In my humble opinion, I think a 650 is too dang small for even a stock marine 460. Std Cobrajet style cast iron intake and all, some pretty good hot rod stuff put in those things from the word go.
750's are what Ive seen come on every one I have or had, ( that's, wait 1,2....3....oh wait that one sank....ok 3,4,5, plus two from the Sleek.....yeah, 7 of them here right now.....
) and only one did I try an 850 on that didn't seem to make a huge difference seat of the pants. It did pick up 100 rpm, if you want to split hairs. Granted this was on a 21' cruiser too.
The larger carb is not going to "burn up" more fuel, it's simply able to flow more air, to which you CAN and SHOULD jet up for if the engine is working well with the increased airflow, it's gonna run lean then. Then, yeah, you're gonna burn more fuel.....
You haven't mentioned at what rpm the boat tops out, and what your plugs are telling you. You might well be able to pick up some rpm, maybe a lot depending on where you're hitting 60 at, but it's not gonna be a huge MPH increase. What you will gain, however is better tuning, better cylinder to cylinder distribution on the lean/fat problems, and much crisper throttle response.
Sell the 650 to a small block user or get the other one for a Tunnel Ram......
Ray
