rod from the pivot point of the arm, the more fuel. Check to make sure the pump shoots fuel the same time throttle is opened, should be no clearance between the pump arm and the pump top. If you still want to change pump, see below.
The following steps will change the pump:
1. Go to parts store and purchase either a kit or the pump itself, presonally, I'd buy the kit, Step 6 may result in a torn gasket.
2. Remove engine cover, air filter asembly.
3. Remove secondary needles and hangar.
4. Disconnect pump linkage by removing clip, remove choke pull-off vacuum hose from base, and disconnect choke rod if divorced choke.
5. Remove front two bolts holding carb to intake manifold, remove eight screws holding air horn onto carb body. Keep track of length to hole.
6. Carefully pull straight up with air horn, gasket may tear at this point, depending upon age.
7. Remove pump, replace with new pump. Make sure edge of new pump does not curl up upon insertion into well.
8. Reassemble in reverse order. Make sure primary needles seat into their respective jets. Pump rod may require adjustment.
Good luck, be sure to have a cold drink when finished.