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Clouser

Non oxygenated fuel and outboard performance

10 posts in this topic

Decided to try this fuel on a 1967 Johnson 9.5hp outboard. Motor starts with just a short pull of the rope which is good but bringing the throttle down to idle or trolling speed it does not want to slow down as it has in the past. The low idle speed has been adjusted as lean as possible before it quits. Does this grade of fuel burn hotter than regular gas to the point that the slow idle can not be achieved? By the way the low speed is controlled by an adjustment knob on the face of the motor that makes it easier to fine tune fuel mixture. Perhaps I need to go back to lower octane fuel.

Using Amzoil at 50:1

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Non-oxy 91 octane actually burns slower than 87 octane. From your description, it sounds as tho the idle mixture is set to lean. The way I set idle mix is, while at an idle in gear, slowly lean it out until you reach the highest rpm before dying , then slowly richen it until it just starts to "stumble" lightly then go half way between those 2 points.

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so if 91 octane burns slower than why do drit bikers brag about eh 100 octane giving them so much more power?? also with that motor i would stay shy of ethonal gas cuz they have cork floats and the ethonal will eat the varnish off the cork and ya got a mess! try what boat fixer said and make a rich batch of seafoam..

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Slower burn equals a more efficient burn. The base stock for hi perf gas is much better to begin with. Not the same junk they force us to run for everyday use.

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so if 91 octane burns slower than why do drit bikers brag about eh 100 octane giving them so much more power?? also with that motor i would stay shy of ethonal gas cuz they have cork floats and the ethonal will eat the varnish off the cork and ya got a mess! try what boat fixer said and make a rich batch of seafoam..

Much higher compression requires higher octane (i.e. slower burning gas). If you put regular pump gas in a "race" motor that requires high octane gas, your fuel burn will occur before TDC (top dead center) and you will hole the piston everytime. The dirt bike riders you refer to have either stock motors or have had their moters modified to run compression ratios in the 12:1 range (versus probably 9 to 10:1 for regular octane). There are other modifiers to this such as 2 stroke versus 4 stroke and valve timing on the 4 stroke but this is a general rule of thumb. Generally speaking you should run the gas your motor was designed for for BEST performance. If your motor calls for 87, run 87, running premium will not gain you anything (it won't hurt your motor either), you may actually lose some peformance. Hope this helps. Paul

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As Bruce said, if you lean the idle mixture the engine will run at a higher RPM.

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Okay, as I lean out the mixture it does not slow down the RPM enough and at that point it simply shuts down. I guess the most obvious answer it to return to regular fuel since I had no problems with it but was concerned with all the bad hype with oxygenated fuel.

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Kind of sounds like a dirty carb to me.

But, given your experience, I'd try the switch back to regular gas before I tore the carbs apart.

marine_man

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I agree with MM above. One clue from your original post is the 1967 vintage motor and carb. Your symptom of the low speed adjustment not having much effect sounds like a dirty circuit in the carb. Unfortunately, the ethanol in modern fuels can attack the polymers in the carb causing them to break down, and that may be what's plugging up your carb.

Try switching back to a gallon or so of non-oxy gas and mix a little fuel injector cleaner in it to see if you can dissolve out the clog. If it clears up, throw away the gas and mix up a fresh batch and run it for a while. The cleaner should not stay in the system too long or it may cause problems of it's own. I have done this with older motors and it has worked well.

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your outboard has no need for that fuel as long as the carb has been rebuilt and you are running a newer hose and bulb. I have run many gallons of the cheap stuff in my 65 Johnson 6HP and she idles all the way down and starts on the second pull. Its just not worth the extra money for the 91 octane.

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