Outboard leaking petrol via exhaust?

Keiron

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I have an 8hp Johnson outboard that appears to be using an excess amount of fuel. We did 17nm over the weekend and used maybe 10litres of fuel which sounds a lot. The engine is on a 23' Hunter Duette and the boat was run at 5knots in calm conditions with no wind. Also when stopped a small petrol slick appears around the engine on the water. Not a huge amount of fuel but continual.

No obvious leaks from the engine of fuel supply. Difficult to be sure but it appear the excess fuel may be coming from the exhaust, is this possible? I stripped the carb off yesterday to check the float needle wasn't sticking and that looks fine. Anyone have any suggestions what the problem may be?

Cheers Keiron
 

stevebrassett

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I have a 9.9 HP Suzuki, which burns about 5 litres of fuel an hour at 5 knots. Yours doesn't sound excessive to me. Is it a two-stroke, and how old is it? If the bores are worn, it is quite possible for unburnt fuel to get past the piston and pass out into the exhaust. Someone who is more of an expert will give a much better answer soon.
 

Keiron

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Thanks for the ideas, sorry should have said its a 2 stroke, built in 2001 but not had much use. I have probably clocked up 30 hours a year over the past 3 years (we prefer sailing!!). Prior to that, the unit was hardly used. I guess it's possible that the rings are sticking then...

K
 

VicS

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My 6hp Evinrude (2 stroke) uses about 1/2 gallon an hour pushing a 19 footer at less than 5kn. So 10 litres in that distance, although more, may not be excessive. Slow down a bit and you may use a lot less. Is it the "sail" version (mine is) with a suitable prop for the job. If the model number includes the sequence J8S then the S stands for "sail"

If the carb float and valve are ok and adjusted correctly then I cannot see how it could be flooding. When idle presumably the tank is lower than the carb so there will be no possible flow unless the tank is pressurising in the sun with the vent closed. Disconnect the fuel line and see if it makes any difference.

It'll need quite a bit of fuel entering the engine before it appears from the exhaust I think sufficient to make starting again difficult.

The high fuel consumption is the reason I have a 6 gallon tank.
 

tross

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5 Knots - you must be running the engine flat out.

The slick you see on the water is unburnt fuel you are pouring in to it. Basically the engine can't rev enough with the boat speed you get to use it all so it has got go somewhere.

I have a Horizon 23 and throttling back to 3.5 -4.0 makes a BIG difference to the consumption with my 8HP Yamaha.
 

tugboat

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Why do you think the slick is petrol? It is quite normal for 2-strokes to display such a slick, it is 2-stroke oil from the fuel mixture which is expelled down the exhaust. There tends to be oily residues in the exhaust pipework which will slowly wash out if the leg is left immersed. Basically this oily mess is why 2-strokes are being phased out. I don't think you need to worry about a leak.
 

Lakesailor

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Coo. My 70 HP Johnson uses about 1 gallon an hour at 5.5mph (15ft launch). It's only running at 1500rpm, but it doesn't seem as ridiculous an engine choice as I thought it would be at first.
I would reckon that Keiron is approaching his displacement max speed (probably 6 knots) and as you say winding back on the throttle will drop the revs, but hardly effect the speed. A handheld GPS will confirm that acurately.
 

Keiron

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Thanks for the advice. The slick really only appears at tickover. I guess the fuel consumption may not be so bad then. In fact the engine will push the boat at 5kn at about 2/3 throttle. It;s quite a light, flat bottomed boat. I'll try throttling back a bit and see if that improves the economy. Looking at the model number its not a saildrive engine so a change of prop may also help..
Cheers K
 

VicS

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I think you will find there are 3 alternative props for your engine. In inches, diameter x pitch they are 8x7, 8½x9 and 9¼x6½.

Second one is the "standard" prop and the third the one most suited to your needs. It is what is fitted to my 6 hp Evinrude Yachtwin and the 8hp Yachtwin.

It is virtually impossible to determine the pitch but the different diameters should enable you to determine which you have. I would not spend a shed load of cash changing it though.
 
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