Re: 1979 Mercury 9.8hp
I doubt if the 89 octane gas made a difference, but 87 is fine. With lower compression engines, the higher octane can actually fire less efficiently. It's made to work in higher compression engines. But, I doubt if that would make a night and day difference like what you're describing.
You can get an OEM service manual for your engine. I'm not sure if they have one for yours, but I got mine at oldmercs.com, and I also have an OEM "owner's manual" which has a different mix of info in it and I find handy.
I've found the Seloc service manuals for the little mercs to be OK, so maybe you can find one locally, quickly.
OK, there are others on this board more expert than I am, but this is what I'd do if it were mine, more or less in order (in other words, I'd work through this until it helped, and might not do everything else until I got the inclination to check out the entire engine).
The fuel filter is probably right near the fuel pump and that would the first thing. I don't know where all this is for your engine, but for mine, the fuel line comes into the engine area (under the cowling), and attaches directly to the filter on the starboard side of the engine (right side as you're facing forward in the boat). It could be a tiny cannister type thing, or as simple as a little screen inside a disk shaped plastic bracket/cover. Then, the next thing in line is the fuel pump which could be a flat outer metal part which sandwhiches rubber diaphragms and gaskets. From there the fuel gets pumped directly into the carb.
I'd see if the primer bulb sucks flat and/or if pumping it gives you the RPMs and speed when trying to run at high speed. If it sucks flat, there's something restricting fuel before that point - air vent closed or clogged, clogged pickup inside the tank (could be intermittent from debris in tank), etc. If pumping it helps, it's probably a fuel pump issue and that's generally very easy to rebuild, like a 10 minute job and your local Merc dealer should have the rebuild kit and it should be cheap.
Double check spark plug specification # and gap. Is it possible you accidently changed the gap when you cleaned them? Use the actual recommended brand whether that be Champion or NGK, etc. I doubt brand could make a night and day difference, but in the long run is probably best. Local dealer can give you this info if you're not sure.
Check compression in both cylinders. Should be >=100psi and fairly even (within ~10%) If one or both is really low, it could make a night and day difference. Report results and if bad, a decarbon procedure might help.
Check actual strength of spark using an adjustable spark tester. This is the one I have:
http://thexton.com/index.cfm?pageid=42&search=404, available many places on line or at auto parts stores. Thers's other similar ones and somewhere you can find home made versions.
This will tell you the condition of the ignition system. Weak spark in one or both cylinders can cause a night and day difference. If you report results, and they're not good, folks can advise you on how to figure out which parts might be bad, how to troubleshoot, etc. A lot of this stuff is very easy to fix.
Since the guy said he cleaned the carb, that might not be a problem. However, it could have gotten messed up and cause your problem. The service manual will tell you how to take apart, clean, adjust etc.