Most trash pumps secure the impeller by one of two means. If there is a bolt holding the impeller on, then it's straightforward...remove the bolt, slide the impeller off the keyed engine crankshaft. If there isn't a bolt, the impeller probably threads on the crankshaft. It's setup to self-tighten as it runs.
Remove spark plug, turn engine by hand until piston is at bottom of cyl. bore, feed soft nylon rope through plug hole, rotate engine by hand slowly until piston comes up and compresses rope, thereby preventing engine from rotating further, place a block of wood against impeller, strike with a hammer in direction of normal engine rotation.
Caution....most impellers are cast. They can fracture.
