2.7 MPG on Inline 6 - 90hp 1982 Merc. Is this normal?

[ST]

That's about the average I get from this motor; it's on 17 ft Center Console fiberglass, 3 persons, dual batt, 10G tank and 100lbs Yamaha 9.9 kicker; leisurely cruising on plane at about 3000rpm. <br /><br />From what I've been reading, this mileage seems low; isn't it?. So far I only replaced the spark plugs, but looks like no improvement on the mileage. What other things I could do to this engine (other than repowering) in order to improve it's gas mileage? <br /><br />BTW; the engine looks powerful enough still, no problem pulling a skier.I hate to waste gas unneccessarily...

 

[jrampey]

Re: 2.7 MPG on Inline 6 - 90hp 1982 Merc. Is this normal?

Yep seems a little low to me... I have an 1983 150 and get about 4 mpg @ wot running 55 mph on a 19' bass boat weighing 2200-2500 lbs.<br /><br />Check your timing and maybe rebuild carbs.

 

[LubeDude]

Re: 2.7 MPG on Inline 6 - 90hp 1982 Merc. Is this normal?

Originally posted by 84champion:<br />I get about 4 mpg @ wot running 55 mph on a 19' bass boat weighing 2200-2500 lbs.<br /><br />

Man, I want your boat. I have an 18' Bass boat with a Merc 150, and I can burn 10 Gallons of gas so fast It amazes me sometimes, no where near 4 MPG. Id have to rate it as Gallons per hour.

 

[JasonJ]

Re: 2.7 MPG on Inline 6 - 90hp 1982 Merc. Is this normal?

I think you are fairly close to typical for what these inlines get. I have a 125 horse Merc that gets me somewhere in the 2.5 to 3 mpg range. The only thing you can do to reduce fuel consumption is to get a more efficient engine or reduce the weight of the load your current motor is seeing. Also, the setup of the boat/motor is critical. The right pitch prop, load placement, trim of motor, trim tabs, condition and style of hull, these are all factors.<br /><br />Just remember, the cost of a more efficient motor must always be considered. It is pointless to buy a motor that will give you 1 more mile per gallon if you will never recoupe the cost of that motor in fuel savings. In anotherwords, an inefficient, paid for motor is always cheaper than a more efficient motor that costs you $9000 unless you just want a new motor. See what I mean? Good luck....

 

[MercFan]

Re: 2.7 MPG on Inline 6 - 90hp 1982 Merc. Is this normal?

Please remember to rather state gallons per hour rather than a distance figure. If your hull lies 3feet in water at WOT and I'm plaining all the way my MPG will kill your's by a 100 miles, but per hour we'll be using exactly the same. If we have more or less the same duty cycle/throttle opening.<br /><br />In summary. A particular engine model will use (more or less) exactly the same amount of fuel at full load, full throttle regardless of the hull it's mounted to, if measured over time instead of distance.<br /><br />It's a lot harder to compare "on water" consumption figures than "on road" consumption figures, but I believe that a "per hour" figure is more comparable than a "per mile figure"

 

[phatmanmike]

Re: 2.7 MPG on Inline 6 - 90hp 1982 Merc. Is this normal?

consider your self lucky with an inline that gets MORE than 2 mpg, i had a 115 on my 17 foot center console and only got 1.5mpg at WOT, but on the other hand, it went nearly 50 mph<br /><br />and that 150v6 in the second post... different type of beast, no comparrison, apples to oranges.<br />plus its on a bass boat, easier to plane than any center console. always gonna get more mpg.<br /><br />inline 6 mercs are VERY famous as not being fuel efficient. remember the inline 6 mercs started in the 60's and didnt really change all that much by the time they were put to rest in the late 80's. they require LOTS of fuel just to get started, and they reuire extra fuel to feed cylinders 1,3 and 5. remember , that fuel has to fight gravity after leaving the carbs.<br /><br />not the most efficient design in boating history. and before you bash me for hating inline mercs... i have 5 of them right now. a 115, a 85hp, 2 50hp's, and a 40 hp. and they are NOT for sale. i love my mercs.<br /><br />MPG has way more to do with boat setup and hull design than that beast hanging off the back.<br /><br />DONT CRUISE AT 3000 RPM. 2 strokes get AWFULL milage at mid throttle rpm range. youll actually find your SWEET SPOT at about 3800-4300 rpm's.<br />the motor will actually "sound" different once you hit the perfect rpm sweet spot, and your boat will run easier here.<br /><br />back to topic... clean the bottom of your hull. inspect your prop.maybe get a different prop, what are you running now? get a setback/jack plate. get some smart tabs. lose that 100 pound yamaha. check your hull for waterlogged foam. get new reed valves. run at 3/4 throttle. stay home.these are all ways that can save you some gas. not that all of them sound fun.<br /><br />just my 2 cents

 

[ST]

Re: 2.7 MPG on Inline 6 - 90hp 1982 Merc. Is this normal?

Hi guys,<br />Thanks for all those suggestions and sharing of what you know; I (and many others in the same situation like me; I am pretty sure...) really appreciate that!<br /><br />Btw; my hull is clean and smooth; but the prop is 12-17p aluminum. No jack plate; but has Stingray Hidrofoils. I don't think I have water in the hull (luckyly), it never drips water; no sloshing noise either. <br /><br />But I need the kicker though; last Summer it already saved me from paddling for 7 miles (or waving other boater), caused I missed read the gauge on the tank : ; thus: I run out of gas!Also, I use it for trolling for Striper as well.<br /><br />You think I should change to SS prop (what kind specifically?) and new reeds can improve the gas mileage? But I read somewhere that changing the reeds requires a powerhead removal? $$$!

 

[phatmanmike]

Re: 2.7 MPG on Inline 6 - 90hp 1982 Merc. Is this normal?

there are a few different reed setups that are more" efficient" than the stock reeds. i have heard of people using boysens reeds, but ive also seen others using reeds from a 1350 or similar. the reeds comment shouldnt really be used for gaining MPG, i was just chattering. although it could.<br /><br />take of the foil, try it without it. trims tabs do the same job, but actually do it a LOT better. try looking into some smart tabs. i have them on my boat.day and night difference.<br /><br />but the main thing is NOT to cruise at 3000 rpms.try 400 rpms.<br /><br />what are your WOT rpms?<br /><br />have you checked your timing?<br /><br />as for sloshing, you wont hear it, it will all be obsorbed into your foam. <br /><br />remember, whatever you do to this boat and motor, its still gonna be an inline merc, and those are NEVER good for mpg's.<br /><br />2 more cents

 

[emckelvy]

Re: 2.7 MPG on Inline 6 - 90hp 1982 Merc. Is this normal?

Your 1982 90 HP is probably a 115 rated at the prop to be 90. '82 was about the time they changed over from crank-rated to prop-rated.<br /><br />That being said, the old formula for calculating full-throttle fuel consumption on a crank-rated 2-stroke outboard was roughly HP/10. So your motor should theoretically burn about 11.5 gph at full throttle.<br /><br />A stainless prop of the same pitch as your existing prop will probably gain you a few hundred rpm and be more efficient. You do need to verify what your full-throttle rpm is now before you change props, to ensure the boat is propped to the correct max rpm. Maybe 4800-5500 rpm if I recall. Best to prop to the max rpm so the motor isn't working as hard.<br /><br />And changing reeds (such as Wiseco fiberglass) isn't going to get you much, unless you like tearing the motor apart regularly to replace broken reeds! I don't think this motor has small reed stops, either (such as the old '70-'71 1150), so you're not going to gain anything there.<br /><br />Anyway, best you can do is to ensure the motor is set up properly on the boat and you've got the carbs and timing properly synchronized so you know the motor is running at peak efficiency.<br /><br />Maybe throw some synthetic gear oil in the lower unit, don't know if there'd be measurable difference in economy but certainly less friction can't hurt!<br /><br />HTH.......ed

 

[Clams Canino]

Re: 2.7 MPG on Inline 6 - 90hp 1982 Merc. Is this normal?

Originally posted by myoutboards:<br /> there are a few different reed setups that are more" efficient" than the stock reeds. i have heard of people using boysens reeds,

NEVER use Boysen reeds in an inline, fine for the V6 but they do not work well at all in the inlines.<br /><br />-W

 

[ladano]

Re: 2.7 MPG on Inline 6 - 90hp 1982 Merc. Is this normal?

I build 4wheelers on the side, and I would never put boysen reeds in one of those.

 

[emckelvy]

Re: 2.7 MPG on Inline 6 - 90hp 1982 Merc. Is this normal?

Yeah, Boysens, that's it, er, I forgot!!! Don't use those either!!! Nyuk.

 

[Texasmark]

Re: 2.7 MPG on Inline 6 - 90hp 1982 Merc. Is this normal?

Sorry to disappoint you guys, but the '89 115 I ran on my 17' Ranger would run all day on the 10 gallon built in tank that came standard with the Ranger. It (the Merc 115 6 cyl) had direct charge, which I assumed was Mercury's answer to OMC's Loop charging till the patent ran out for OMC. The boat was padded and the engine had trim so you could set her up real nice on top. Goose her outta da hole then throttle back to about 4500 and "life is sweet".<br /><br />My 2 c<br /><br />Mark

 

[ST]

Re: 2.7 MPG on Inline 6 - 90hp 1982 Merc. Is this normal?

An update:<br />While I was testing my rectifier (yes, I had a bad rectifier too! Thanks to Ed Mc. and others), I saw some gas spraying out from under the "top safety bracket"; upon removing this bracket, I discovered a tiny leak on this small black tubing. Here's the photo:<br /><br />

<br /><br />I hope, this leak contributes to the cause of my low mileage. Wonder what's the purpose of this tiny tubing for? It runs from the top, along the side and down to the bottom of the engine inside the cowling. Thanks!<br />02/16/06 Image edited...st.

 

[rodbolt]

Re: 2.7 MPG on Inline 6 - 90hp 1982 Merc. Is this normal?

not much, it only lubricates the top and bottom bearings. <br /> but 11 gallons an hour or so at or near WOT is about right. <br /> you need to figgure your fuel consumption in gallons per hour and forget the MPG. MPG is to much affected by outside factors that dont seem to affect GPH as much.<br /> also helps when you know about how long the engine will run on a given amount of fuel for the day your fuel gauge starts lying.

 

[ST]

Re: 2.7 MPG on Inline 6 - 90hp 1982 Merc. Is this normal?

Thanks rodbolt,<br />and how does the center bearing (if any) get it's lubrication from? Btw; where do the mixed gas go to after it lubricates the bearings? Sorry, it's basic, just curious, and no manual at the moment. The next outing, I'll try to get the GPH data instead...

 

[rodbolt]

Re: 2.7 MPG on Inline 6 - 90hp 1982 Merc. Is this normal?

center mains get lubed by crankcase pressures and gravity. needle bearings dont need a positive film and do better if not swimming in oil like sleeved bearings.<br /> after lubing the crank,wristpins, rods and pistons it goes into the combustion chamber where its converted to fire,smoke and magic .<br />another common cause of heavy fuel consumption on the late seventies and 80's inline six motors was the techs that eliminated the backdraft feature on the fuel bowls. I do it all the time or used to.<br /> its a nice feature but sometimes can lead to a lean fuel mix and smoked pistons. <br /> so the cure was to block of the back drag tube with a bb or a roller bearing and let it ride. it can create a rich mixture at certain RPM's but it prevents a lean mixture at certain RPM's.<br /> how the back drag set up works was really quite simple.<br /> the only reason fuel flows from the fuel bowl to the carb main nozzle is a 10 mile colum of air above the bowl placing pressure on the fuel and the vacum created by the upward moving piston at the aft end of the main nozzle. what merc did was run a tube from the aft end(low pressure) of the venturi to the fuel bowl and placed a calibrated jet to vent the bowl. this lowered the atmosphric pressure on the fuel in the bowl. it made the engine run a bit leaner, sometimes to lean. merc is not the only one to ever do this. and in certain hull,throttle and load conditions can create an excessivly lean air/fuel ration creating extra heat in the combustion area that lead to other issues.<br /> most of them I eliminated.

 

[timmathis]

Re: 2.7 MPG on Inline 6 - 90hp 1982 Merc. Is this normal?

At WOT yes it is normal. Sometimes you have to pay to play!!!

 

[mjbrueck]

Re: 2.7 MPG on Inline 6 - 90hp 1982 Merc. Is this normal?

You also may want to up your cruising rpm's a bit. From your description (FG 17' with kicker and 3 people), I'd guess the 90 hp (if propped right) is barely on plane at 3000 rpms. Your hull should run more efficiently when it's fully planing (i.e., smallest wake).

 

[ST]

Re: 2.7 MPG on Inline 6 - 90hp 1982 Merc. Is this normal?

This is what I've done so far:<br />1. Removed Stingray Hydrofoil.<br />2. Fixed the leaky fuel line that lubricates top and bottom bearing.<br />3. Change lower unit oil with Synthetic Mobil-1.<br />4. Did the decarb routine with Seafoam spray.<br />5. Fixed a bad rectifier (this probably have nothing to do with mileage?)<br />But, I haven't launched the boat for the trial run yet, It'll be interesting to find out. <br /><br />Other things I may pursue:<br />- Change to SS prop (what kind?)<br />- Go to 1 battery instead of 2 ?; maybe... Any comments on this? (saving weight: approx. 50 lbs-size 24).<br /><br />You're right mj; that 3000 RPM was barely brought the boat on plane; I will run it to about 4000 rpm on my future cruising...<br /><br />Thanks for all your suggestions....!