When you say you fiddled with the throttle body screws, did you mean the brass idle air control screws or the throttle butterfly synchronization screws(these are on the linkage between throttle bodies and marked with blue paint)?
If it was the idle air screws, you can get back to a decent adjustment by first removing them one by one and cleaning the bore they screw into with carb cleaner. Then screw them in until lightly bottomed and then back them all out 1 1/2 turns.
Your spark plug doesn't look all that bad, seeing as how you really haven't been able to get any extended run time on the engine. The engine looks like it is running a little fat, but that may be the result of not maintaining normal operating temperature for any length of time.
One common problem is the engine coolant temperature sensor. A failing sensor or dirty/loose/corroded connection will make the fuel injection control think the engine is cold and enrich the mixture. With the engine at operating temperature, the resistance between contacts #10 and #13 on the engine control plug should be about 250 ohms. A cold engine will read about 2500 ohms.
About the rail: when the soak is done, pour out the vinegar in the rail through a coffee filter to catch whatever comes out. Do a good rinse with lots of water and if you can, blow it out with compressed air followed by a rinse with 91% isopropyl rubbing alcohol to remove any water that may still be in it.
