RE: Setting up an OS 1.60, cline, bisson pitts, 18x6w
Listen, here's another thing, How have you placed the regulator?
Is it fixed to the body of the engine with a zip tie and a short pipe to the inlett nipple?
Floating free with a short pipe to the inlett nipple.
Attatched to the firewall with a long pipe to the nipple etc etc?
Believe me, I've seen them all used and working well, it all depends on the engine as it and the regulator placement have to compliment each other.
Still try it with a larger prop. Anything, just get the R's on 8,500 or fractionally under/ over.