My rinker 236 used to consistently run 57-60 mph. 502 magnum carb. Best speed ive seen is 63.0. This year the boat will only do maybe 53mph. Same prop ive been running 23 plus mirage. used to see 4600-4800 rpm , now it can only turn about 4500. The boat comes onto plane slower than i remember also. I just changed the seawater impeller, all the fins were broke off. I'm not sure i recovered all the pieces could this be causing it to lose some power?

Simple solution after I spent thousands of dollars trying to fix my boat and then getting it back and having it top out at 56 when it used to top out at 70 it is bad gas, or 10% Ethanol. When you see a tree hugger punch the *****.

when you were in there doing the sea pump you did not happen to knock off a plug wire by any chance?

Boat would be running hot if you had a restriction from a piece of impeller. Assuming you are running on all eight cylinders and the bottom is clean, you might do a leakdown test. Might be time for a valve job.

No fins? How long did you run it hot when you lost the impeller?

Start with the water sep/filter change, plugs cap and rotor in case it is something simple.

I agree if you have water blockage from the old pieces you engine would be running hot still, what is the temp gauge reading?

Compression and / or leakdown test may be in order.

Remember that outside air temperature rising kills power. I don't know if you are running in very hot conditions in comparision to previous results.

Sounds like your running on 7 cyls