Re: 1984 Mercury 90 hp inline 6 lower unit noise.
Before you tear into anything, check your remote control cable adjustment. Remove your shift linkage from your remote shift cable at the engine. Rock your remote control back and forth and stop in N. Do the same with your engine shift linkage. Hook up the center connector at the engine and look at the brass barrel and the slot it is supposed to fit into. If it's not perfectly aligned run the barrel up or down the cable till it is and reinstall it. Then go out and if you still have noises then you can start worrying.
The thing confusing about this is the rpm. Having noises occur at this rpm and none others is confusing. You have another prop? If not can you borrow one for long enough to test? Is your prop nut torqued....about 55 ft-lbs is the number?
Course at 1200 rpm, things in the water are in high turbulence. The blades aren't spinning very fast, the engine is just above idle and may not be running all that smooth, there is a lot of irregular turbulence in the water.
I have a heavy SS high pitched prop on my 3 cyl engine. Being 3 cylinders it has a lot of time between ignition pulses at idle and just above. When I am at/around that speed, I get a lot of clunking from the lower unit. If I increase rpms a few hundred or so, it quits as I have put enough pressure on the prop to stop the "clutch dog" from rattling. No other place in the rpm curve do I have any noises.
If I remove the prop and put on one with less pitch, like going from my heavy 24P to a smaller blade hence lighter 21P I don't have those noises........................................Nothing wrong with the clutch dog. It's just getting impulses of thrust and none etc. and it is moving back and forth, which is what it is designed to do, and making noise in the process.
The clutch dog is an overrun clutch.....not really a clutch just gears with teeth shaped like a saw blade, whereby when you cut the throttle on the engine, the prop can continue turn (screw) through the water while the engine is running at a lower speed. If it weren't for that, if you were running at WOT and chopped the throttle you would have to pick pieces of windshield plastic out of your teeth. The same kind of critter is used on early farm tractors to keep the mower from running the tractor through the fence when you tried to stop it.
HTH,
Mark
