Fuel Efficiency

Strapped on the 1966 20HP Johnson last Saturday and hit the lake. Runs like a champ, although it seems to be a gas hog. Made a 3.5 mile run, each way, and was on fumes back at the ramp (3 gallon tank). I'll be bringing my extra tank next time for sure. Just used to my little 8 that would run all year on a few tanks of gas. Granted, it was my first time out to a bigger lake with no speed limits, mainly ran 3/4 - full throttle, which will suck down the juice. I'll run a can of Sea-Foam through it this weekend, but does this seem out of the norm? BTW, we lit up the crappie, they were mostly under the limit though. My buddy caught a 9 lb bowfin and I brought in a nice 5 lb channel cat :lol:

There is a ratio of Hp to fuel. IIRC with a 20 hp motor you're looking at 2 gallons per hour a WOT.

You're right she's a gas hog. I ran a 77 25hp evinrude on the back of my 16' jon and never had that bad of gas mileage. But if she's running as good as you indicated and after the seafoam I'd just live with the gas mileage but move to having a 6 gal tank in the back in stead of two tanks. Down the line if you want to worry about savings on fuel look at a 4stroke. The Nissan 18hp I have I can run WOT and it just sips gas.

Those 50-60s vintage motors are thirsty, but it's obvious the fish don't mind. Enjoy. :LOL2:

Thanks for the replies guys. I thought it was sucking down the fuel at a pretty good clip. My other tank is a 6 gallon, so good there. It just doesn't fit nice below my deck like the 3 gallon. I'll probably put my 8 HP on when fishing the 5MPH lakes and just use the 20 in bigger waters. It sure was nice to be able to hop up on a plane though, didn't have a GPS, but with 2 people and gear I bet we saw 20 pretty easily. po1, I'll be looking into a four stroke for my next motor, I like how quiet they are. And yes, the fish could care less!

2 strokes have about a 60/40 efficiency rating. They've gotten better in later years, but that's near what your's is.

That rating means that for every 100.00 of fuel and oil you put through that engine, 60.00 is burned, and 40.00 goes out with the exhaust.
Now you know why folks brag on their 4 stroke engines as far as fuel goes..

Your fuel consumption seems high.

I have a 1979 Merc 20hp & get about 7 mpg.My 1976 15hp Evinrude gets about the same.

Would do a little more checking on that. I just ran a 1957 Johnson 18hp on my 15'6" Gheenoe at an Antique Outboard meet this weekend. Used the GPS as I had never run this boat before. Ran a total of 8.71 miles and burned a little over a gallon of fuel. Mostly WOT as I was playing with props and other boats out on the lake. As far as your engine goes I would check the high speed jet size to make sure some brain surgeon before you didn't drill them out or install larger ones to "make more power". You could also have a minor leak at a fuel pump diaphragm that the engine would still run with.

Pappy said:

Would do a little more checking on that. I just ran a 1957 Johnson 18hp on my 15'6" Gheenoe at an Antique Outboard meet this weekend. Used the GPS as I had never run this boat before. Ran a total of 8.71 miles and burned a little over a gallon of fuel. Mostly WOT as I was playing with props and other boats out on the lake. As far as your engine goes I would check the high speed jet size to make sure some brain surgeon before you didn't drill them out or install larger ones to "make more power". You could also have a minor leak at a fuel pump diaphragm that the engine would still run with.

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Love a Gheenoe, they don't cause your engine to burn much fuel :wink:

I just picked up a 13' Riverhawk at a great price.

Pappy said:

Would do a little more checking on that.

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Pappy, would you suggest the decarb process that was mentioned on here? Basically adding an entire can of sea-foam to one gallon of gas and run motor while spraying sea-foam directly into carb.

Johnson/Evinrude has a product called "Engine Tuner" that has been on the market for several decades. This is what I would recommend for you. I do not believe this will solve your mileage issues but will de-carbon your engine better than anything I have seen to date. If you are running a heavy load in your boat then your mileage will suffer, period!
Bobg - You are correct about a Gheenoe being efficient. This model, with my 18hp runs about 28.4 with just me in it and a full tank and livewell. Not bad for 54 year old technology that probably equates to around 14-15hp at the propshaft. I am currently selling my 13' Gheenoe, since I found the Classic I have not used the 13 at all.

Pappy, just pointing out that boat type, hull design, load, boat engine set up, all affect the gallons per miles.

Smaller boats are going to get expensive quick, with gas prices going the way they are.

I've got a 63' Starcraft Jupiter, rated for an 80 hp. Stripped out the weight. The bow was butchered out when I got it. It's powered bu a 97' Johnson, that was just taken out of the box, last summer. It'll do 26.4 on the GPS with just that 25.

Before

After.

If you need a 6 gallon tank, i have one i will give you. Just pay shipping. I used it 1/2 a season and realized that 6 gallons lasts me 35-45 minutes so I upgraded. Send me a pm if interested.

Pappy said:

Johnson/Evinrude has a product called "Engine Tuner" that has been on the market for several decades. This is what I would recommend for you. I do not believe this will solve your mileage issues but will de-carbon your engine better than anything I have seen to date. If you are running a heavy load in your boat then your mileage will suffer, period!

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Thanks Pappy, I'll check it out. I threw in my six gallon tank this weekend and hit the lake, also brought the GPS. My boat is a 1440 MV and with about 180 lbs of mods, two people, gear & full livewell it hit 21.6 MPH wide open on the GPS. That was max, usually clocked about 20.7. That seems pretty good for a '66 :lol: Went through about 3 gallons, the GPS logged 12.3 miles, some of those were drifting and trolling through the cypress stumps though. So, the mileage may not be all that bad. I'm just used to dinking around on a 170 acre lake with a 5 mph speed limit, when I hit a bigger lake and opened it up a bit she drank a little more fuel than I'm used to. I also added a can of Sea-Foam to the tank.

bulldog said:

If you need a 6 gallon tank, i have one i will give you. Just pay shipping. I used it 1/2 a season and realized that 6 gallons lasts me 35-45 minutes so I upgraded. Send me a pm if interested.

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Thanks man, but I have one.

Ictalurus said:

Thanks Pappy, I'll check it out. I threw in my six gallon tank this weekend and hit the lake, also brought the GPS. My boat is a 1440 MV and with about 180 lbs of mods, two people, gear & full livewell it hit 21.6 MPH wide open on the GPS. That was max, usually clocked about 20.7. That seems pretty good for a '66 Went through about 3 gallons, the GPS logged 12.3 miles, some of those were drifting and trolling through the cypress stumps though. So, the mileage may not be all that bad. I'm just used to dinking around on a 170 acre lake with a 5 mph speed limit, when I hit a bigger lake and opened it up a bit she drank a little more fuel than I'm used to. I also added a can of Sea-Foam to the tank.

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Wanted to bump to get input/response on speed data.