If I had to guess what is going on in your motor, it is probably either or both of these....
1. Static compression ratio too high with not enough cam to bleed off excessive cylinder pressure. Using what you perceive as the proper ignition timing probably isn't helping any either.
2. Piston used in rebuild was short compression height, leading to excessive piston deck height and lack of squish. Squish "jets" a part of the fuel/air mixture from the coldest side of the chamber over into the spark plug area when the piston is coming up to TDC on compression. This homogenization and turbulence of the mixture allows a higher static compression ratio engine to operate on lesser fuels which might be the best available at the pump. (Think 11.0:1 on 91 octane fuel as a for-instance)


Using higher octane fuel and no longer having the two issues I described, better mileage too. Thanks again!