changes for elevation

[hunterfishercamper]

I have a 1963 18hp Evinrude Fastwin that I will normally run at elevations of 5000'-10000', I'd say 80% of the time. I will be making trips to areas between 1500' and 3000' several times a year. I'm guessing this engine will run WOT until I get to where I want to fish (10-30 minutes) then troll for several hours, then WOT back to camp.

What is the best way to compensate for the loss of elevation? At 10000' I am losing 30% of my power and even at 2000' I am losing 10%.

I will be setting the carb at my house around 5700' so I was figuring on setting it just slightly lean. I can always fine tune it while the motor is running on the lakes. I only run 89 octane (our super unleaded) with sea foam in small engines, never anything less. When I get out of state I run 91 with sea foam.

The manual calls for Champion J4J, AC M42K, or Autolite A21x all gapped at .030". I had been planning on setting the points exactly at .020" (really devoting my time to getting this right on), upgrading to the next hotter plug, then gapping the plugs at .035". Also, with the wide selection of plugs today, is there a better style (ie: ez start, eco, etc) than the old ones? I want to deliver a nice strong spark that will be sure to ignite every molecule of fuel and burn every atom of oxygen.

 

[Chris1956]

Re: changes for elevation

On my Mercs, a main jet change and idle adjustment was called for on altitudes above 5K feet. I would think the J/E motor would be the same. Someone will be along to answer that question.

I have never heard of changing spark plugs for altitiude. The J4C is the std replacement plug for J4J. make sure the points are clean, the coils are crack-free and the wiring is clean and tight. Also check the plug wires for arcing and cracking.

 

[steelespike]

Re: changes for elevation

First off high octane is actually harder for the motor to operate.Our 87 octane is plenty.
Adjust your high speed needle valve at elevation and just slightly toward rich
so as not to skimp on lubrication.
I wouldn't reccomend a hotter plug if you will be running extended wot.
Some do suggest .035 for trolling.If there is a thermostat be sure it is working
You will surely need a lower pitch prop.A Tinytach will help determine what may be best.
When adjusting the low speed avoid setting too lean.
Of course keep weight at a minimum and well balanced fore and aft.

 

[hunterfishercamper]

Re: changes for elevation

The engine is getting all new points, condensers, plug wires, and if it needs it I will also put on new coils. I am replacing the thermostat while I have it easily accessible.

I am curious about the higher octane making it harder on the engine, I have never heard that before. I know my truck gets 2-3 mpg better if I run 87 vs 85. Though the only time i notice a difference with 89 is above 8000 feet.

 

[hunterfishercamper]

Re: changes for elevation

I just did my research on the fuel, that is interesting. I know many mechanics who says always run super in small engines.

 

[nwcove]

Re: changes for elevation

im admittedly no expert on this topic, but you should be able to compensate for the elevation change just by setting the high/low and keeping an eye on the plugs. i wouldnt set it lean under any circumstance.

 

[fishrdan]

Re: changes for elevation

If leaning out the carb for high elevation, make sure you reverse those changes at low elevation so you don't run the OB lean and burn it up, melt a piston. I would have different props for high and low elevation, as SS suggests.

I also agree of not using high octane fuel at high elevation. Gas stations at high elevation sell a lower octane gas than at low elevations. Around here, regular unleaded gas available at high elevation (5000') is 85 octane and regular gas available at low elevation is 87 octane. I've even seen 83 octane fuel at really high elevation.

 

[64osby]

Re: changes for elevation

All the small engine guys I speak to say to use mid grade. I use this in my 4 stroke lawn mowers / garden tractors. It has been good for me but science says it is a waste.

Higher octane burns or ignites slower.

F_R posted in a different thread today.

BTW, high octane gas ignites slower than low octane. That's why it doesn't detonate in high compression engines. High compression makes more power IF the fuel doesn't detonate. Also the reason there is no advantage in running high octane fuel in a an engine that doesn't require it for high compression.​

 

[hunterfishercamper]

Re: changes for elevation

From what I have read it seems that you actually get more power from a lower octane fuel, i tiny fraction of a percent more. Since it burns faster, it expands faster, pushing the piston down faster. The lower octane fuels are supposed to carbon up less too since they burn cleaner. I also read that running 100% gasoline (no 10% ethanol mix) will help, especially at higher elevations, so I may have to give that a shot at some point.

I am not going to set it lean. Its just too risky. Since these engines are simple to adjust I can always adjust it while I am running it. I'll just make sure to mark where the normal setting is.

 

[kfa4303]

Re: changes for elevation

yup. Hi octane gas is for modern high compression motors, not low compression motors like these old outboards. You want to delay ignition by using high octane gas in a modern engine so that maximum compression can be built up in the cylinder before it fires. However, if you use the same gas in a low compression motor it will fire too late (i.e. slightly on the down stroke, which results in less compression, therefore less power) which is less efficient. Good 'ol 87 octane with TCW-3 oil is fine. No need to run Sea foam each and every time either. If you're worried about lubrication just add a bit more oil to the fuel, which is not a bad idea on older motors any way.

 

[steelespike]

Re: changes for elevation

If a motor is running a little hot or is timed wrong higher octane will lessen
preignition.
The Ford ranger 4 cylinder I think in the 80s or early 90s were rated for 87 but would ping
or knock dealers said no problem to run it that way if you don't like the knock run a grade higher gas.