Reading your last post, I forgot to mention that I'm using a GEM box muffler with our engine.
If you're using a pipe, I'll bet you'll need more fuel!
Another thing that may have affected the way the engine runs is the amount of Ethanol that is now in our fuels! When this engine was built (1974), the gasoline available did not have ANY Alcohol fuel added and was probably containing some lead!
Let me know how your engine runs when you get it tweaked.

Terry