Yamaha 15HP outboard issues

[GordMay]

You might have a pinhole (air) leak in your fuel supply hose or primer bulb.

[Captain Bill]

This sounds like a dirty carb to me. I have the same engine and with the same symptoms it's always a dirty carb. With debris floating around the bowl the clog is irregular and can often go for a minute at high speed on the fuel in the bowl. The choke creates an extra fuel path through the carb and allows it to keep running. There is a vent on the top of the carb in these engines and when the float needle valve is stuck open the fuel overflows out the top of the carb and causes the fuel sheen on the water. It also causes an overrich condition which can stop the engine. You will need to take the carb apart and clean it throroughly. I've become quite expert at it and can do it in about 45 minutes. It's not that hard to do. Make sure you get the fuel inlet needle valve seat really clean. Buy a can of spray carb cleaner before you start and use the small tube that comes with it to blast all of the small passages clean. (wear eye protection and do it in a well ventilated area as the spray tends to come out of unexpected places) I use a piece of paper towel sprayed with carb cleaner to wipe the seat and the needle valve. Just spraying it never seems to be enough. Make sure there are no pieces of the paper towel left in the parts. The holes in the jets are very small and must be clean. Hold them up to a light and if the holes don't appear perfectly round they are still dirty. I sometimes use a single strand of wire from a fine brass wire brush to loosen deposits in the jets. Use it gently as you don't want to scratch the sides of the holes. Then flush the jets with the carb cleaner. Odds are you will not require a carb rebuild kit to complete this task, but you may want to have one available anyway. Mine was once gunked up so bad this did not even work. I took it to a Yamaha shop where they soaked it overnight in an ultrasonic carb cleaner. They charged me $75 for an hours labor.

[Vasco]

Gord has a good point. A hole in the fuel line will do this. Check the fuel filter and see that it's full of gas when it stalls. Also if it'll run with the choke out it's usually the carb. Those jets are very delicate and need to be removed and cleaned by soaking. Running wire through them increases the hole and is not advised. Blowing carb cleaner through sometimes works but not often.

[ontherocks83]

Because a gummed up carb can cause these symptoms and is a common cause of them. You really do need to start there. I believe capt bill is right but as vasco said I would definitely shy away from sticking any foreign object into a metering orifice. I know they look like just a hole but they are engineered to be that shape and size. The tiniest of scratches can have a negative effect.

[john540]

I had similar symptoms with an intermittent air leak where the fuel hose connects to the motor.new o-rings fixed it. It also would leak fuel when I primed it, sometimes.

[DITB]

Thank you for all your responses, it is very kind of you!

I will let you know once it's up and running again. It does indeed seem like it's just the carburettor that needs cleaning.

[DITB]

I now emptied the fuel line, cleaned the fuel filter (again), and I emptied the carb from the drain screw. Pumped in some isopropyl alcohol, and let that empty again. Gave away the fuel to someone who can use it (i.e. smarter than I am), and cleaned out the fuel tank entirely.

Yes, there was some water in it, and a little black smudge. That alone could have been the only cause, don't know why I didn't see it the first time, but I find it kind of hard to empty the fuel tank completely. Next time I will use a simple pump to pump from the bottom of the tank (before it is empty), making a lowest point, to get water and other sediment out.

Btw that tank is a 3 USG tank, so I'm happy with the size - just need to burn it more often!

I went to the gas station and I found some STP fuel system and carb cleaner. Filled the fuel filter half way up with that, as well as added some to the fuel tank before refilling with new gas. Pumped a bit into a container to check, and that was all clean, no sediment at all.

I also want to get my hands on some STA-BIL additive, but here in Hong Kong it seems the only option is to buy it online. Or the shops that sell it don't advertise online!

I just took it on a shorter trip, and it seemed OK now. Waves were too big for comfort, single handed in that small dinghy it was hard to run it at full speed continuously without getting a good beating!

For all your help, I hope some others can benefit, there were some really good feedback here (and some funny posts as well )

[darylat8750]

Hope your problem is solved. If not a real carb cleaning would be my next order of work. If you have a car or truck just empty the outboards tank into your car every couple of months and put fresh fuel in your boat.

[a64pilot]

Be careful burning mixed fuel if your car has a catalytic convertor.
If it's not mixed you should be fine

[darylat8750]

Originally Posted by a64pilot

Be careful burning mixed fuel if your car has a catalytic convertor.
If it's not mixed you should be fine

I may be wrong but I thought that model is a 4 stroke.

If the high speed jet is clogged just running carb cleaner through the gas and spraying in the carb while it is running seldom works for me. The overnight soak seems best.