Timing

[Joker2]

sounds complicated, maybe I'll just leave it. I am getting 4200 RPM's max running a 15-1/2 X 15 Alum. prop recently repitched for more RPM. Yesterday I got 29mph @ 4200, 2 adults 4 small children, 35 Gall. fuel, new plugs. Feels to slow based on numbers I have seen on this site. tried compression check to see if I got all 6 cyl. firing all the way, comp. tested wasn't holdng, I'll get another one. Also hearing what other people are running sounds like the 15" prop should be a 14-1/2 X 17 or 19. Any thoughts