2009 mercury 20hp 4 stroke not running right

Stand Up said:

Lower octane fuel has other junk in it, like corn oil so to speak that gums up after the fuel has sat for just a few days. Highest octane has way less of a chance for that. I use 92 octane.

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Not sure where you get your fuel but we have a big refinery right here in town, a number of friends work there, additives are added to the tanker truck when it is filled. There is no junk added, those are just stories !!! The only difference between Shell and Marathon is a small bag of additives added to the tanker as it is being filled with what ever octane fuel is being purchased. Been using 87 octane fuel with ethenol since it came out. Maintain your stuff and you wont have any issues.. The only crap getting into the fuel is from the station where you but it.. If your getting bad fuel, go to another station. Comming from the refinery, all gasoline is the same, all meets the same standards !! Burning premium fuel in an engine designed for regular will lower its performance!!!!

Airshot. I don't mean "junk in the fuel" like pieces or flakes of junk but rather the fuel contains things that gum up in various carburetor uses. I can burn anything in my Ford F -150 but if I use low octane fuel in the motorcycle or lawnmower etc. it will gum up the jets. I use the highest octane available at the local gas station for small motor use and I ended the problem.
They add ethanol to fuels these days and Ethanol is a domestically produced alternative fuel most commonly made from corn. It is also made from cellulosic feedstocks, such as crop residues and wood—though this is not as common.
You can run any fuel you wish in your equipment as it's your choice. Oh, and burning premium in your gas engine will do it no harm.
I believe my advice is relevant and given with experience.

Not any harm, just lower HP and performance by using a higher octane fuel than the .motor was designed for! Gasoline is a distilled product just like alcohol, no crud or particles are getting in. The alcohol cleans the system, folks that have issues, are those that have dirty fuel systems to begin with.

Many of the premium grades of fuel have cleaning additives added to there fuels. NO and Shell are two that I know for sure. But for the extra price of premium fuel you could add your own and you might get better performance!
Both my wifes and my suv's get worse mpg with premium fuels, had to use them some time back as our local stations had no regular for about a month. Do what you feel in necessary, but any performance mechanic will tell you not to expect better performance using premium when an engine is designed for regular...

Pappy said:

I just read through this entire post. One thing that has not been mentioned is the fuel line itself. Today's fuel lines have an inner liner. Some of the earlier lines were prone to have this inner liner crack, split, or come loose INTERNALLY restricting fuel flow with no visual indication on the outside of the line. The silver colored lines were famous for it at one point.

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I think they may have corrected the gray/ silver fuel lines issue. The boat I bought last summer had those gray fuel lines, fairly new ones at that. Replaced the inline fuel filter so had a chance to inspect the inner liner where the problems were. No inner liner now, just the gray line only. Examined mine carefully and found no issues, but watching my new filter carefully !! About once a month I squeeze some out into a clear container just to check. Hopefully that issue is solved !!