I have the option of running a 650 or a 750 for my olds build it's around 9.1 to 1 .030 over forged pistons my intakes a low rise old offy 360 and I'm running the 280h marine cam from comp. Any ideas on jet size? I'm lucky enough to have a pile of carbs lol

Your combination isn't particularly unusual, I'd recommend starting with the original jetting for the carb you use as a starting point.  Make changes as needed based on performance and plug readings.

Your combination isn't particularly unusual, I'd recommend starting with the original jetting for the carb you use as a starting point.  Make changes as needed based on performance and plug readings.

when it was stock, it had the 650 on it and it ran pretty damn good, just didn't know if I could get a bit more out of it. I'm aiming toward low and mid so the smaller seems like it'd be better but hey I don't know if Itl love the 750 more I'm hoping someone has a similar setup as mine

when it was stock, it had the 650 on it and it ran pretty damn good, just didn't know if I could get a bit more out of it. I'm aiming toward low and mid so the smaller seems like it'd be better but hey I don't know if Itl love the 750 more I'm hoping someone has a similar setup as mine

There should be no reason that your engine wouldn't use every bit of a 750.  As far as jet boats go there isn't a low to be concerned with, it needs to idle and make power from 3000rpm on up to the point the jet won't let it go any more.

There should be no reason that your engine wouldn't use every bit of a 750.  As far as jet boats go there isn't a low to be concerned with, it needs to idle and make power from 3000rpm on up to the point the jet won't let it go any more.

ok then I'm getting the 750 on for starters 6.5 pv 70f 80r for jets

Depending on the cam you might want to drop the PV to a 4.5.

Factory Q-jets on Olds 455s were 810 CFM stock! On cars! With smaller cams! Why go smaller on the carb? An 850 DP works great on a Crosswind dual plane intake for the Olds. I have a new one, and I have an Olds O4B intake too.  Start with the stock jets and change the power valve to a 45 (which actually starts the enrichment at 4.5" mercury. The Comp 280 is one of the better (but not the best) jetboat cam with log exhausts. TIMINATOR