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Mariner 40hp 4 Cylinder Fuel Consumption

T

TGBTG7701

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I just installed a Mariner 40hp 4 cylinder 2 stroke on my 16ft bass boat and noticed it really went through some gas. I had a Suzuki 40hp 2 stroke on the boat before, it barley used any fuel, but the Mercury can burn it up. I use to run about an hour and burn a little over 1 1/2 gallons with the Suzuki, but the Mercury used over 3 gallons doing the same. The Mercury seems to be set dead-on, I bought it from a Mercury outboard mechanic, the motor does not seem to run rich, carbs are in sync with each other, runs smooth and has plenty of power, cranks first hit of the key. The Suzuki was loud, revved hard but did not use hardly any fuel. Is this a common thing with the old 4 cylinder 2 strokes?
 
johnnygjr said:
Newer engines are more efficient than some of the older 4 cylinders of old. How do the plugs look?
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The plugs look great, but I did change them. It had Autolite plugs, but I put in some Mercury recommended NGK plugs, I have not ran it on the water sense the plug change. I have always used NGK plugs in my outboards with good luck, have not seen much information on people using Autolite in outboards.
 
Autolite's "were" the OEM plug on many Merc's years back. They are still a good alternative if NGK's are unavailable (if the spec is NGK).

Some Merc's and a number of the newer models come with Champions - the models that were designed to use autolite/NGK do not always run so well if you cross ref to a Champion plug.

As far as gas usage. General rule of thumb is you should burn about 10% (in gallons per hour) of your total horsepower when you run at full throttle.

So with a 40 horse you can expect to burn 4 gallons per hour running @ full throttle. You can save gas by running at about 2/3 to 3/4 throttle - the point where the motor has reached maximum timing advance, but there is still some throttle left - that can cut max consumption by up to 40%.

And while I do agree that the newer motors burn less fuel at rpms "less than max" - and specifically at/below 3500 rpm for the 4 strokes, if you are heavy on the throttle they still suck gas like their old 2 stroke counterparts.

When you wind up a brand new 150 horse 4 stroke Merc it still "drinks" about 14 gallons per hour (13.9 measured on a Tracker Targa V-18 WT). That is only about 5% less gas than an old XR6 would burn on a similar hull - hardly the "fuel savings" that are bragged about.

To compare your Merc against your Suzuki you would need to match rpms, total weight in the boat, weather conditions (high heat/humidity = burning more gas) etc for it to be a fair comparison. Because if you were running full throttle with a 40 Suzuki and only burning 1.5 gallons per hour you better resurrect that motor, because it was a "one of a kind"...
 
Thanks for the reply, I understand a lot more now about what effects the fuel consumption. I do have another question though about something else. I took the boat out and ran it yesterday. When I brought it home I decided to look at the plugs, going from the top to the bottom, the top two had a little bit of oil on them, but the bottom two had nothing, looked brand new. Should not all four have a little bit of oil on them?
 
johnnygjr said:
Do a compression test, Then try a cylinder drop test
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From top to bottom (138, 135, 132, 135) I did a drop test, the RPM was very close as each cylinder was tested, also installed a inline spark checker and got the same results for each cylinder. Ran the motor again today, and the bottom two looked fine, all four have just a little oil on them. Not sure what happened, but it appears to be running well.

Does anyone know what the idle RPM should be, and in gear RPM? The engine seem to idle a little high, and wanted to make sure I set it correctly.
 
600-800 rpms as long as timing is correct and engine does not stall when placed in gear. As far as engine running better,plug wires might be seated better on plugs now!
 
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