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Old 03-11-2009, 10:30 PM   #1
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Default Merc 5.0 lacks power at altitude

2000 2400 SCR with 5.0 with Alpha 1. I live in San Diego and at sea level the boat operates great, last couple of years I've taken it to Lake Powell in August and the boat is a dog! I've tried every prop size known to man with no success. The engine just will bog down when any throttle is applied and acts like it's gasping for air. Should I rejet or consider swapping carburetors?

I had this same problem my 1900 Maxum I had that had a 150 Merc outboard on it. Sea level the boat was great, when I took it to Lake Powell nothing.

I believe Lake Powell is about 3800 - 4000 feet

Any help would be appreciated.
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Old 03-11-2009, 10:40 PM   #2
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at those altitudes..the carb shouldn't be struggling...but ...if you have the 2 bbl carb then yes it's gonna choke alot....before doing anything to the carb while your up there...check to see if the choke plate is in the full open position....if not..see if it's binding any and try to work it loose...

if that still doesn't help..then you may need to rejet for higher altitude..which means a larger air jet instl....which will lean out at sealevel....so then you'd have to richen up again...one way to solve the problem is fuel injection...it will adjust the air/fuel mixture at any altitude...


oh...and welcome too the zoo...


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Old 03-11-2009, 11:35 PM   #3
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Thanks
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Old 04-08-2009, 03:00 PM   #4
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I have the same problem with my motorcycle when crossing the mountains.
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Old 05-15-2009, 04:09 PM   #5
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Default Re: Merc 5.0 lacks power at altitude

This sounds like a carb problem. It's esactly the same with dirtbikes and snowmobiles too (the extent of my experience with carburator tuning, but the principals apply to any carburated engine). I would contact a dealer in Lake Powell and ask them what the standard jetting for the Pilot and Main are in that area for your boat. He should be able to tell you the setting for the pilot screw and idle adjustment too. Compare to the standard jetting at sea level. I would bet that you're running rich. There needs to be a balance in the air to fuel ratio delivered to the carb. The pilot and main jets are staying the same when you relocate the boat, but the air is thinner at altitude causing the fuel in the fuel to air ratio to increase.

Once you have the right jet sizes and settings and you've purchased the jets, rejetting should be fairly straightforward.

Steve
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Old 07-22-2009, 07:32 PM   #6
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Default Re: Merc 5.0 lacks power at altitude

Okay,
Just back from Lake Powell and the boat ran great! I dropped 1 size in jets and took an old 3 blade prop and had the pitch changed from 14 to 13.
Finally, after years of struggling with power, I was able to get up on a plane without any problems.

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Old 07-23-2009, 02:07 PM   #7
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Default Re: Merc 5.0 lacks power at altitude

Awesome!!!!!

Remember a Carburated Engine loses 3.5% power for every 1,000ft. of elevation even WHEN properly jetted for elevation. The curve will go up far more if left jetted for sealevel. So a boat jetted for sealevel can loose 17.5% power (based on a 5% loss/1000 ft.) or more when brought to 3,500 ft. Proper rejetting for altitude will minimize that loss. So, for example a 225 HP would loose 40 HP , or more, if jetted for sealevel and run at 3,500 ft. I've seen many people spend a significant amount of money trying to capture and increase far less HP.
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