Type of gas used in a 4 stroke engine

gmarshall43

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May 5, 2002
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I have a friend who has a 2 year old 4 stroke Mercury motor and has been having trouble with it. He was told it was bad gas and he had been using "marine gas".
I'm not quite sure what that is but he said its like normal gas for autos but missing something in it. Can someone tell me what marine gas is and is it bad to use standard regular gas in one of those 4 stroke motors?

Thanks,
Gary
 

Gyrene

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Mar 28, 2014
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Only thing I can think of is WAAAYYY back most gas had lead in it and because boats exhaust in water, the lead would get in the water so boat gas had to be lead-free - we called it "white gas".
That is way over 30 years ago. Now it is hard to find gas with lead in it.
 

garbageguy

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What may be missing from the "marine gas" your friend spoke of, is ethanol. It is added to most "gas for autos". There are several considerations and opinions if its OK to use in any machine. That information is readily available - fun reading...
 

Frank Acampora

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It is not what may be missing, but what may be accidentally added: WATER. Many years ago, marine gas was lower quality and lower octane rating than auto pump gas.

NOW, the new four stroke engines are ethanol compliant and it is OK--preferable even on just a cost basis--to use regular 87 octane auto pump gas
 

NYBo

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What Frank wrote. Regular "automotive" gas should be just fine as long as the ethanol content is 10% or less.
 

undone

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There is no exact definition of "marine gas", it's a general description and could possibly include, no road tax, no ethanol, premium fuel (high octane), or just what they sell in the pump at the marina.

Modern small 4 strokes have problems with any fuel due to having very small jets and being tuned to run as lean as possible, plus not being legally adjustable by the servicing dealer or you.

Any debris in the fuel, water, dirt, gunk, bits of dissolved fuel line (which the gray Merc fuel line is known to do) can plug or limit the flow through these small jets and/or the no longer adjustable low speed needle. There is a plug over the needle that can be removed and a screwdriver can be used to adjust the screw for easier starting and possible running issues.

This needle is illegal for the dealer and you to adjust (EPA), this law was put in place 1997 or 8, and then over time the emission regulations were tightened and the motors were required to run leaner, which may result in hard starting and a poor idle. If the carb you have happens to run a little too lean for your motor the only option is for the dealer to put on a new carb and hope it helps, they can't adjust the one you had.

The solution is to drill out the plug and turn the screw.

Now if this motor did run well at one time it's just something partially plugging the low speed circuit in the carb.

Also, these Mercs tend to be hard to troll with on small boats because of the lean jetting, it can cause the motor shake a little more, plus it can be hard to control the RPM's at very low speeds. Slightly richening the idle mixture screw can help with these problems.
 

Fleetwin

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Nov 23, 2011
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As far as "marine gas" goes, the only thing that makes it marine is that its available at the dock.

It is most likely the same stuff you get at your local gas station . With that said, it probably has ethanol in it and that attracts water.

You don't say what Mercury engine it is but it is probably fuel injected.

I don't doubt that a bad load of fuel could have made its way into the tank.

The best way to guard against that is by adding a 10 Micron marine fuel/water seperator. See below.

http://www.iboats.com/Water-Separati...-view_id.45101

The filter canisters are spin on/off and should be replaced at least annually. Cheap insurance.
 

Sea Rider

Supreme Mariner
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Just use fresh E or non E high octane fuel you can buy at any gas station, engine will work much smoother and keep combustion chamber much cleaner than running standard fuels.

Happy Boating
 

cannonman

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Jun 20, 2010
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Many engines (but not all) will operate fine with fresh fuel that contains 10% ethynol. Here in Missouri, the powers that be have allowed 15% ethynol to be sold at gas stations. Use of this fuel will actually void the factory warranty on many brands of engines. Most of the time the gas station employees don't even know what percentage of ethynol is in the fuel they sell. The only way I have found to prevent an issue is to buy non-ethynol premium fuel, at an considerable higher price per gallon. I know a lot of people will say they have no issue with using E10. Beware of the E15 or E85 as it is sometimes called.
 

Fleetwin

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E 85 and E 15 are not the same thing. the numeric designation ( 15 or 85) is the amount (%) of ethanol allowed in the fuel. It can be up to that amount but not to exceed.

As an example, E85 may be 78% Ethanol. E15 may be 13% ethanol.

Unless you have a vehicle designated as "FLEX FUEL" DO NOT run E15 or E85.

The EPA allowed E15 for sale not caring that it is is ruining vehicles not designed for it.

No outboards are "Flex Fuel".
 
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