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Old 03-01-2014, 03:37   #1
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Yamaha 15HP outboard issues

For my dingy, I have a 15HP Yamaha outboard, manual start. (Model F15BMH 6D4K S 1000777).

I took it out for a 15 minute trip today, where there was no problems at all, but coming back ... oops ... I had taken the fuel tank with me, and when I came back, I started the engine before I connected the fuel tank. The engine ran for about 3-4 minutes with just the fuel in the hose and carburettor, then I connected the tank before the engine quit. Sailed at low power for about 5 minutes (out of a harbour), and then as I increased power, it then quit half a minute or so.

I managed to start, but it quit again. I pumped the fuel hose many times, and did all sorts of things. When I got back, finally, I took it out again just to trouble shoot it. It seemed I could run it either at idle or low speed, while in gear, but any higher speed, it would quit. Either after a few seconds, but at times it went for several minutes before it stalled. I then found out that it would run without stopping, when the choke was about an inch out.

I looked at the fuel filter, just from outside, but didn't have any tools. Wouldn't this be the most likely problem - dirt in the fuel filter?

Because at first I thought it was air pockets in the fuel line, but I think I can safely rule that out.

Any thoughts (for a rather inexperienced ) ?
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Old 03-01-2014, 04:04   #2
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Talking Re: Yamaha 15HP outboard issues

It sounds like your high speed jet is cloged. Take carb off be carefully with plastic linkage when you pull them off tell break .Take carb off then take bowl off and you can get to jets. Very easy get some engine Turner and soak jets in also spray into carb then flush out with carb cleaner and air.Very easy you can do it yourself .
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Old 03-01-2014, 04:13   #3
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Re: Yamaha 15HP outboard issues

+1
These are typical symptoms of a blocked carburettor. They are usually very easy to disconnect and clean, the biggest challenge is not dropping any parts into the water.
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Old 03-01-2014, 04:21   #4
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Re: Yamaha 15HP outboard issues

Thanks, ironworker and noelex77 - those were quick answers!

So if the engine at times could run for many minutes at full speed with no problems until ... suddenly it quits, would that conform to the carburettor jets you refer to?

One final thing to add ... the engine is run just once or twice every two months, and not for too long. Going through the fuel in the tank takes about a year, as this boat is only used for shorter trips.
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Old 03-01-2014, 04:33   #5
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Re: Yamaha 15HP outboard issues

If engine runs at full speed and then quits It's not the jet .Might be fuel filter or your hand primer bulb is bad and loosing pressure when it quits running reach back and feel bulb and see if it's tight. Might change gas for some fresh also or put sta be in it.
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Old 03-01-2014, 04:47   #6
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Re: Yamaha 15HP outboard issues

I still bet a gummed up carb from old fuel.
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Old 05-01-2014, 22:23   #7
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Re: Yamaha 15HP outboard issues

I pulled the fuel filter off, cleaned it with isopropyl alcohol, and dried it. Remounted and went out with the engine cover off. Just like a few days ago, after about a minute at full throttle, the engine dies down. If I pull the choke out in time, the engine can keep running, so I did that in order to motor back into the marina.

As I idle the engine, the choke can stay in, and if I try to increase the RPM slowly, I can hear it starting to change the sound, and further increase kills the engine.

When I moored up, I saw a layer of fuel on the surface of the water. I know it only takes a few drops to look like a lot, but it was quite an area.

Inside the engine, it seemed dry, couldn't see or smell fuel coming out. Someone told me it seemed like the fuel came from the exhaust (???)



So for the minute or so that it runs at full throttle, it's absolutely fine - normal response to throttle and no misfiring, lack of power or wrong sounds. But then it suddenly dies off.

I am quite sure (from what I can deduct with my limited experience with engines) it's fuel starvation by lack of flow into the float bowl at higher engine speeds. So when the engine runs idle or close to idle, enough fuel comes in, when it's at high speeds, it empties the float bowl down to where it cannot draw fuel any more.

Applying the choke partly should explain that.

But how about the fuel spill - could the fuel I am missing at high speed be that same fuel being dumped into the water (through the exhaust?)

Unless there is a clear diagnosis here, I will tend to just "call a friend", i.e. $$$
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Old 05-01-2014, 23:32   #8
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Re: Yamaha 15HP outboard issues

Can we assume you opened the vent on the tank??
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Old 06-01-2014, 00:52   #9
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Re: Yamaha 15HP outboard issues

Quote:
Originally Posted by donradcliffe View Post
Can we assume you opened the vent on the tank??
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Old 06-01-2014, 01:27   #10
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Re: Yamaha 15HP outboard issues

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Can we assume you opened the vent on the tank??
How would that explain the fuel spill, or that the engine runs fine if the choke is an inch out?
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Old 06-01-2014, 01:41   #11
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Re: Yamaha 15HP outboard issues

How about old, worn or incorrectly gaped spark plugs?
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Old 06-01-2014, 03:01   #12
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Re: Yamaha 15HP outboard issues

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How about old, worn or incorrectly gaped spark plugs?
That wouldn't explain that the engine runs fine for a minute or so, then suddenly quits.

So I took it out again just now.

No leak, it must have been some puddle from another boat that I just happened to end up in when I shut down. This time, no spill at all noticed. But same symptoms.

Just for the sake of the funny suggestion above, I removed the fuel cap entirely to ensure no vacuum. Same symptoms.

So if there are not other suggestions, I'll have someone else take the carb apart and clean it.

How's that fuel additive mentioned here - does it really work wonders on a four-stroke?
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Old 06-01-2014, 03:02   #13
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Re: Yamaha 15HP outboard issues

Btw, can anyone tell from that picture I posted above, whether it's diesel or gas/petrol?
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Old 06-01-2014, 04:45   #14
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Thumbs up Re: Yamaha 15HP outboard issues

Ditb get some starting fluid and go back out and runenguine.WHen it starts to quit spray fluid directly into carb and see if engine speed picks back up .If it does then it's fuel related and not electrical.CLean carb and run again if still acting up c compression .
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Old 06-01-2014, 05:14   #15
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Re: Yamaha 15HP outboard issues

The four stroke Yamaha motors are very prone to gumming up in the fuel system. It could be in the pump the carb or both. You should be running stabilizer in the fuel and I wouldn't run a five gallon tank with your light use. Get a two gallon tank and change it our more often. It is likely that the fuel pump is gummed up and works some time and not others. They also are prone to corrosion issues in the fuel pump due to water. I would consider putting a small racor filter on the fuel line as well.
Jay
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