Re: adjusting carbs for high elevation use? 5500-9500 foot elevations
So I got a used 1991 18ft Capri bayliner with a 90Hp Force outboard on it
I think you might be fighting a loosing battle since an engine looses 3% power for every 1000' of altitude. That 90HP outboard will only be putting out 65HP at 8800', and 65HP on a 18' boat is severely underpowered, probably 50% or less of the hull's rating. My old 16.5' Seaswirl with 140HP (102HP after compensating for altitude) fought getting up on plane at 8800', but was able to after rejetting and re-propping. It was slow to plane, but was still able to get up an plane in about 100 yards. The SeaSwirl had a 19P prop for 1000' and 15P prop for 9000'.
With that said: If you are going to be at 5500-8800 I would try jetting for 5000 and see how it does at both 5500 and 8800. It's going to be running rich at 8800, but it's a real PITA to re-jet a bunch of carbs (multiple times a season) so I'd try to find a happy medium. I normally run at 1000', but have jetted for 9000' (Fish Lake, UT) and it's kind of a hassle jetting the carb for high altitude and then turning around and jetting it back to sea level the following week.
Have you re-propped yet? I would try this first and see how the boat reacts at 5500'. Re-propping might work at 5500', but I think you will need to re-jet and re-prop for 8800' for best performance and the ability to plane. I'm no expert by any stretch, but would hope you could have the outboard jetted for 5000' and run fine at 5500', then while running at 8800' you would need to swap to a prop with less pitch for 8800',,, If you have enough HP.
Honestly,,, I think the outboard is lacking the HP to plane your boat at 8800'. While fishing up at 9000' I've noticed that most of the boats that can easily plane are boats with a high HP/weight ratio (max HP hull rating), boats with a low HP/weight ratio were not able to plane.
