Yamaha 50hp Four Stroke Unhappy

[thomshap]

I have a rib with a 2004 50hp 4 stroke Yamaha that had been sat in a river with less than 20 hours use and as far as I can make out had never been opened up as the river has a 8 knot speed limit.

I had a full service a couple of weeks ago and took it out yesterday, I was having a problem with it dieing when I went to engage gear and at a low RPM and it did not sound healthy but when it I gave it some throttle it was fine, it seemed to clear itself so I headed out of Chichester to open it up (had arranged to meet a Questor at West Pole which turned out to be a good plan).

It was running fine, though seemed a little underpowered for a 50hp on a boat that size, 4.5m, after about 10 mins the problem came back, it would not idle at all and it sounded terrible at low RPM so we towed it back into the harbour, where as before it would come back to life it remained sick every time I tried it, periodically, on the trip back to the marina.

I have a couple of possibilities, one is a faulty low pressure fuel pump but I'm not sure if I have one, it's not fuel injected and as far as I can see there are 2 outlets from the main fuel pump, dividing then going straight to the carbs. The second, Richards idea, is dodgy HT leads or plugs, but if they were at fault then would it still run ok at higher RPM?

It's off back to the engineer next week but would like have some sort of idea.

 

[Kawasaki]

50hp on a rib that size should make 25kts with ease
Yep, if the HT side was dodgy I would expect trouble at high speed not low speed.
If the rig/rib feels underpowered it sounds like one cylinder is not functioning correctly maybe
This would also give you idling problems too.

 

[Cret]

I've had similar problems a couple of times on different outboards (both yams but that's irrelevant really).

The first time on my old 3 cylinder 2 stroke 75 'enduro' engine I removed the carbs and cleaned them. There was fuel algae in the carb float bowls and it was sucking into the jets and blocking the fuelling.
It manifested itself by idling ok and revving freely in neutral but under power it would only go a very short distance before bogging down. Then back in neutral it would be ok. And repeat. Managed to limp home as it was only half a mile out to sea.

Cleaned the carbs and it was absolutely fine.

Then more recently with 1999 4 stroke 50 yam, like yours it had been sat a long time and had very very low hours on it. Fuel had been left in it for quite some time and had left varnish deposits in the carbs. This showed up by a gradual drop in performance whereby it would start/run/rev ok, but just seemed weak and wouldn't propel the boat like I knew it should.

Had to get engineer to sort it this time to remove the varnish deposits with ultrasonic cleaning tank and some special solvent. After he did that and reset the carbs the engine ran like it was straight out of the showroom.

Good luck anyway - hope that helps!

 

[thomshap]

Thanks, that all sounds very similar, same engine too - it's off to Emsworth to see the mender in the morning and I'll pass on what you said - I report back tomorrow.

 

[Cret]

Good stuff - I think when you explain the symptoms in detail a decent mechanic should know the direction to look in. It's entirely possible it's electrical instead, but I'm just relating from similar behaviour I've encountered that it was fuelling problems both times, and it seems a convenient coincidence that you say your engine has been sat unused for some time.

Always best to ensure carbs are empty of fuel unless you're going to be using the boat again imminently, although I still forget sometimes myself!

 

[thomshap]

All fixed now, though a lot poorer.

Seems you were kinda right, because it was left with fuel in for so long there was a build up of sludge in the fuel lines, when I opened it up it was flushed through into the carbs and stopped two of them working.

Temperamental thing, my old Mariners have seen much worse and still start after a couple of pulls.

 

[Cret]

Glad it's sorted out now even if it's frustrating now to know that such a simple & avoidable thing as leaving fuel in can cause such problems.

What tends to happen with problems like this is one of a few things in my experience with engines in general:
1) If it's really old/left a very long time then carbs (particularly for very small engines) can have bits actually blocked by varnish or gum, and this can stop them running entirely in worst case, or sometimes block tiny passageways that might just affect idle or pickup mix.
2) If it's been left less time then you can just get reduced power output potentially from restrictions to fuel flow, albeit not quite blockages. Can be tricky to identify as a problem when it's like this.
3) If there's bits of gum in the carb(s) that is loose, it can run ok on idle, but when you put open the throttle & put load on the engine, the sudden demand for lots of fuel can cause gum to get sucked into a fuel jet, blocking it, but it drops back out again when you ease back off the throttle, until you do the same thing again.

I've also seen jelly like stuff form in carbs in the past but not quite sure what the cause has been of that.

Glad all is well though.

 

[FishyInverness]

Just a thought for the future - Soltron (and similar) do a fuel stabiliser aimed at preventing exactly this buildup in petrol engines left idle for long periods of time - might be worth keeping a bottle to hand, think it retails somewhere around the £20 mark.

 

[mikeshrimper]

Outboard servicing Chichester Area.

Good to hear your problem has been solved it certainly appeared to be a fuel problem make sure you have a good filter in the fuel system.

The best Outboard Engineers in the Chichester area are:

Home Marine, Tarquin Yacht Harbour, Thorney Road, Emsworth. PO10 8PB - Tel: 01243 374125.

They only repair and service Outboards - they don't touch inboards - and Bill Mitchell of Home Maring really knows his stuff when it come to Outboard Engines.

 

[thomshap]

Thanks, Bill does know his stuff, knows me pretty well now too, that's where I took it for it's service and subsequent clean out, good of him to fit me in as he's a busy man.

 

[Cret]

If you get hold of a decent (and reasonably priced!) outboard engineer keep hold of them!

I've dealt with a few shysters in my boating days when it comes to this stuff but the guy I've used recently is superb. A real professional and passionate about what he does.

Unfortunately I think he's looking at slowing things down now though to have a bit more retirement than he has at present.

 

[thomshap]

Well the bloody thing let go again yesterday, fortunately only one cylinder this time, last time it was two and it would't start, so we managed to limp home. Same thing, I'd been pottering about all day but as soon as we opened it up it started to sound rough.

Since last time I have replaced all the fuel line and installed a new fuel tank with fresh fuel, I'd like to think it was electrical but last time Bill stripped down the carbs, cleaned them and it worked fine.

My engineer is a good one, everyone speaks very highly of him and he knows his outboards, everyone also tells me Yamaha make great outboards, maybe they give the **** ones so Selva to stick their name on.

 

[Cret]

That's a real bugger, sorry to hear that.

Interesting that you say it was just down one cylinder. Were you able to check or identify which one caused the problem?

If so, the first thing would be to check for spark on that cylinder when it's under load.
I think to do that under load you either need the boat tethered, or with the outboard in a drum of water or something. You can a little inline spark tester which flashes when there is spark for just a few quid to show this up.

If it sparks fine at low load and idle etc but starts to fail when the engine is opened up and under load it could possibly be a fault with the ignition coil for that cylinder maybe.
Don't know how old your engine is but coils can break down over time and it often manifests itself in this manner.

If it's not that and the cylinder in question is behaving ok at low speed/revs then I'd suggest there might somehow still be a bit of crud in it that is getting sucked into the main jet for that carb when you open the throttle much.

Or the jet could be restricted in general, meaning enough juice gets through that carb for it to idle ok, but not enou fuel gets through once it's opened up.

Might be worth running a compression check on all cylinders if you have one, just for peace of mind.

Sorry you're still having grief though. Know how frustrating that is!

 

[thomshap]

Thanks, when the cylinder went it went for good, even at low RMP or out of gear, I'm going to find out which one has gone this morning, last time it was the bottom two, then I'll swap a good plug with with the one from the dodgy cylinder and see if it's a plug, if that doesn't work then it's off back to Emsworth with it. Bill ran a compression check last time and everything was perfect before I took it home.

The engine is 10 years old but under 20 hours and was never really used by my friends, which I guess it where all my problems stem from.

I actually don't mind too much, we stayed safe, got back ok and it didn't ruin the day, when you get a problem like this you learn a lot of information pretty quick which can only be a good thing.

The only thing I've not tried is a big hammer!!

Just been out and all four are running now so it definitely a problem under load, would a weak spark plug cause it, there was one that was sparking dry when it failed last time.