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I've got a question for those of you who are mechanically minded. I've got a 6 hp two-cycle Johnson outboard trolling motor that uses mixed gas. The manual says to use a 50:1 mix of gas and oil, but when I was at Fish Lake recently, that mix didn't work very well. My motor kept stalling out. The guy in the boat house said that they usually mix gas at a 100:1 ratio because of the altitude (elevation 9000 feet). I added some unmixed gas to my 50:1 gas-oil fuel and brought it up to more like 80 or 90 to 1, and the little motor ran just fine. This solution brings up two questions.
1. When I run the motor with a leaner oil mixture, does it reduce the lubrication that the mixed fuel provides and that the motor needs? In other words, am I running any risk of damaging my motor?
2. When I return from Fish Lake (elevation 9000 feet) and go out on Utah Lake (elevation 4500 feet), should I put more oil back in the mixed gas? Then when I go back to Fish Lake should I add more gas to the mixture? This could get confusing!
I would appreciate hearing from anybody who knows about such things--either through physics and chemistry classes or mechanical experience.
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[size 1]1. When I run the motor with a leaner oil mixture, does it reduce the lubrication that the mixed fuel provides and that the motor needs? In other words, am I running any risk of damaging my motor? [#bf0000]In short, yes, your motor is made to run with the mix laid out in the book. The altitude difference doesn't make the friction in the motor less, does it? The only difference elevation makes is you get slightly less O2 into your carb. You can use the air/fuel mix screw on your carb to get more air and less fuel, but unless you have a really old motor, it should not make a difference what altitude you're at.[/#bf0000]
2. When I return from Fish Lake (elevation 9000 feet) and go out on Utah Lake (elevation 4500 feet), should I put more oil back in the mixed gas? Then when I go back to Fish Lake should I add more gas to the mixture? This could get confusing! [#bf0000]If you decide to run a 100/1 ratio at altitude then by all means add more oil when you get lower. My sleds are oil injected and they have no altitude senor, why do they run just fine as high as 12,000 feet? Without getting into the theory of partial pressures of O2 (since Gshorthair will correct me if I'm wrong), the matter of a few thousand feet means nothing as far as engines go.[/#bf0000][/size]
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[#bf0000][size 1]Call Lee at Lee's Marine, he a pro mechanic and knows way more then me. [/size][/#bf0000]
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[#bf0000][size 1]I just fix the stuff (airplanes not boats), that don't mean I can spell.[/size][/#bf0000]
[size 1]I would appreciate hearing from anybody who knows about such things--either through physics and chemistry classes or mechanical experience. [/size]
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I also had the same problems with my old motor.It would always run too rich.I didn't change the mixture ratio but I did adjust the lean / rich screw to add a little more air at the higher altitude.The engine I have now is a Yamaha that has a 100 to 1 ratio and I never have to adjust or change anything at altitude.
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The other guys are right on. It's the air/fuel mixture not the gas/oil mixture that is the problem. You have two solutions, adjust the richness or rejet the carburator. Since you want to use it at multiple elevations rejetting the carb isn't a very good solution. Hopefully your carb is easily accessable and you can merely make a quick adjustment at launch and be on your way.
I've had similar problems with my older gas auger and I used to work at a golf course at elevation (7000) and we had to rejet everything we had with a carb to make it run right.
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I've run Amsoil 2 stroke oil at about 80:1, but it says you can run it at 100:1. I didn't notice any problems, and it seemed to run better, but I've also got an older trolling motor, so I'm not as worried about burning it up.
NAPA used to carry it, but this year they didn't have it so I went back to regular 2 stroke oil.
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I used to have the same problem at Fish Lake with my old 1953 Johnson 5 hp. I.d even adjust the mixture leaner & all that but after a while the thing would start acting up. I only experienced this problem if I was trolling. A guy in Salina told me to try one or two steps hotter plugs. Problem solved. What he told me was that the cold water at Fish Lake kept the plugs too cold & they would begin fouling.
I absolutely WOULD NOT run a leaner oil / gas mixture unless I had an oil that was formulated to run at those mixtures and even at that, if I did I would mix a little to the richer side just to be on he safe side.
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Ok I have a question for all of you. As long as were on the subject of gas/oil mixture does any one know what an old craftsmans outboard motor would use. I had an old old old craftsmans motor that looks like it was use on Noah's Ark and no owner manual, This thing has the gas tank on top of the motor and you have to turn the motor around 180 degrees to go in reverse. Any body have an idea what oil mixture to use?????????
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The best mixture at sea level, air/fuel is 14.7 to 1. leaner it goes in to a lean misfire, any richer and you dump raw fuel out the exhaust.
think of o2 molecules as basket balls the ideal ratio is still the same at altitude but the air density is less so you need less fuel because you have fewer of those o2 balls.
The object of the two cycle oil is to provide cylinder lube after combustion which means liquid that does not fully burn.
keep your oil/fuel ratios the same and turn your air/fuel mixture screw in to achieve a smooth idle.
20+ year mechanic
RF
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I'm kinda an old timer & I remember asking a top notch mechanic that's old enough to be my dad about that for my old 53 Johnson 5 hp.that you turn around for reverse. He probably still does 99% of the outboard repairs in Ely NV. His advice was to mix the oil according to the directions on the container. Now the instructions on my motor say to mix 16 to 1 using a good grade 30 wt. motor oil. Go figure?? I've followed his recommendation for over 30 years and that old Johnson still purrs like a kitten.
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I am educated in physics of nuclear nature....but I am in the middle of trying to figure this exact fiasco out with my 7.5 horse 49' champion. it runs fine in my driveway at a 25:1 ratio but when i get up to say deer creek it putters out at lower idles right away and after a while begins to not run at any thing other than "full throttle". I have a rich/lean adjustment screw on the front of my motor but as i dinked with it I had no luck in finding the "sweet spot" to get my motor running smoothly again. I am going to bump my mix to a 40:1 added to my 25:1 so i will have somewhere in the neigborhood of a 30 to 35:1 ratio and see if that does anything, some others mentioned a trick of just trimming up the motor a bit thats to say lift it a little like you would in shallow water to not bump rocks with the prop and that DOES put a less direct load on the motor at the prop and may let it run a higher RPM to keep the exess fuel/oil "blown out" of the cylinder, i can see how this works, but I can also see more pollution out of a probaly higher pollutant emittin motor as i ejects unused fuel and oil from the motor into the water.....i am trying to address my issue with a solution that will burn what i feed it, not trick it into thinking its burning it. course the trim trick would be great for gettting a slower speed out of the motor as it would have to draw water to the prop to be churned instead of just cutting into it as normal.....its the equivalent of either driving a screw straight into wood or doing it on an angle its harder to begin with but in the end you just have an angled screw in the wall.
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sounds like your air bleed jets on your idle circuit and or dirty fuel passages on the same
RF
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Thanks to everyone who replied to my post. My question has been answered, and my motor may have been saved. I'll go back to the prescribed mixture of gas and oil and adjust the lean-rich screw accordingly. THANKS!
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