You asked about 9.9 OMC engines, here is my comment as I have owned several since 1978:
Well, despite all the comments to the contrary,
I have a 1975 Johnson 9.9, with a 15hp carb, and it runs like a champ. I use it to troll, it ticks over like a clock, idles smoothly, starts first pull. Has plenty of torque, will plane a 12 foot boat with three adults in it at 6800 feet of elevation, and since I have owned it (1980) it has been faithful for every season and never left me stranded out on the lake. Prior to putting on the 15 carb, it ran great as well, but the 15 carb helped it with some more power, especially at altitude. The engine sits in a barn under snow for 7 months a year, and always starts right up. In 2011, I replaced the fuel pump and fuel lines, re-sealed the lower unit, and replaced the powerhead with a used one off a 1975 Evinrude 15. The difference was amazing, even more power, and a much smoother idle and trolling. It also now gets much better fuel economy. The reason I put on a new powerhead was because in 1982, I took the head off to inspect the cylinders and saw severe scoring from a previous seizure that was due to the thermostat corroding closed, as the prior owner ran it in salt water on his skiff and never flushed it. Despite the damage, I ran this engine hard for the next 29 years and it never gave me any trouble...good old bulletproof OMC, made in USA, never say die!
In all the years I have owned this engine, I set the points twice, and both times they were within tolerance, but I like to have them perfect. My engine ignition coils developed minor cracks over time, but they still work great and throw a fat blue spark. If fact, you can put your palm on the flywheel, and turn the flywheel, and she will often fire right up!!
One trick I learned with this engine is to run B7HS plugs instead of the colder B8HS plugs that are specified. It is common knowledge that these engines will foul the standard plug, especially if you troll or run low speed all the time, and you are in colder climates. All the marina operators at my lake equipped their 9.9 rental fleet engines with one stage hotter plug. Another problem is that the thermostat can stick open, or it's seal can leak, and over cool the engine, which will foul plugs, so be sure the engine block gets hot enough that it is uncomfortable to touch for more than 5 seconds.
Another piece of common sense is that you need to slightly lean out the low speed mixture for trolling and low speed, and open it a 1/4 turn for cold starts. Now, if someone does not know enough to use the low speed mixture adjustment(its there for a reason) and you don't run a hotter plug, well it's your own fault if the engine acts up and you can't figure it out.
There was an earlier cylinder head with 7 bolts on the thermostat cover instead of 8. There was a change made that re-routes the water circulation in the head for more even temperature, that is the 8 bolt version that came as part of the warranty retro-fit kit by OMC. Both of my 1975 powerheads came with 8 bolts thermostat covers however.
By contrast, I had a 1982 Johnson 9.9 with electronic/CDI ignition and had nothing but problems with the ignition system; it blew out two CDIs, had problems with the throttle linkage sticking, and the plastic parts like the transom clamp levers eventually cracked and broke, as did the cam for the carb on the mag plate. I sold this engine as it was problematic and nowhere as reliable as my 1975.
One thing I do know, and that is to avoid the plastic top carbs that came on the later versions of the 9.9 and 15, as the warp and leak...see Leroys ramblings for more details.
So, despite all the naysayers here, as a longtime owner of one of the early 9.9 engines, I have nothing but praise for them and will never sell mine.