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1981 40 hp performance questions high altitude

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greg1

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"I have a 1981 40 HP outboard

"I have a 1981 40 HP outboard on a 14 ft john boat. At Sea level it ran about 29 - 30 MPH with just me, and with another adult it performed at about 28 mph.

I have only run it at high altitude (5000+) twice and I only perform at about 24 mph by my self and barely 15-18 mph with another adult. What can/should I do to adjust for the altitude?"
 
First you will never get the s

First you will never get the speed at 5000ft that you had at sea level. The engine has lost at least 20% of it's hp.
The best thing is to change the pitch of the prop for a lower pitch that allows the engine to turn the correct rpm.

The carburetor jetting can be changed so less fuel is let into the engine but this is tricky.
If you change the jets make one number changes & check the color of the spark plugs until the plugs are an even dark tan to mid brown color.
Some people adjust the timing ahead but I do not advise this on a 2 stroke engine.

The rpm needs to be checked & if it is 400 or 500 less than max rpm you can try the jetting first & see if it improves any. Any lower rpm will need the prop changed first before changing the jetting.
 
they change the jet vent or ma

they change the jet vent or main vent for alt changes
2500-5000 .002 smaller than standard
5000-7500 .004 smaller
7500-up .006 smaller
the vent jet is the one on top with the hose.
need carb part # for the right part
 
the cheapest fix would be just

the cheapest fix would be just change the pitch of the prop u can change jet sizes but its a lil more expensive
 
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