Changed my impeller plugs and fuel filter and everything seemed to go smoothly. Engine was running great before I performed the maintenance. Put muffs on and let the engine warm up. Put it in gear and everything seemed great. Put it in reverse and the lower unit was clunking every few seconds. It was almost like the gears were broken in the lower unit. Forward it seemed good but reverse it was not. I turned it off and restarted only to hear the same thing again.

Mercury 115HP 2 stroke.

Wondering if I need to take it in and what I could have done wrong changing the impeller to have caused this. Please help!

I've got nothing but keep us informed when you find out what it is.

Ttt

Shift cable adjustment. Not fully engaging into reverse. Adjust cable slightly toward reverse.

Actually if you have a stainless prop this is normal when running on earmuffs cause there is no load on the prop. ....OPTI

Ttt

Do you have a Stainless prop have you checked the gear lube check the bottom drain plug has a magnet on the plug well some did some didn't and if you have gear damage the metal from the damage will collect on the magnet. Now you will have some I call it fuzz on the magnet that's normal but if you put the fuzz between your thumb and index finger and rub it between your finger and your feel small chunks there is a problem. I have heard them rattle with a alum prop also but not a loud.....OPTI

Opti is correct a "clunk" is normal when not under a load and it is still possible your shift cable needs a minor adjustment. Cables can wear over time and stretch slightly and need to be adjusted back within specs.