Excessive fuel consumption

[pirateofcapeann]

I have a 1994 Mariner 8hp with a 25 inch shaft pushing a 28 foot wooden sailboat with a sailboat prop. The motor is mounted on the transom with a pull-up mount. Up until this year I have averaged 5 - 5.5 knots at cruising throttle (the point where the RPM no longer increases) and have consumed approximately 7/8 gallons per hour. Lately the motor has started to run only slightly perceptibly and inconsistently rough and has started to consume close to 1-3/8 gallons per hour at 3.5 - 4.5 (at worst) knots at cruising throttle. I replaced the needle valve and cleaned the carburetor, changed plugs, performed all the sync and link adjustments, replaced the lower unit lube and completely cleaned the fuel lines and installed an external fuel filter but there has been no change. The motor still starts and runs dependably but I just can’t figure out this problem. Any ideas?

 

[Laddies]

Re: Excessive fuel consumption

Ck the float level and the primer for leakage

 

[Silvertip]

Re: Excessive fuel consumption

Perhaps the hull is fowled with marine growth which creates a great deal of drag. Just one other comment that relates to fuel consumption. If your 7/8 gallon/hr fuel measurement is accurate (and I believe it is) you just proved again that even on small engines, fuel consumption at WOT is as simple to calculate as placing a decimal point one position left in the HP rating. 8 HP = 0.8 g/hr at wide open throttle. d

 

[kitebuz]

Re: Excessive fuel consumption

Silver - I wish that calc proved true for my old '89 Evinrude 120's. They burn @ 18 gph ea at WOT 5500rpm) - & I've got twins :%

That's on a 23', deep V hull though, pushing @ 5.5k lbs at 40 mph - that said, they will burn @ 9 gph ea at 3900rpm & 13 gph @ 4200rpm.

Maybe it applies more to EFI engines??

 

[Silvertip]

Re: Excessive fuel consumption

Ouch -- Are you monitoring with a flow meter or just time and gallons pumped at refill? Granted, some of the older engines (although yours are not that old) and obviously the size of the boat, hull type and condition, its load, and water conditions affect fuel flow. Generally though, on most loop charged motors, and even the fuel injected motors, that calculation comes pretty close. I have a flow meter and my 75 HP Merc burns exactly 7.5 g/hr at WOT on a 17 ft walleye boat.

 

[Silvertip]

Re: Excessive fuel consumption

As for the EFI engines, go to Yamaha's web site and pick any boat, any engine (two or four stroke, carbed or not) in their performance tests. You will probably be surprised to find that it doesn't matter a great deal what technology is used -- WOT fuel burn is very close the 10% rule I just described regardless. It takes so much fuel to make so much horsepower -- its in the lower speeds and midrange where the technology provides some fuel savings -- not so much at WOT. Obviously there are slight differences but not the drastic difference as in your case. Now here's something for you to chew on. I have no Idea what the max HP your boat is rated for but I suspect your fuel burn would be less if you had two 150's or 175's. The theory is that an under powered boat has an engine(s) that are working harder (more throttle required) than a higher powered boat at lesser throttle. So your boat at WOT burns 18 g/hr each at 40 MPH whereas the 175's just loafing would burn a fair bit less than that. As a test, check Yamaha's web site and see if they list your boat or one similar in size and weight. See what the fuel burn is with a 175 at the same speed you are running and not what the 175's RPMs are at that speed. Keep in mind you are looking for fuel burn numbers on the 175 that are in the speed range that you are running wide open. The 175's would not have to run wide open to reach that speed so its that burn rate you are interested in. Then do the same for 150's. Prove me wrong -- I'll eat crow and admit it's my first mistake in 2007. d

 

[Silvertip]

Re: Excessive fuel consumption

I did your work for you. Here's the link to an actual test of a World Cat 248 with an as tested weight of 6466#. Powered by twin Yamaha 150's.

http://www.yamaha-motor.com/assets/...tin_otb_2StrokePerf_HPDI_150hp_0188-WCC-Z.pdf

At 5400 RPM these engines burns 28.7 g/hr or 14.35 g/hr each at WOT. Speed at WOT was 46.8. At 4000 RPM fuel consumption was down to 15.5 g/hr or 7.75 g/hr each. Speed at that throttle setting was 33 MPH. So you see, more power = better economy. Your poor economy is not due to an inefficient engine -- it's due to a boat with too little power. You didn't indicate what your speed was at 3900 RPM but this example really shows how an underpowered boat can burn lots of fuel.

 

[OldMercsRule]

Re: Excessive fuel consumption

Silvertip: Your insights in this area are very valuable, and have taught me a lot, hope the others listen to you. Thank You! JR

 

[steelespike]

Re: Excessive fuel consumption

Pull one plug wire at a time to see if it is running on 2 cylinders.

 

[Texasmark]

Re: Excessive fuel consumption

Hey Kite. Are your Rudes cross flow? Bet they are and there's your reason why Loopers became so popular so fast.

I never put a ruler to it, but having run the old crossflows (up thru 125 hp) and then a Direct Charge (115 hp) and finally Loopers (thru 90 hp) and finally a 4 cycle (120 Mercruiser) there is a noticeable difference in fuel consumption.....was for me anyway. The loopers ran more like the 4 stroke and I assume that's what Silver is using for his statement....beins most engines of today are looped, injected, or 4 strokes.

Mark

 

[JasonJ]

Re: Excessive fuel consumption

Pirate, I recommend doing a decarb (I assume it is a 2 stroke), and check the compression. The rings may be sticking and causing a reduction in compression. You would lose power, and waste more fuel because you woule be giving more throttle to try to go the speeds you used to. Good luck...