Poor fuel consumption...I think?

[petrolhead]

OK guys, I've done some reading up of old posts on this subject and I'm pretty sure my Johnson 18 is getting awful mileage!<br />It's on my 11' Glen-L Squirt which I took out on the river today with myself and the wife aboard, and in no time at all I was running low on gas. The speed limit is low on this stretch of river, so at no time was I exceeding about 40-50% WOT, and in 2 hours I had used up approx 16 litres ( 4.25 US Gallons) of gas.<br />According to the original manufacturers specs I should get 2.5 to 3 hours at WOT from a full 6 US gallon (23 litre) tank, but as you can see I'm getting nowhere near that!<br />The carb has been rebuilt, and I tuned it up the best I could using advice I found here and in my manual, could it just be that I've got the fast needle setting way out somehow, would it still run OK if it was far too rich?<br />Maybe, as I don't need the full power of this engine most of the time, I might be better off looking to change it for something smaller and more economical?<br />With UK fuel prices I'll never be able to afford to use it if this is going to be the norm!

 

[cajun555]

Re: Poor fuel consumption...I think?

petrolhead, from what I've read on these threads, 2 to 3 mpg at wot out of these old motors, another is fuel consumption based on hp. You take 10% of your h.p. rating at wot per hour. In your case would be 1.8 gal per hour. My old 30 is close to the 3mpg. Also weight is the issue, were you going up or down river. I learned long ago boat outboards are not the most efficient things.

 

[petrolhead]

Re: Poor fuel consumption...I think?

I tried that 10% calculation on my motor, 1.8 US Gallons per hour would be acceptable at WOT, but I'm using a fair bit more than that at less than half WOT!<br />Up or down stream makes virtually no difference here, current is almost unnoticeable, but FWIW I did about the same distance upstream as down so it would average out anyway.<br />Based on OMC quoted WOT figures of 2.5 to 3 hours per 6 US Gallon tank, that equates to around 2 US gallons per hour. I'm using more than 2 gallons per hour at maybe 40% WOT so it seems to that something is wrong somewhere.

 

[rustyoletoy]

Re: Poor fuel consumption...I think?

My engine does the same thing as yours, Petrolhead. I have a '91 25hp and WOT lasts about an hour and 15 minutes before I've sucked back a 6 gallon tank. Half throttle and taking it nice and easy gets me about 3 hours. I've gone out with friends with similar boats, one has a old 20hp Merc and the other has a brandnew 25hp Nissan. Both get significantly better mileage than I do. I've tuned my engine according to the manual and everything seems right with it other than the mileage. Still haven't been able to figure out why it is bad and I'm hesitant to keep leaning the engine out since it is at the proper specs that the manual called for.<br /><br /> Sean

 

[Paul Moir]

Re: Poor fuel consumption...I think?

Exactly how far out is the high speed needle, and is your exhaust particulairly smokey? How do the spark plugs look/compare?

 

[cajun555]

Re: Poor fuel consumption...I think?

Something else is on the older motors they have a drain relief valve that dumps fuel overboard. This fuel collects in a couple of areas in crankcase, then dumps down l/u. I just modified the cover for this to collect the fuel in a container to see what I was dumping. Lastnight I collected 3oz. of fuel. Probally used about 3/4 gal. of gas. If this is the case you will see the rainbows in the water when sitting still. On one post some guy did the same thing and collected 1:6 ratio. Quite a bit of fuel.

 

[petrolhead]

Re: Poor fuel consumption...I think?

I don't see any drain relief valves in my parts catalogue exploded view, so I assume this motor has none?<br />Paul, I started with my needle 3/4 turn out as specified, but after adjustment I'm not sure how many turns out it is exactly, but it's one of the first things I'll be checking in the morning before I go out again, that and the condition of the plugs.<br />I wouldn't say my exhaust is very smoky, I've seen worse, but then I've seen better too!<br />I'll run another tank through tomorrow after making adjustments and see if it's any better. Can I adjust the high speed needle without running at WOT? The problem is at WOT I'd be doing about 30 mph, and that's way over the speed limit here, someone would notice for sure if I was going that fast, mind you I don't think the river police boat goes that fast so as long as I don't run out of gas....

 

[petrolhead]

Re: Poor fuel consumption...I think?

I think I solved my problem, I retuned the carb this morning and now have the fast needle set at 3/8 turn open, yesterday it was set at about 3/4 turn, it still runs happily on this new setting, exhaust is much cleaner, and I just pulled my plugs after 1 1/2 hours and no sign of running lean, so I guess that must have been it. I covered the same distance today as yesterday and used less than half as much fuel!<br />Todays problem though I spun the hub! Had to come home at no more than 1/3rd throttle otherwise it would start to slip. I'm going to look at it later and see if it's something I can fix, if not it may be new prop time if I can find somewhere that does props for this motor at sensible prices.<br />I'm wondering if maybe it was slipping a bit yesterday, which would have contributed to the fuel consumption problem, if it was I certainly didn't notice it but I suppose it could have been.

 

[Paul Moir]

Re: Poor fuel consumption...I think?

Sounds like you're on to the problem. You can set the WOT on your engine without speeding along by securely tying your boat to the dock or whatever. I guarantee this will generate lots of interest in you and your boat, but hey - it works.<br />Good luck with the spun hub. I'm interested to hear how you make out!

 

[petrolhead]

Re: Poor fuel consumption...I think?

Paul, don't you think that 3/8th setting for the needle sounds a little odd though? It does to me, I expected the setting to be very close to the 3/4 turn starting point, with the taper angle on the fast needle I expected it to be far more sensitive than this.<br />Oh yes, I pressed the hub out of the prop earlier and the rubber bush disintegrated, literally it just crumbled to wet soggy mush! Not sure how I'll fix that, are new rubber bushings available to press in, or will I need to mould one?

 

[G DANE]

Re: Poor fuel consumption...I think?

You probably spun the hub because it runs better now. When you hit the point where its at the edge of running lean, it will make the most power.

 

[petrolhead]

Re: Poor fuel consumption...I think?

I reckon you're right G DANE, also because the boat was tied to the dock at the time which must create more stress on the prop bushing than if the boat was moving?<br />I'm going to phone around this morning to see if I can find a local repair shop that offers a rebushing service, if not, or if it's really expensive I may just mould a new polyurethane bush and press it in myself, its a simple tubular bush on these motors so should be fairly easy.