1995 Mercury 115hp outboard running rich

I have an intermittent issue with my 1995 Merc 115hp outboard. I think the problem is that it is running rich, but I don't know for sure. The most obvious symptom of the problem, is a change in the sound of the motor while cruising. The sound change is best described as a lower pitched droning sound, kind of like when you would turn the choke on when a motor is running. When the problem occurs, there is no noticeable change in RPMs, but I feel like there may be a slight drop in power. As I said, the problem is intermittent, but also erratic. When it occurs, it can last a few seconds, or several minutes. I have replaced the spark plugs and fuel filters. I'm confident it is not a fuel quality issue, as I have completely drained the fuel tank and many times. I also believe that I am using quite a bit more fuel since this problem began, but that may just be my imagination. I have had this motor for 20 years, and only put 10 to 20 hours on it each year. I am meticulous about winterization at the end of each season. ie: fogging and fuel stabilizer. I have never had the carbs rebuilt, and I'm wondering if that might be where I should start?

Sorry, I should have mentioned that it's a 2 stroke. I believe this outboard is referred to as the 2+2. ie: runs on 2 cylinders at low RPM, and then the 2nd & 3rd cylinders kick in at around 1500 RPM.

If you are going to do the work yourself buy a FACTORY manual. Then do a compression and spark test and test the stator and all related components.

Cylinders 3 and 4 kick in at around 1800 RPM I believe.-----All 4 cylinders have spark all the time.-----Bottom carburetors kick in at + 1800 RPM.

Where is a good place to buy a factory manual for the boat (1990 Fisher 18 GT) or the motor? I have a Mercury 115 hp 2 stroke motor. Thanks.

Did you figure it out yet?

Sounds like you may be having a cylinder dropping out. I have a similar problem with the same engine mine would intermittently drop a few hundred RPM at speed. I found a coil issue, cracked . Check the Ohms readings on your coils. Measure resistance between the 3 wire connection point 2 at a time. I found the the point between the spark plug wire and the + side of one coil was measuring different than the the other three. if i understand the book right it should be 900 to 1100 ohms my bad one was 256k ohms. The other 3 coils were 0.93k ohms (930ohms) . When i pulled the coil out of its rubber pouch i could see the crack.

I just received yesterday a new coil. The brand new coil measured 245ohms So back it goes, i have another one coming Monday, hopefully it will measure correctly.