If you got another 100 hp without changing anything except run'n it, then there was/is something wrong.
LOL Maybe 'WAS' but I can assure you it's not 'IS'. And if it fixed itself, 'WAS' wasn't very bad.... It could be that I didn't get all the 'three year old' gas out the tank when I siphoned it. It runs like a top now. Over 300 miles yesterday without a miss.
Personally, I'd be trying to find out what the problem was to make sure it didn't happen again, or happen on the next engine, or cause another problem, and also to see if the problem that was limiting the hp is gone completely, and the engine is now working at its full potential.
Given the change, I'd have a look at the mechanical advance to make sure it is moving freely. I've seen them stick at both extremes of the range, and with three year old gas or even fresh gas, a mechanical advance that tends to stick anywhere but especially at full advance is opportunity knocking - for an engine builder.
