Random Rough idle, and cruising

I have a 1985 Nautique with the 351 engine.

For starters last summer I had my holly carb re-built and replaced my wires and plugs. This past spring I replaced my distributor with a new electronic Mallory distributor and also replaced the fuel filter/separator.

All summer my boat ran smooth until about a month ago, I started to notice that at cruising speeds or any speeds that it seemed like the engine was vibrating a lot but it was subtle. My boat sat in the boat lift at my cabin pretty much all of August until I went to use it this past weekend. It took longer to start and was running even rougher than it did a month ago. To the point it wont even idle without sputtering until it completely just dies. Even when in gear the entire boat vibrates. So here are a couple things to note.

Before I started it this past weekend I added Rec90 fuel to the boat.. I wanted to try it out on my boat instead of using fuel that contains ethanal. At the time of fueling my boat with Rec90 fuel my tank was nearly empty. I want to rule out the rec90 fuel because like mentioned earlier the boat started to run rough a month before.

Secondly, when I was running the boat in my boat lift and a higher RPM for several minutes I was observing the exhaust... The exhaust on the driver side seemed to be very whitish/suddsy more than the passenger side. It also smelled more like raw gasoline rather than exhaust fumes, I could also see a skim of what look like gas in the waters surface. So my thoughts are the engine is running rich somehow. I didn't want to mess with the spark timing or the carb again since everything was running fine all summer.

Next step was to check all the plugs, I pulled each one and they where all pretty dark and some more dark than others. I started the engine letting it idle at a higher RPM to keep it running and pulled each spark plug wire off one at a time to see if I could sense any engine changes. and not much was observed.

Fourth step I went and installed a new set of spark plugs. It seemed to take a couple min before the boat would even start this time with the new plugs but when it did start it still ran rough as it did before.

So my conclusion is that the engine is running very rich and I am not sure how it could be running so rich in a matter of a month. So before I start to messing the carb again fuel mixture screws I am looking for alternate input as to why this could have happened.

Thanks again.