(shocking) Fuel Economy on (old) 10hp 2stroke

The_Heretic · 06 July 2006, 22:43

Hi all,
Im new to all this, so this could be a real newbie question...
I run an very old (20 yrs) Evinrude 10hp 2stroke outboard on my Zodiac Fastroller340. I got the engine second or third hand and whilst it starts and runs completely fine, Im concerned that the fuel eceonomy is REALLY bad

By my estimate it used about 8 litres of fuel in a 6 mile or so round trip - about 1.5 - 2 hrs running time. That was with only myself and some camping / photography gear on board.

Also, when I kill the engine prior to running up the beach, I notice theres a fair slick behind me, which I really hate to see

I cant stand the though of polluting the very thing Im there to enjoy, so the question is fairly obvious :

How normal is this for an engine of this age ?

I can see there are plenty of technically knowledgeable people on here, so has anyone got a suggestion what can be causing the shocking fuel economy ? My guess was wear n tear on the valves requiring a rebuild - though Im a pessimist aparently

Also, are there any suggestions on what to change or fix to make it better ?

And one more questions if I may...

I am strongly considering just buying a new engine. A shiny new 2-stroke outboard. Specifically a Tohatsu 9.8hp.
How would you say the fuel economy and pollution would compare on a new one of these with my old Evinrude ? And are they as good engines as they seem to be - horror stories anyone ?

Thanks very much. Rather too many questions for my second post on here

Nice to be among you at last

rgds,
Doug.

Nos4r2 · 06 July 2006, 23:37

Were you running at full throttle or close to it for most of the time?

That's about the usage you'd expect from a 10hp for 2 hours use at full throttle.

Go up to 15/20hp if your boat will take it and use less throttle to get the same speed and you'll use less fuel.

Get a 4-stroke and you'll use even less but it'll be heavy. Check your boat will take the extra weight if you do.

Re the oil slick-what 2 stroke oil are you using?

Poly · 06 July 2006, 23:50

Nos is right at FOT this is the expected fuel consumption of a 10HP engine. Rule of thumb is 1 Gallon (not sure if that is US (3.8L) or UK (4.5L)) per hour of use. So you are in the right ball park.

I think he is also right to suggest that a slightly bigger engine running at less than full throttle but the same speed will use less fuel. E.g. my new 20HP yamaha uses less fuel than this sitting at just over 3/4 throttle and around 18-19 mph on a 4m boat with me and a load of gear and fuel.

Andy JC · 06 July 2006, 23:59

Welcome Doug

First of all 20 years is not actually all that old for an outboard, you'll find there are still loads of 2 stroke outboards built in the early seventies still going strong today. It's also suprising how little they have changed in quite a long time, I have an '88 Johnson and a '05 johnson and they're very similar in lots of ways.
That said, the piece of mind from having a new or nearly new outboard is unparalleled due to how much you rely on it when out on the water.

As for your current outboard, it's possible you might just have a fuel leak. It's probably worth checking all the fuel lines, clips, filter etc. Also have a good sniff around to see if you can smell anything, as I'm sure you know petrol's pretty stinky stuff. It could be running too rich which is a simple adjustment of the mixture screw, how does it start/run/tickover? It is of course also possible that you might have more severe internal engine problems but it's worth checking out the obvious first before looking at buying another engine. Even if you do need to buy another outboard it would be well worth considering a 'nearly new' model... oh and go for the 15hp, you won't regret it

Although a little over priced (probably worth around £900) something like this:

http://www.boatsandoutboards.co.uk/view/F106637/

Andy

Nos4r2 · 06 July 2006, 23:59

Sorry-should have said it's US gallons.

The_Heretic · 08 July 2006, 12:47

Thanks all for replies

I guess I got trigger-happy with the keyboard, my boat is a fastroller 310 not a 340 as I stated, so my max engine size is a 10hp. Makes sense what you guys were saying about getting a 15, but 10 is the limit.
A four-stroke would be nice and no doubt cleaner, but they seem to weigh about 30% more than a similar 2. The point being, its mostly just duggy-no-mates in the boat, as none of my friends are interested in leaving the pub for long enough - lol.

As to running time, I must admit, I have the throttle full open most of the time, though Im surpprised that 1hr/USgallon is the norm. I had expected it to be a bit more economical.
I have noticed that once its planing, I can throttle back to about 75% and still hold the plane nicely, so maybe Ill excercise a little more discipline.

Regarding the oil slick, Nos, I've left the carton in my mums garage, so not sure exactly what type, but the ratio is 100:1 so I guess its gonna be pretty mucky. I compare that to the 50:1 on most newer engines - and a wee scooter I used to have.
/reminisce...

Ill take a look at the mixture as Andy suggests, and see if I can find a nice balance there. That, combined with "throttle-discipline" could make for a good experiment.

Really I just didnt like the idea that it was leaking fuel through the engine (worn valves or whatever) and pumping it out into the water.

As to a new engine, well, Ive been looking at the Tohatsu 9.8hp 2-strokes. Aparently theyre pretty good, and a decent price. Whats more theyre light, which is good for when Im using the boat myself, and also just man-handling the thing on my own.
Anyone had any long-term experience with these engines ?

Thanks again, for the info guys, I feel a lot happier now, and Im not panic-saving for a new engine

rgds,
Doug.

Nos4r2 · 08 July 2006, 18:22

Quote:

Originally Posted by The_Heretic

Regarding the oil slick, Nos, I've left the carton in my mums garage, so not sure exactly what type, but the ratio is 100:1 so I guess its gonna be pretty mucky. I compare that to the 50:1 on most newer engines - and a wee scooter I used to have.
/reminisce...

Really I just didnt like the idea that it was leaking fuel through the engine (worn valves or whatever) and pumping it out into the water.

you're running on 100:1? Evinrudes of that era should be 50:1.

You should be using marine 2 stroke oil as well (I assume you are?) rather than motorcycle/chainsaw type 2 stroke.

You won't be leaking fuel through the engine-if it's running on both cylinders then it's pretty much ok. If there's a major problem with cylinder/piston wear it'll start showing by an inability or reluctance to tick over in gear while in the water.

I assume you're not killing the engine while it's still revving are you? That'll chuck unburnt fuel out of the exhaust and cause a slick.

wavelength · 08 July 2006, 19:51

gallon and hour for every 10 horsepower we used to reckon on outboards when they were all two strokes (I've been at this far far too long!) Figures about right then. The words economy and outboards were not really things you could use in the same sentence then.

The_Heretic · 09 July 2006, 17:52

Right, Ill double check the ratio requirements in case Im using too much oil as you suggest, Nos. And no, Im not killing it whilst revving - that would just be vandalism

1gal / hr / 10hp that sound like a good rule-of-thumb, thanks John

OK, thanks guys. Next weekend Im gonna take it apart and replace the gaskets, take alook at the spark plug etc...
From there I should be able to figure out how its running (rich / lean etc).

Im off on the West Highland Way tomorrow, but I should be back saturday.

Youve all been a great help, thanks again to all replies.

rgds,
Doug.

Neil Harvey · 09 July 2006, 18:14

It's unlikely you're actually leaking fuel, especially if you manage to keep going at full throttle. A fuel leak in the line etc means equally you could suck air in and hence the engine will stop running. So that's probably not the case.

If you're mixing 100:1, you're not using enough oil IF it's supposed to be running on 50:1. At 100:1, every litre has 10ml. At 50:1, every litre has 20ml of oil.

Hope this helps.

Neil

PeterR · 09 July 2006, 19:23

Not wishing to be picky,Oh by the way welcome ,but notice you mentioned worn valves your a mecanic and know 2 strokes dont have valves