[Uhehner]

I agree with the above diagnosis. One more question. Are both props the same size P & D and can you spin the prop shaft by hand. If not you need an engine alignment. You say you changed the risers. Did you also do the elbows? When did you clean out the thru-hull valves? Critters love to live there and reduce the water flow. Also check the fuel tank vents. If they are plugged open the fuel fill caps and listen for a hiss.
A good mechanic should be able to adjust your throttle handles to within a 1/4" of each other. Lastly, a low water flow will cause the exhaust to be loud.
Capt. George

One more thing to check besides the old zincs blocking the coolers is pieces of water pump vanes blocking the flow. Sometimes a back flush washes them out. 
Capt. George