new prop on my scr 2400

[RCTcroptech]

questions for the forum, my mechanic recommend I change my prop to a smaller pitch. With my new prop my wide open rpm is 4500 top mph is 28 but I doubt my speedometer is accurate. as soon as I pull back to cut the rpm down a little she falls back in to the whole, shouldn't this boat stay in top with less than wide open throttle??. I am not one to run wide open all the time, I would like to be able cut the throttle 33% and just cruise...At what RPM should this scr2400 cruise?? and and what pitch prop is correct for my boat at 6200ft elv.
5.7 mercruiser 15 pitch prop. Thanks for the help this new prop has really messed with my boat and I am not sure it was such a great ideal

[mmwjr]

I have to first ask has the carb been rejetted for that altitude?

This is a must as the air is much thinner.

Next question is what drive do you have? Alpha, Bravo?

At 3600 rpm I can cruise over 30 mph with my 2400 SCR 5.7LX Bravo 2 at near sea level.

[RCTcroptech]

Thanks for the reply Mike,
the boat has a mercruiser 5.7L with a Alpha outdrive. She was in lake Powell at the bull frog area elevation is 3600ft. There must be a significant amount of power loss with gain in altitude most of my friend's I know at sea level get allot more out of their boats than we do at this higher altitude, what is the amount of power loss per 1000 ft gain in elevation?
Thanks For reply Bob R
Maxum SCR 2400

[Kevlar7r]

Lake Powell is at higher elevation, but according to the carb builder I had work on my boat, they don't begin rejetting until above 5000'.

[Phillbo]

Powell is at about 3600' but in the OP he mentioned the boat was at 6200'.

[mmwjr]

Bob have you or do you plan to run the boat at lower elevations? If so I would see how it performs there before making any changes.

[RCTcroptech]

Yes the boat is ran on weekends on jordanell res 6200ft elevation.., but we have plans to take the boat to lake powell at the end of the season. Is the a way to jet the carb so that it will run in both locations??

[oldno7]

My general rule of thumb for elevation power loss is 3% per 1000

[RCTcroptech]

Nearly every lake I run in is going to be over5000ft elevation utah lake is 4850 and every thing else but lake powell is higher in elevation.. I am not sure if I should jet the carb for my area or leave it as it is..

[mmwjr]

I would talk with a few mechanics and see what they recommend.

[Kevlar7r]

I think you are going to be permanently challenged on carb jetting. I used to race motorcycles across the country and carried jet kits, and would tear down and rejet my carbs for the particular track. The bikes were further compounded in that we had to run the center 2 cylinders (1 carb per cylinder) richer to maintain even cylinder temps.



You may want to invest in an extra carb jetted for Powell, and swap carbs for there. You are talking about 3000' elevation differences. Might pay for itself in a couple years in fuel savings.

[oldno7]

My daughter and I used to race snowmobiles, anywhere from 3000'-10,000'+.
Carb tuning is a science, I think most boaters are good with a happy medium on tuning.

Elevation is only one factor, heat or cold plays a big part as well as stated by Kevlar in tuning inside(hotter) cylinders different than outside cylinders(exposed to more air).

And then throw a prop into the tuning equation and we can all be thankful for MPI

[RCTcroptech]

Thanks for all the replies and great information, I am having the carb jetted for 6000 ft, if this make the difference, I will get a set of jets for lake powell and change them as needed, , as always a wealth of great information,, thanks to everyone who replied