Fuel Economy question 1979 115 Inline 6

[Skidooosl]

is it normal for a 79 115 I-6 to SSSUUUCCCKKKK through the gas? We were out with a full tank of gas for like 15 - 20 min at WOT and it stalled out... checked EVERYTHING and could not figure out why it would not start again. Just before we tried to get a tow for some reason I bumped the gas tank and noticed it sucked the thing dry!!! switched tanks and it fired right up. These outboards look SWEET but dang... it would take like 239042394823409 gallons of gas for a day's cruise. Does anyone else notice this with the big I-6 Mercs or does it sound like their is something wrong with this one? I know our Yamaha and Johnson will go forever on 6 Gallons

 

[ReeseK]

Re: Fuel Economy question 1979 115 Inline 6

oh ya my 1976 115 sucks gas but I have cyl #3 with no compression.

 

[phatmanmike]

Re: Fuel Economy question 1979 115 Inline 6

yes. they suuuuuck gas likes its going outa styleeeeee<br /><br />i had a 1976 115 on back of a small 16 foot boat that only weighed 800 pounds and i only got 1.5 mpg, but on the other hand, i hit over 50 mph!!!!

 

[ZmOz]

Re: Fuel Economy question 1979 115 Inline 6

I remember when I bought my '74 150hp the guy gave me a 16 ounce bottle of oil. I thought "perfect, that should last for the first few days"<br /><br />Hahahahhahhahhahhahha! I wish I was back in that dream world...

 

[Mercury140-I6]

Re: Fuel Economy question 1979 115 Inline 6

You have to remember, that the rule of thumb is 10% of the prop shaft HP is how much it will burn per hour. Pre 83 were rated at the crank shaft, so essentually, a 115 would be close to a modern 90, so about 9-10 gallons per hour at WOT. My 79 140 does about 11.5 - 12 per hour ar WOT, but really likes 4200 RPM's better, 3200 is a nice clean plane and will run like that for a long time (on my boat).<br /><br />

<br /><br />Craig

 

[emckelvy]

Re: Fuel Economy question 1979 115 Inline 6

It's actually 10% of crank-rated HP=fuel consumption at WOT for a 2-stroke; so an old 115 would burn upwards of 11-12 GPH, whereas a newer 90 would probably come in a bit less but not much.<br /><br />I remember my old 75 HP Evinrude Starflite V-4 could go thru at least 9 GPH!!! Most of the gas I think was just going out the bottom of the motor thru the downdraft carbs. That was one motor that burnt fuel above and beyond the formula.<br /><br />Amazing the same motor with sidedraft 4-bbl carb made 90 then later 100 HP and used way less gas!

 

[Beater]

Re: Fuel Economy question 1979 115 Inline 6

Well this is kinda a good news bad news thing LOL! I just had my 1979 Merc 140 out for the first time. Don't know if you remember but it is the one I posted about that sat for 7 years. Well anyway it ran great but we were only out for an hour or so and damn near used a tank of gas. I figured something was wrong but I guess not. So good news is it is OK bad news is.......well I guess we know what the bad news is! Hey Craig I was wrong my motor will tip all the way up, I just had it sitting on the transom and it must not have been all the way down. How long can you cruise with yours at 3200 rpm and roughly what speed are you doing?

 

[Clams Canino]

Re: Fuel Economy question 1979 115 Inline 6

Stop whining... <br /><br />I've gone through a gallon of TCW-3 in a weekend. No way a little 6 gallon porta-can is enough to do anything "real" besides testing and tuning. I have a 25-30 gallon tank in my boat and can use it up in a day or less depending on useage. <br /><br />Running one of these older critters at WOT all the time is both hard on gas and flirting with trouble. They cruise real nice at about 4000 RPM and are actually pretty good on gas at those speeds. <br /><br />In general remember that pushing a boat is like driving a car uphill - all the time.<br /><br />-W

 

[Mercury140-I6]

Re: Fuel Economy question 1979 115 Inline 6

Beater:<br /><br />I have a 16 gallon built in fuel tank and just cruising around at 3200, I can run for a few hours non-stop. My friends boat is much slower than mine. <br /><br />

<br /><br /><br />His top speed is between my planning and plowing speed. We both went from Priest River, ID to Sandpoint, ID ~25 miles. He burned 7 gallons at WOT and I burned 5. Love that Merc.<br /><br />Craig

 

[Deas]

Re: Fuel Economy question 1979 115 Inline 6

Anybody comment on the economy of newer motors like the optimax, seems to me if they have the improved economy as advertised that a new motor would pay for itself after only 1 or 2 seasons of use.

 

[Mercury140-I6]

Re: Fuel Economy question 1979 115 Inline 6

The way I see it, it takes "X" amount of fuel to make "X" amount of HP. If you want to use 150hp, it takes a certain amount of fuel per hour to make that. In other words, if your baot requires 60hp to achieve a clean plane, it would take the same amount of fuel to run a 60hp at WOT to keep it there as it would a 120hp at about 1/2 throttle. It just depends on how fast you want to go as to how much it is going to burn. I have a 140 (120-125 at the prop) on a 16' light weight boat. I can either run her WOT for just under 2 hours or cruise around for 4+ hours. I prefer to take my time getting there and enjoying the ride. But, if I really have to get there now (Like in an accident situation) I can nail it and get there in a hurry, but pay for it at the pump.

 

[ZmOz]

Re: Fuel Economy question 1979 115 Inline 6

Originally posted by Mercury140-I6:<br />In other words, if your baot requires 60hp to achieve a clean plane, it would take the same amount of fuel to run a 60hp at WOT to keep it there as it would a 120hp at about 1/2 throttle.

In theory, yes, but not in practice. A 60hp screaming along at WOT is going to use alot more fuel than a 120hp putting along at half throttle. No engine is very efficient at WOT.

 

[JasonJ]

Re: Fuel Economy question 1979 115 Inline 6

I get decent economy on my 69 Merc 125. About 2-2.5 mpg if I stay in the 3800-4000 rpm range, which gives me a cruise speed of about 25-27 mph. Forget about WOT, that is unpleasant. I am picking up an Evinrude 150 horse later today. If I get it running well, I should be able to cruise at a faster speed, or a lower rpm, I don't want to think about the fuel economy on that thing...