Would higher altitude affect outboard from lower altitude?

[mountainboy]

I just got an outboard from MN. Johnson 20 model#20CR78C (this is a 78 model, correct?) And It is idling a bit rough. I have a manual for it, but before I get into it further I was wondering if the higher altitude can affect it's performance. If so, what would be the next step? Thanks, MB

 

[radsrh]

Re: Would higher altitude affect outboard from lower altitude?

Yes the higher the altitude will make the motor leaner, before you run it to hard I would go have a talk with a local Johnson dealer and see what they would recommend for a base jet setting based on your altitude. You want to do this sooner than latter the difference could cost you a burnt piston or two.

 

[1woody1]

Re: Would higher altitude affect outboard from lower altitude?

Some manuals list jet sizes for different altitudes you will also probably have to prop to a lower pitch for best performance.

 

[Forktail]

Re: Would higher altitude affect outboard from lower altitude?

"Yes the higher the altitude will make the motor leaner"<br /><br />Sorry, but that is incorrect.<br /><br />Air is less dense at higher altitudes. Therefore the air/fuel ratio will be richer since the fuel metering doesn't change and there is less oxygen to mix with that fuel.<br /><br />This is probably why you are experiencing a rougher idle at higher altitude. Rough idle is a prime indication of a rich air/fuel mixture.<br /><br />The best solution (if this is the cause) is to jet one step leaner (smaller jets/smaller number) with your pilot (or idle) jets. I would leave the main jetting alone. Altitude change isn't generally an issue on newer outboards.<br /><br />FYI temperature change is just as important as altitude change. Colder temperatures cause air to become more dense. This increases the amount of oxygen present in the air and causes a lean condition. Hot weather causes the air to be less dense and contain less oxygen and thus a rich condition will exist. Obviously best performance for any engine can be obtained at low altitudes and cold temperatures. <br /><br />Your outboard will burn a piston due to a lean condition at lower altitudes long before doing it at higher altitudes.

 

[billy-bobby]

Re: Would higher altitude affect outboard from lower altitude?

this is an interesting topic. I usually go fishing at lakes at 3500 ft altitude, but I live at sea level. I just bought a 9.9 Yam 4stroke, and it is tuned at sea level. Should I expect problems when I go fishing this spring? Isn't there a computer control in this motor to compensate and adjust for required optimum fuel/air mix? Thanks.

 

[Forktail]

Re: Would higher altitude affect outboard from lower altitude?

Your Yamaha 9.9 4-stroke doesn't have a computer controlled ignition or fuel system, just basic carburetion. So it will run a slightly richer air/fuel mixture at higher altitudes, but you probably won't notice it. You certainly won't experience problems because of. As a newer 4-stroke outboard, it's designed to run at many various altitudes and temperatures right from the factory without adjustment.<br /><br />Many of the older 2-stroke outboards, like the 78 Johnson mentioned in the original post, are more sensitive to altitude and temperature changes.<br /><br />Most carbureted outboards do have a low speed or air screw adjustment to lean or richen the idle mixture. Higher speed adjustments can only be made by changing the actual jetting. <br /><br />Your going to love that 9.9!

 

[billy-bobby]

Re: Would higher altitude affect outboard from lower altitude?

thanks Forktail, I am looking forward to putting the motor in the water. I have read many of your posts and have learned quite a bit from your information and opinions you provide. As I start my new partnership with this motor, I may pick your brain peridically on this board.<br />Keep up the good work.

 

[Boatist]

Re: Would higher altitude affect outboard from lower altitude?

Mountainboy<br />I agree with Forktail higher Altitudes will make motor richer not leaner. Before you make any changes, what kind of elevation change are you talking about?? I do not have a manual but for idle I think the 1978 20 HP Johnson has a idle mixture adjustment. The high speed jet is fixed. Also for higher elevation I advance the timing a little. If you are only talking 2000 feet or less change I do not think need to change any Jets.

 

[mountainboy]

Re: Would higher altitude affect outboard from lower altitude?

Thanks alot for the great information! This site has been a big help to me. <br />Boatist and Forktail--the elevation that I am at is around 5500 feet. I did not add on my original post that the motor sat in storage for 3 years and the previous owner started it every fall to winterize only. Last night I was looking through my manual and found the section on "High Elevation Modifications" and it does not really tell how to rejet the outboard. It does give the setting--low speed=Adj., high speed=43D. I can honestly say that I do not know what this means. I also do not know really what rejetting means. I can really use more information about how to rejet if needed. Thanks again for everything!!

 

[mountainboy]

Re: Would higher altitude affect outboard from lower altitude?

I was just looking through my manual again and from 1973-1979 it does not list the settings for the 20 hp, but from 1980-1982 it says 20/All, so I figured that I should use the setting that I posted previously. It lists the 25 hp 1978 as 45D would this be the setting instead of the 1980 setting? Thanks again

 

[mountainboy]

Re: Would higher altitude affect outboard from lower altitude?

Boatist--if is do not need to rejet that would be great. I can handle working on the idle mixture adjustment and timing. Thanks

 

[rickdb1boat]

Re: Would higher altitude affect outboard from lower altitude?

mountainboy<br /><br />The Manual is referring to "high speed=43D" as the Jet size.(43D being the number of the Jet you need to use) You need to physically remove the jet and replace it for high speed operation. The "low speed=Adj" means just that. Adjust the low speed needle for best operation. The jets should be available from the dealer.

 

[Forktail]

Re: Would higher altitude affect outboard from lower altitude?

Correct. But make sure you use the right jet...there is a difference in the 43 and 45.

 

[mountainboy]

Re: Would higher altitude affect outboard from lower altitude?

Which jet would be best, 43 or 45? Thanks alot! MB

 

[rickdb1boat]

Re: Would higher altitude affect outboard from lower altitude?

I would stick with the high speed jet you have now and try to smooth out the idle by adjusting the low speed needle. The jet will have no effect at idle and you probably don't need to change it anyway. As forktail said, it will only run a bit rich, if at all. It sure won't hurt anything to run it jetted the way it is...

 

[mountainboy]

Re: Would higher altitude affect outboard from lower altitude?

Thanks again!!

 

[marty_scher]

Re: Would higher altitude affect outboard from lower altitude?

mountainboy<br /><br />You might be still considered a "flatlander", compared to where I run my Johnson, at 8500 ft. <br /><br />I worried how my new 2001 40HP would run after being factory adjusted in Wisconsin. I could not even find jets and did not want to void the warranty. My boat mechanic said there should be no problems. Seems to run fine at both 8500 and 5500 feet (two lakes, 40 miles apart). Top speed difference is minimal.<br /><br />I agree with the others, to concentrate on the idle adjustments and not worry too much about jetting.<br /><br />Marty

 

[kglinz]

Re: Would higher altitude affect outboard from lower altitude?

Even with proper jetting you will lose 4% of your power per 1,000 feet of altitude unless you turbo or supercharge.

 

[radsrh]

Re: Would higher altitude affect outboard from lower altitude?

Sorry I stand corrected, what I listed was backwards I typed that right before I left town for the weekend and relized it was wrong. Sorry for the misinformation.

 

[rons boat]

Re: Would higher altitude affect outboard from lower altitude?

Fun listening to you guys and I decided you needed a pilot to keep you straight though most said it right. When you gain altitude you must lean the mixture as it is too rich and improper burning takes place and wastes fuel, but does not harm the engine except maybe fouling the plugs. A normally aspirated engine like your engine should be tuned for the higher altitude in which you normally run the engine for efficent smooth operation. This means after leaning your carbs you don't want to forget when you return to sea level to richen the mixture agian as this is the point damage could occur. Airplanes unlike outboards have a pilot controled mixture to keep your mind straight as every flight requires a change in altitude unlike your boat. HA Ha