the air is denser so more oxygen in the cyl= more power and will lean out the mixture slightlyLast I checked, the timing was on right unless it somehow jumped? But i dont understand why it runs good on cooler 50 degreeish days but runs not as good on days like this.
if you have the time I would pull them all one at a time and have a good look, just to be sure ones not fouledIll pull a plug tomorrow, is there a plug that would show the best signs? The motor is 42 years old but its been rebuilt and the heads only have 5k miles on them along with the ignition pieces too.