[Christ]

Estimating lower on the scale from my figures earlier, the sqrt of 3 is 1.72, at 2500/2=1250rpm.

So based on the information I read:
~12hp @ 5000
~3hp @ 2500
~1.72hp @1250
~1.31hp @ 625

My engine idles at 900.

Anyway, based on what you posted from Robert, (4hp @1500), its more like 50% than 75% of total friction. However, we're gleaning data from 3 different engines,.as well. No good way to tell for sure without actually testing.

I believe the balance shaft actually spins at twice the crank speed on some engines, as well, further skewering the possibility of getting usable data from guesses.