A few thoughts:
On my 50 HP merc which is newer than yours, there are several linkage rods between carbs and throttle assy that are connected by plastic bushings. I had one of them break and the linkage wouldn't move thus all I got was idle speeds. You can check to see if your throttle is opening with motor turned off just look into the carb throats.
Other than that you probably have either have an air leak somewhere or your carbs need to be rebuilt. I know that a lot of boaters have used after market fuel pump kits successfully but after trying two rebuild kits I finally bought a Quicksilver fuel pump kit and no problems since. Air leaks can be anywhere from inside the fuel tank pick-up tube to the carbs. Check to see if the carbs are tight to the engine. So look everywhere. I took the fuel filter off and replaced it with clear tubing and ran the motor on muffs to see if there were any air bubbles indicating a leak. Of course this is just for testing, I made connectors from 38 special ammo brass and small hose clamps LOL! I would suggest replacing every single inch of fuel line and also the vacuum line to the fuel pump.
If you rebuild the carbs make sure you ID them and put them back in the exact right place. When I did mine I put ID tags on the top, middle and bottom carb and I took pictures every step of the way to make sure I put them back together correctly. Simply spraying carb cleaner will not help and may make things worse you need to take the carbs completely apart and replace parts from rebuild kits. The carbs need to be disassembled and soaked then blown out with compressed air.
Do simple things first then move to the complex. I spent over a year trying to get my motor to run correctly at all RPMs. Running good on muffs isn't the same thing as running good in the water, your motor can run great on muffs with one cylinder not firing because there is no load on the motor. As long as you have good compression then the problem has to be either fuel or spark and most of the time it's fuel but it can be a bear to find the actual problem. Also, the tach on my boat is off by about 100 rpms so I set idle speed using a hand held lazer tool I bought off amazon for about $15.00
I keep a full set of carb gaskets in addition to carb rebuild kits and fuel line on hand, also another fuel pump rebuild kit plus the quicksilver zip tie clamps, all this on hand just in case. Also I keep on hand a rebuild kit for the water pump and the little washer gaskets for the screws that you drain and add gear oil to the lower unit. Get a copy of the factory service manual for your exact motor this will save you much headaches.
