Would old petrol cause this?

[VicS]

Just to be clear guys, starting is NOT a problem, first pull and off it went merrily. The ONLY issue, is that if I throttle back from half power, the engine dies about halfway.
Obviously, it makes it a bit difficult to engage gears then, I have to hold the thing down to put it in reverse as it wants to jump up, and more to the pint, I will be manouvering at speed in the marina etc, which is really not ideal.


By the way, regarding shutting off the fuel and not stopping the engine, the ob is fed from a separate fuel tank with no on/off tap, so that's just not possible, unless I'm missing something?

You are saying that you have a remote tank, rather than an integral tank with a fuel tap. and that you therefore cannot run the carb dry ?

If that is so you simply disconnect the fuel line and let it run until the carb runs dry ! it's what everyone does with remote tanks !

You do not appear to have confirmed that you have dumped all the old fuel and replaced with fresh. If You have not then do so without further ado.

However not only would I replace the old fuel I would strip and thoroughly clean the carb, concentrating on the slow running jet and passages

 

[steve yates]

Thats it vic, does everyone else do this? I must admit I have never seen it done, and I thought that all I would be doing is possibly splashing fuel around by disconnecting it while in use. It's gonna have to be kept in an upright position somehow or everything from the primer bulb onwards is just going to piss all over the cockpit surely?

No, I haven't replaced the fuel yet, as I previously said, I'm a long way away now, and want to plan best use of my time. If anyone had definitively said, oh yeah, old fuel always caused me a similar problem, then I would be fairly confident about just swapping fuel next time I'm up.

Doesn't really look like that's the cause though, so I'll plan on removing the ob and bringing it back south for further investigation, and leave the fuel where it is for the time being.

 

[PeteCooper]

Steve - I have a very similar setup to you. When I moor I unclip the petrol pipe from the engine - no more than a drop spills. I then let the engine run as I finish off various other tasks and eventually it runs to a stop.

 

[VicS]

Thats it vic, does everyone else do this? I must admit I have never seen it done, and I thought that all I would be doing is possibly splashing fuel around by disconnecting it while in use. It's gonna have to be kept in an upright position somehow or everything from the primer bulb onwards is just going to piss all over the cockpit surely?

No, I haven't replaced the fuel yet, as I previously said, I'm a long way away now, and want to plan best use of my time. If anyone had definitively said, oh yeah, old fuel always caused me a similar problem, then I would be fairly confident about just swapping fuel next time I'm up.

Doesn't really look like that's the cause though, so I'll plan on removing the ob and bringing it back south for further investigation, and leave the fuel where it is for the time being.

I have been doing it for years. The ball in the hose end fitting seals it as it is disconnected

Some engines appear to run happily on old fuel , some dont. Never tried old fuel, other than what has been stored in a full, tightly sealed metal cans in my Evinrude but the Seagull is not too happy with old fuel and the lawnmower wont go at all on last years fuel!

Until you try fresh fuel you cannot be sure that you are making good use of your time

Have you checked other basic things.

Compression ?

Spark ... with a spark tester not just at the plug electrodes,

valve clearances?

Always worth trying a new plug too.

BTW Service manual here: http://162.144.28.33/lib/mercury/manuals/4-5-6.html#/0

 

[lenten]

Steve - I have a very similar setup to you. When I moor I unclip the petrol pipe from the engine - no more than a drop spills. I then let the engine run as I finish off various other tasks and eventually it runs to a stop.

-----i always unclip the petrol pipe from the tank saves having stale fuel in the petrol pipe

 

[JumbleDuck]

I had problems with rough running and cutting out - obvious symptoms of fuel starvation - that caused me to dismantle and clean the carb and filters several times to no avail. It turned out to be due to corrosion on the connection between the coil earth wire and the engine engine.

There's a general principle that all obvious fuel problems turn out to be ignition problems and all obvious ignition problems turn out to be fuel problems.

 

[BelleSerene]

You do not appear to have confirmed that you have dumped all the old fuel and replaced with fresh. If You have not then do so without further ado.

However not only would I replace the old fuel I would strip and thoroughly clean the carb, concentrating on the slow running jet and passages

And it's easy to forget that 'dumping the old fuel' should include cleaning and airing the tank or jerry can you keep it in. As that gets used and topped up through the seasons, some of it can be far older than you imagine.

 

[Lon nan Gruagach]

Just an anecdote but....
We were just given a well sealed container (proper green plastic 5 litre) of old, maybe very old petrol for a lawnmower. Whilst the mower did start and run, it seemed to have significant trouble with long grass. Mixing in a bit of new fuel has it performing better.
Conclusions: even a well sealed container won't necessarily prevent fuel going off. It didn't gunge anything up, we'll not enough that new fuel couldn't fix it.
As I say, a one off anecdote.

 

[steve yates]

And it's easy to forget that 'dumping the old fuel' should include cleaning and airing the tank or jerry can you keep it in. As that gets used and topped up through the seasons, some of it can be far older than you imagine.

Thats no problem, the old tank has a hole in the bottom patched with mastik and gaffer tape, and the current priming bulb and pipes got splashed by something caustic and are patched with more tape I have a new tank and lines sitting in the cabin waiting to go, so will just bin the entire old system.

Interesting that folk go to these extents when shutting off thier ob though, I have never seen it done nor heard of it being done until now, which just goes to show how easy it is not to know what you don't know! I won't bother doing it under normal use, but will certainly do so at the end of a cruise when she is next used properly.

 

[VicS]

Interesting that folk go to these extents when shutting off thier ob though, I have never seen it done nor heard of it being done until now, which just goes to show how easy it is not to know what you don't know! I won't bother doing it under normal use, but will certainly do so at the end of a cruise when she is next used properly.

That's it

It is not necessary to run the carb MT every time. At the end of a cruise/ week/ weekend sailing is adequate, instead of leaving a bowl full to evaporate. It is no major hassle with an integral tank with a tap and even with a remote tank you'd disconnect the fuel line anyway in most cases before stowing engine and tank in a locker or loading into the car boot.

The real knack to cultivate , with the dinghy outboard at least, is judging when to turn the fuel off so that the engine splutters to a stop as you approach the hard/ slipway just before you need to tilt the engine to prevent it grounding.

 

[steve66]

I have just had an outboard in with exactly the same issues. The owner asked me to replace the carb with a new one. The fuel had been in the tank less than 6 weeks. I know this as I serviced it 6 weeks ago. I took a sample of fuel and could see it was stale ( fresh fuel is clear, stale fuel has a yellow/brown hue). To prove a point I fitted a new carb and the symptoms were no better. I emptied the tank and drained the system and poured in pump fresh fuel and fitted the old carb .Started first pull and all the other running issues that are identical to the OP's engine disappeared. I cannot stress enough use pump fresh fuel in modern outboards

 

[AngusMcDoon]

That's it

It is not necessary to run the carb MT every time. At the end of a cruise/ week/ weekend sailing is adequate, instead of leaving a bowl full to evaporate. It is no major hassle with an integral tank with a tap and even with a remote tank you'd disconnect the fuel line anyway in most cases before stowing engine and tank in a locker or loading into the car boot.

The real knack to cultivate , with the dinghy outboard at least, is judging when to turn the fuel off so that the engine splutters to a stop as you approach the hard/ slipway just before you need to tilt the engine to prevent it grounding.

Some carbs, like one of mine, have an easily accessible bowl butterfly headed drain plug. After running the engine until it stops quite a bit is still left in the bowl which dribbles out on opening the drain valve. Not all engines have that feature though. More often it's a not very accessible slot headed plug. To do a proper drain for winter layup it's best to use the drain plug, even if it's a bit of a fiddle.

 

[steve yates]

I have just had an outboard in with exactly the same issues. The owner asked me to replace the carb with a new one. The fuel had been in the tank less than 6 weeks. I know this as I serviced it 6 weeks ago. I took a sample of fuel and could see it was stale ( fresh fuel is clear, stale fuel has a yellow/brown hue). To prove a point I fitted a new carb and the symptoms were no better. I emptied the tank and drained the system and poured in pump fresh fuel and fitted the old carb .Started first pull and all the other running issues that are identical to the OP's engine disappeared. I cannot stress enough use pump fresh fuel in modern outboards

Oh.

Ok, thanks. I will replace the tank and try it with some fresh fuel. Can I just clarify? I have only ever asked about one single running issue, there are no "others" and specifically, it starts easily, first pull, every time. And runs fine, until you throttle down ( I always start it at half throttle)
That's my ONLY issue, it cuts out at low revs. ETA: I did have a look in the fuel tank, the petrol seems clear to me, is the yellow brown hue quite obvious?

 

[VicS]

Oh.

Ok, thanks. I will replace the tank and try it with some fresh fuel. Can I just clarify? I have only ever asked about one single running issue, there are no "others" and specifically, it starts easily, first pull, every time. And runs fine, until you throttle down ( I always start it at half throttle)
That's my ONLY issue, it cuts out at low revs. ETA: I did have a look in the fuel tank, the petrol seems clear to me, is the yellow brown hue quite obvious?

You told us in your opening post that you are trying to use very old / stale petrol and that when tried on different fuel by another it ran OK. Surely that should have been sufficient to persuade you to change the fuel

In a later post you told us that your current tank and fuel line is all patched up with "mastic" and sticky tape ! There is now a good chance that you have clagged up the fuel hoses, the filter, the fuel pump and the carb with adhesive from the sticky tape and the mastic if it is not 100% petrol resistant.
My advice is to get rid of the old tank, contents and the fuel line. Then replace the filter and thoroughly clean the connector, the fuel pump, the carb and the interconnecting hoses to prevent any future problems from these sources.

The service manual in my earlier link has all the necessary information for stripping and cleaning the fuel pump and the carb

 

[steve yates]

"You told us in your opening post that you are trying to use very old / stale petrol and that when tried on different fuel by another it ran OK. Surely that should have been sufficient to persuade you to change the fuel "

It was certainly my initial thought, hence why I asked the question. I couldn't do it at the time and am away for quite a while. When I get back I will have very limited time. I asked if it would cause this specific problem for a reason and most of the answers, until steve 66 posted, seemed to say no, it would cause starting and other running issues too, none of which I have. That matters, because I my original plan of changing fuel and then moving the boat under engine to it's new moorings wouldn't work, and maybe I should plan on taking the ob away, cleaning the carb etc and then bring it back on my next trip north. (obviously changing the fuel too)

Now steve66 suggests that old fuel does cause exactly this issue and fresh petrol will solve it immediately, so I can plan on moving the boat after all on the next trip north. Which is good Plus I learned more than few other things in the thread too!