jetting for higher altitude?

[sun runner]

Hi, new to the forum and just bought a 82 sun runner with a 120hp merc/gm 4 cyl I/O
boat runs and idles great but wont get up on plane if more than 2 people in it.
I got the boat from a local dealer that took it in on trade for a camper. boat came out of California,
I am thinking it may need the carb rejetted for the altitude 4400 ft in slc, utah.
could this be my issue, already tried a new prop but didnt help much. went from a
15.5" 19p (3 blade) to a 14.5" 17p (4blade)
my only other guess is jetting as the motor runs great at idle and with 2 persons on board. planes out a little under rpm at 3800rpm but have not got it to plane with new prop.

 

[jtybt15]

That extra blade causes more drag. Even that 15.5x19 is quite a bit of prop for 120 HP. What size boat? What gear ratio?

I have a 20' with 180 HP and the 15.5x19 is too much prop...but it's an old engine/boat. I use the 16x16.

Re-propping is the recommended method but reading the plugs will give you a better idea how the fuel/air is burning. You can try leaning it out some. What carb?

Oh yeah, OB or I/O

 

[sun runner]

1982 17' open bow
2.5L 4 cylinder I/O with 1.98 ratio gear
carb is rochester 2bbl

 

[jtybt15]

You should be able to turn that prop with that gear ratio but you should also see how the old prop performs or try out new props. It just may be that you won't be getting your rated HP at that altitude.

The plugs will probably show rich (black soot) but as long as it's not so bad that it's fouling the plugs, You're probably OK with jetting. You'll be putting the engine under more load than at sea level so a richer setting is preferable than lean (too much air) which can cause detonation.

Yeah, go slightly lower pitch prop rather than change jetting.

 

[GeeBee]

Reality is, with the temps at mid day out there, he is closer to 6500' elevation (density altitude). WOT properly leaned is only going to produce about 80%, if that.