I have a 115 2cyc merc, 94 mod. I have had an issue of clogging up the fitting of wp discharge hole for the water pump. I took it off and got a 3/8 hose barb to some threaded fitting, and mounted on engine. This opened the discharge from about a 1/16 to about 1/4 or 5/16. I had a friend tell me that the cooling system requires this back pressure to cool properly. From my research, I have never found that, but I asked BPS parts, and he said he didn't suggest this. Does the cooling system require a back pressure, and would I have issues if I increased the diameter of discharge hole to about a 1/4"? I kept the fitting, so I can replace it. I also put a clear line on, so I can see if it is clogged.

I ain't no guru but I think your thermostat(s) and pop-it valve provide all the back pressure it needs. Once the water leaves the block via the thermostats opening it is free flow. Don't know that hole size would matter. When the t-stat is closed, no tell tale. T-stat opens, tell tale starts. That's the way you know it's warmed up enough to run wide open.

I have a 70 Johnson and I have done the same to mine. I haven't had any problems, the water comes out warm.