I have a 33OL with the carb HP500's. Up until this past weekend they would turn 5000rpm all day... The last couple of times out the port side engine would run up to the 5000 mark but immediately start backing off like a knock sensor was pulling timing out... It would still run good up to about 4000 rpm so I just stayed below that. I'm due a tune up because this is approx 180 hrs since the last one. It starts and idles perfect and doesn't miss at all. Could the timing have crept on that engine causing the knock sensor to retard it??Do these engines even have knock sensors?? I'm pretty sure they do. I'm going to replace cap and rotors and plugs and reset the timing.. I was just curious if this had happened to anyone else... THat engine also has a new fuel pump. I will check the fuel pressure too. Could be leaning out causing a knock...

what is the stb engine doing ...........high heat and humidity could cause a pinging issue.....is the fuel good

edit: what ignitions are you running

If it was fuel or weather I would think both engines would do it... I'm running the stock mercruiser ignition.

check timing on both engines, have you swapped ignition modules side to side to see if it makes a difference

SOunds like fuel starving up top...

you can verify timing but I doubt that's it, no knock sensors on them with carbs.

I would check fuel pressure through out

TB 5 with knock control, ran with carbs............you dont need EFI to run knock sensors

I just replaced that fuel pump last year. It just quit running on me. I have a new back up I keep that I will swap out and see if that cures the problem.. Running lean is all I can think would cause the timing to back off like that.

whats the fuel pressure say

Did not know they did them with carbs.

Do the tune ups first. A few years ago, one of my 502mpi's ran perfect up to 4000rpms then would start missing. Felt like it was hitting the rev lmiter.
Changed dist cap(had corrosion) and rotor and it was fine to 5100rpms.