I've got a 26' Chaparral with a 496 MAG with 65 hours on it. Up until last night, I could always turn the motor 4,800 RPM...even a tad higher in really cool weather. Last night I could only turn 4,400 RPM and have lost about 4 MPH on the top end according to the speedo. Boat starts/runs/sounds fine but does feel more sluggish out of the hole. Cruising RPM's and speed appear to be unchanged. I did a quick diagnosis looking for obvious items but everything seems to be in order. Anyone else with a 496 ever encounter this? It will be next week before I can get my Merc tech buddy down to the dock with his scan tool. Thanks!

I lost 400 RPM's on an engine years ago. Spark plug wire fell off. Dont ask how but I would put it back on and an hour later same damn thing. Changed all plug wires next day.
Probably something simple

Had The Same Thing Go Wrong An A 454 Took The Ground Off Of Batt And Reconnected Problem Solved.

Prop bent ?

Bravo III drive and both props are perfect. Doesn't sound or feel like the motor is missing but I will check the plug wires. I'll give the battery disconnet a try as well. Keep the suggestions coming!! BTW, it doesn't start cutting out or anything...just tops out at 4,400 RPM.

Dirty Hull?? I neglected mine last year and I lost even more than that.

probably not overnight though. I would vote for a spark plug wire fell off.

Guardian mode?

Hull is clean. Boat sits on a Poly Lift out of the water. Hull has not developed a hook either...at least not overnight!! I think guardian mode wouldn't allow it to spin up to 4,400. Going to check the plug wires Monday when I get back from LOTO.

i had it happen to me twicei have a 496ho. wire just fell off but when this happened i knew something was wrong you could really tell but the second time i dont know what to tell you. i ran the boat one day and i lost 200rpm and a little on speed and i didnt know why told my mech he took it out and called me an idiot cause he said it ran fine. i checked again and the boat was fine. never happened again. he still makes fun of me til this day and tells me i need to buy jet ski cause i sure dont know how to drive a boat. maybe it was just the water, maybe temp. could be anything but like it was said before, i bet its something stupid. good luck