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burr56

Hi octane or ethanol?🤔

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 Hello all and hope you had good holidays.  Ive read bout the problems with "corn gas" and have a catch 22 thing on my mind. I can run ethanol-free gas bur can only get 91 oct.  Motor company (yamaha) recomends 87 oct. but it is 10%ethanol. I ran the 91 this last summer and seemed to run good on it. So which way would be best-91 and no corn or 87 and run a ethonal additive?  Thoughts?        Burr

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Corn free is usually 91 octane and is perfect.  The recommendation of 87 is usually recommended as a minimum.  Truth be told, many will bump up their octane for corn gas.

The rule of thump, is to run an octane just high enough to avoid ping, but it usually isn't an issue.  My Yammi runs best on 91 corn-free and my water separator like it too

 

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In general a motor is a motor whether it be a boat motor or a pressure washer or a zero turn mower... ethanol free is better despite the octane.   A couple things comes to mind and that is (1) treating the ethanol with gas treatment or (2) mix your gas, like 5 gal ethanol with 15 gal 91 Octane, but that would be a pain.    I'd use 91 octane before ethanol.

 

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The only difference between lower and higher octane gasoline (given the same formula i.e. 10% ethanol, no ethanol, etc) is that the higher octane gasoline burns slower.  This prevents engine "knock" which occurs when the fuel ignites (due to compression and heat) before the spark plug fires.  This is most likely to occur at higher RPM and/or heavier load.  I'm sorry that I don't know more about boat motors, but as for car and truck engines the use of higher octane gas in modern engines is nearly pointless as all modern engines have computers which adjust air/fuel mixture and ignition timing to provide the optimal environment for combustion.  Back when we had points and vacuum advance distributors high octane was a must for performance engines.  I suspect that if you have a modern four-stroke boat motor (mine is a 2016 Merc 115 4-stroke) you can run any "normal" octane you choose with no significant effect on combustion.  If I could find any ethanol-free gasoline in the 87-97 octane range I would definitely use it in my Merc.  But unfortunately, I live in Illinois where ethanol-free is a lynching offense.  

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All right it seems that the 91 oct. is the way to go. By the way i should say my outboard is 40 hp 2 stroke yamaha going on 21 yr old. This is on a 1998 g3 17ft aluminum bass boat. I guess i should tell you im kinda old school for a lot of you. Angus, I can remember setting those points and plugs you mentioned and know a little about the octane and compression relationship. I do not have much experience with 2 stroke outboards so thought i would get some feedback. Thanks for your 2 cents worth.Happy new year!

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3 hours ago, burr56 said:

All right it seems that the 91 oct. is the way to go. By the way i should say my outboard is 40 hp 2 stroke yamaha going on 21 yr old. This is on a 1998 g3 17ft aluminum bass boat. I guess i should tell you im kinda old school for a lot of you. Angus, I can remember setting those points and plugs you mentioned and know a little about the octane and compression relationship. I do not have much experience with 2 stroke outboards so thought i would get some feedback. Thanks for your 2 cents worth.Happy new year!

2-stroke, well now you are taking me back 35 years to my motorcross racing days. 91 should be fine in your 2 stroke. Just check your plugs after you’ve run a couple tanks through to make sure you don’t need to adjust the mixture to compensate for a slower burning fuel. 

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Well, i pulled plugs and they have the look of running a little rich-bit darker than they should be . That could be my goof as oil mixture might get a little heavy on oil, but not to extreme. Running plug that manual calls for - should i go 1 stage hotter plug?

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I have a 150 Pro xs that says 87 only... but my manual actually says 87-91. On another popular bass site one of the merc pros says never use higher then 87. I followed this for a season and a half and I was having rough idle issues even after a full service. I always treated my fuel and only realy filled with what I would use in a week. I checked my plugs and noticed they were gapped incorrectly. This didn't smooth out the idle much. I then made the switch to 91 gas and problem solved. I will alternate between 87 and 91 with an additive and so far so good. 

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As I went thru my Merc manual ( 2019 4 stroke calif compliant ) US & Canada

 

87 octane minimum,  Premium 92 octane is also acceptable .   Butanol fuel blend up to 16%  (USA Only) is acceptable.  Ethonal  or Menthanol  should beware and no more then 10% and Merc does not know how much much of this type fuel it can withstand.  (I suspect they are also talking about the boat mfg as well... fuel lines and other components )  BT shows 15% next to the fuel cap...  And of course Merc recommends fuel additive.

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