Outboards operating at an elevation

[keninaz]

I have had a few outboards in my time but admittedly the last small outboard was in the mid-1980s. It was a 15 horse that I operated at sea level.
I recently purchased a boat, motor and trailer combo for fishing high elevation lakes in AZ,
I am not getting the performance I think I should out of the motor so far. I am aware I am losing some 15% the total power and due to elevation and I am about change props to a 6.5 pitch (On order) from the factory 8.75 that came with the motor. But even with the lightest load I can put in the boat the boat weighs in around 600# gross and and I only get some 3900 RPM with standard prop on it. This should get me up to around 400 more RPM but still not near the 5700 or so specified for WOT by Mercury.
Both cylinders fire, the motor seems to run fine too. Of note I did take the boat down with a slightly heavier load to 1700' one time I did not have a tach installed then but the RPM was not coming up then either, I could tell. About 6.5 MPH as measured by GPS is about all I could get out of the boat regardless of trim.
Just curious for those with experience at elevations of 5000' and above (my elevation). Do you lose RPM like this regardless of what you do and what I am seeing is the reduction in power due to the elevation?

 

[Sea Rider]

You can lose as much as 15% engine power for every 1 K meters of elevation. The issuse with your engine is pitch and carb jets. The poor speed you're obtaining is due to poor engine performance at elevation which increments hull drag slowing down the combo while trying to plane.

Can play with prop pitches all you want but don't think you'll ever reach max 5700 rpm at elevation if those numbers are the max wot range for that outboard.

Happy Boating

 

[keninaz]

As of today I am almost certain that I have my problem solved.
Today our granddaughter is staying with us and wife wanted to take her for a ride in the boat has at 6 years old this is her 1st time ever in a boat.
So I added well over 300 pounds back to the boat with the extra passengers and gear and while boating I had the passengers sit in the very front of the boat.
So now the Lowe V1467 WT that weighs some 262# and is 13' 11" long is back where I would normally use it fishing and such.
Much to my surprise versus the results I had as suggested with the latest boat possible and only obtaining 3900 RPM the boat was able to reach nearly 4200 RPM and the boat actually planed and maintained that plane quite easily. I did not have the GPS with me to monitor speed but I would estimate that I obtained somewhere between 8-8 1/2 mph
The Dealer also called me and told me that my prop for correction at altitude is in stock and that is an 8.9 X6 .5 which when installed should raise the rpm's perhaps another 300 to 400 RPM.
I noticed yesterday that my plugs were pretty dirty and fouling due to the low RPM range I have been in so perhaps with better balance and load and the new prop I will get enough RPMs that they will burn clean. I could always go to a hotter burning plug to correct this problem however.
I have a few posts going so I will post this in all of them as it may help someone else along the line. I never figured that adding so much weight to this boat would increase the speed. Lessons learned.

 

[Sea Rider]

Don't want to spoil the nice party. Did you need to maintain full wot to maintain plane with that new load ? If so, bear in mind that if with new prop and same load engine achieves + 300-400 revs still will be lugging the engine. Won't be even close to factory safe min wot rpm range. Lugging an engine is going from a dead stop up hill on second gear, too much stress on pistons, connecting rod and crankshaft than will shorten engine life or even ruin it.

Happy Boating

 

[ondarvr]

Also the dirty plugs can be from lugging it, plus running too rich from the altitude.

 

[keninaz]

Well I assume that so called lugging is what results with lower RPM caused by elevation? I don't know for sure but I am assuming that the reduced RPM is due to the elevation. That being true the only thing you can do according to Mercury Marine is reduce the pitch of the prop and I am doing that now. But I still calculate that I will be somewhere around 800-1000 RPM below WOT.
So if the fouling is due to the jets that you cannot change what other option would you have other than to change to a hotter running plug like the old days to keep the motor running clean?
And the bigger question to me is why won't a new 4 stroke run clean, or doesn't the industry care about those of us that live in the mountains????

 

[ondarvr]

You can change the jets, most manufactures offer a high altitude kit. Or you can get them separately.

 

[keninaz]

Mercury does not offer jets for high altitude.
And today, with a full load in the boat, all but the trolling motor itself, and the new 8.9x6.5 prop that Mercury Marine said to use at 5000' I took the boat out and it hit 5400 RPM max and over 12 MPH max. Steady speeds against the wind and all was 10-11 MPH easily.
Problem finally solved.
The only other thing that I did was at idle it was loading up a bit so I drilled the plug they had installed over the fuel screw for idle and adjusted it. It now starts better and idles better and does not load up at extending idling.

 

[ondarvr]

Turning the screw effectively change's the jetting at lower RPMs, it's technically illegal, but it frequently needs to be done even at sea level to get small four strokes to run right.

If you check around you can probaly find a one size smaller main jet, look at the parts diagram, or pull it out and see what size it is. It will cut down on the carbon build up and possibly run better and gain a little power.

 

[keninaz]

Technically it would be illegal for me not to adjust the motor as it would not be running right in compliance with the law. Ever think of that?

 

[ondarvr]

The law states that nothing can be done to the carb, as in a adjusting it, it can be cleaned and set to factory specs, but nothing more. It makes no difference whether you are making it run better or not. That's why dealers are constantly replacing the carbs on small four stroke motors, they are not allowed to adjust them. The carbs are set to meet emission standards when new. This started in 1998 or so.

 

[keninaz]

Point is that above 3000' the engines do not meet emission standards as produced by the factory and they don't run well as a result.
Most of us don't operate at elevations of 5000-7000' like I do.
And the only thing I adjusted was the idle fuel which is only in effect at idle. You can adjust nothing else.
If you were to rejet the carb, which requires disassembly, I would have to rejet if and when I went to a lower elevation or put the motor at risk.
Don't care much about the law, we don't smog cars here either.

 

[ondarvr]

I'm not concerned about the law either, just stating what it is. The idle screw is covered due to the law, but most people here remove it so they can get the motor to run better.

Yes, if you return to lower elevations you need to change the jetting back for it to run well. Some people change the jets as needed, others just put up with the poor performance at altitude because they don't want to mess with it. It depends on how much you use it at high elevations.

 

[keninaz]

As 95% or more of my fishing is in the 5000-7000' range I have done what I could to get better performance out of the motor to suit my needs. As is should I go to Lake Pleasant which is 1700' or Lake Havasu which is nearly sea level I will have to change props back to factory specs. As Lake Powell is at 3700' I think I can run the same setup for elevation that I run here.
Just problems we have to deal with but I got tired of the plugs fouling at idle and near idle and the motor stopping when I returned to idle and restarting. I hot rodded for many years building many of my own motors/cars and got used to multiple carbs on many motors over the years so I certainly am not afraid to make modifications.
I have found it funny that the buyer of small outboards is rather discriminated by the makers of motors. The manufacturers do put devices on larger motors to compensate for idle and such, just not the smaller motors.
Frist time I have ever had to deal with elevation problems and it was a learning process. I was born/raised at sea level and that is where most of my boating experience has been.