50 Mercury at altitude

[clovisbobca]

1986, outboard 50 Mercury is running sluggish at 5,000 elevation. Marine shop suggested putting on a 14 pitch prop to solve 90% of problem. Shouldn't jets be adjusted for altitude?

 

[tmcalavy]

Re: 50 Mercury at altitude

Hope someone can answer this question. I also run a Merc 50 hp at altitude, about 3,500 feet, and can't get over 20 mph on the lake...boat full or with just me aboard. Prop is a 10.5 diameter and 13 pitch. Came with a 15 pitch, but its so beat up I just keep it around as an emergency spare. Can't tell you rpms until I get my tiny tach installed, but can only make about 20 mph, according to speedo, at wide open throttle. Gets on plane pretty quick, loaded or unloaded, just not as much top end as you'd expect. I know you can tune auto/trucks for altitude...how about outboards, as clovisbobca suggests?

 

[Terry H]

Re: 50 Mercury at altitude

You might notice a little diference when going up one or two jet sizes, and that should be done if you only run at one elevation, but proper tuning of propeller will help more...just a Thought

 

[martyscher]

Re: 50 Mercury at altitude

Assuming similar designs with Mercury, My Johnson manual states that you should consider both jets and/or prop configurations when running at altitude. Other than that it does not say much.<br /><br />I agonized over this issue when running my 01' 40HP at 8600 Ft.<br /><br />Since I was told there was minimal chance of engine damage and I get 19+ MPH, I decided to leave carbs and prop as is (for now). I did not want to be adjusting carbs on a brand new motor.<br /><br />If you feel comfortable adjusting the carbs, you might try some different jets. Otherwise, I'd suggest a different prop first.<br /><br />Let us know how it goes.

 

[FlyBoyMark]

Re: 50 Mercury at altitude

You MUST rejet.....Mercury will tell you the proper jet size for altitude. The engine has to be LEANED out considerably. You will gain almost 20% more power. At 5,000 feet your producing about 75%-80% power than at sea level. Because of the loss of power you will have to reprop 1"-3" less pitch. If you do not lean out the engine your only wasting fuel, fouling plugs and carboning up the engine. Idle air bleed screws MUST be adjusted too or the engine will idle rough and load up also.<br /><br />JUS' remember, coming back down ya gotta rejet again.........orya' burn it up.