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Old 19 July 2015, 18:43   #1
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Mercury Outboard Engine fire

Hello, New to the world of ribbing, family history of boats and water borne activities and I've decided it's my turn.

I have a 4 meter Avon rib. It has a Mercury 50 thunderbolt 2 stroke outboard on it. Today, arrived at the slip, boat into the water. I noticed I had driven the 6 miles to the slip with the fuel line attached to the tank. I then primed the fuel just until the bladder went hard.

Fired up the engine, then boom. loud explosion, not enough to blow casing off etc, however a small fire started. It was quickly extinguished with a powder extinguisher.

Boat out of the water and returned home.

Carbs and bottom end cleaned out of powder. Primed fuel and engine fired up no problems.

Obviously the explosion was due to fuel from somewhere. I checked for leaks - none.

I did noticed the engine had two cards and no air filter? Is this right, direct air entry?

Can anyone offer advice. Has the fuel been primed too much and as a result there has been excessive fuel which has leaked out the entry to the carbs and in turn starting the fire?

Any advice would be gratefully received.

Thanks,
Jamie
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Old 20 July 2015, 01:08   #2
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it could be one of your carbs is overflowing the needle valve in the float chamber can get a bit of crap in the seat and stay slightly open this sometimes does not always happen but it will cause the carb to drip and maybe the flooded carb caused a small backfire when you fired it up (excuse the pun)
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Old 20 July 2015, 01:10   #3
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Hi, you will get some fuel leakage/spillage under the cowl with squeezing the bulb, I always do, but it still needs a spark to ignite it. It may be a good idea to have a good look at your spark plug leads make sure they are in good order and tight, check for cracks in the insulation and make sure there are no bare wires anywhere that could cause a spark. As for the air filter, there should be a strip of foam rubber around the front cover, this is the air filter. hope this helps..
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Old 20 July 2015, 02:02   #4
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Get this checked out professionally. In 30 years of boating I've never (touch wood) had an outboard catch fire, and I've had some old motors before. It's one thing having a small fire on a slip, quite another at sea.

With the cowl exposed, where exactly was the fire on the engine, and what damage was done?

The carbs would be the first thing I'd look at, followed by HT leads, fuel and vacuum hoses, the fuel pump and then the wiring, including chaffed or exposed cable.

If you're looking for a replacement engine, here's a few long-shafts 50hp engines:

http://www.seamarknunn.co.uk/download/usedob/used.pdf

Second Hand Archives - West Wales Marine

Buy Used Outboards tohatsu 50hp power trim, oil injection, long shaft 50.0HP Outboard at Bill Higham Marine

Good luck with this.
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Old 20 July 2015, 03:42   #5
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Here's what can happen with a fuel leak!

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Old 20 July 2015, 03:54   #6
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few gaskets and a t cut and that will look like new
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Old 20 July 2015, 14:58   #7
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few gaskets and a t cut and that will look like new
It will be on ebay with the line - "not running, but just needs a service"

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Hi, you will get some fuel leakage/spillage under the cowl with squeezing the bulb, I always do,
is that normal? I've never noticed this - it doesn't sound right to me.
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Old 20 July 2015, 16:49   #8
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no shouldnt leak from priming - thats what float valves are to prevent. my force with tilotson carbs spills a small amount of fuel out of them when the engine is tilted up - a problem im currently trying to resolve
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Old 20 July 2015, 23:57   #9
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I'd be surprised if most carburated two strokes don't spill a bit of fuel into the tray now and then. I'm yet to pull a cowling off one and not get that familiar "wiff". The 6hp Yam I had was the only one that actually caught fire (put out) and I traced that to a bent cowling bracket that an HT lead was arcing over to. Sorted bracket and replaced HT lead.
So I'd look for the ignition source because you're never going to guarantee there'll be no petrol fumes under the cowl.
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Old 22 July 2015, 01:33   #10
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Worn Neddle / seat in the carbs can allow fuel to leak past when the tank has pressure caused by expansion.
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Old 22 July 2015, 07:36   #11
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My best guess here is that the engine will have been tilted for towing? In which case all conventional carb float valve logic goes out the window, as gravity also plays a significant part i nthe ioperatio nof that valve. Coupled with the ambient level in the carb being at 45 degrees or so to "normal".

Fuel takns if the breather is shut and properly sealed (I guess you donlt have Hulk tanks? ) then the expansion of the air with the heat of the sun as you tow could be enough to auto pump. The only thing stopping it is the float valve which as above is about as much use a sa chocolate teapot at that angle.


Re. the explosion, depending how much had been pumped into the cowl and depending on where it pooled / ran when the engine was moved to "operating angle" there is a starter with a sparky set of brushes, and could also have been a backfire.....


I would agree with checking your electrics, but also look for scorch marks roud the inlet passages too.....



I guess you may have an older version of mine. There is no air filter per se, but should probably be an air box?

If it's a "Brown stripe" livery on the cowl, does the top of the cowl attach to an open square box? that could be an air filter?

You got a pic? if it's the vintage I think it is the air intake body holds the cowl on! - Mr Boatjumbles in Bellshill (google will find his number) has one breaking for spares at the moment....... I'm going to collect a bit for a decrepit Johnson tonight or tomorrow, I can ask if he still has the air box if you want.
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Old 23 July 2015, 10:31   #12
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Thanks

Guys,

Thanks very much for the replies. Stripped down the carbs and head, thereafter removing a mound of fire extinguisher powder!!

Found there to be a good bit of dirt and grit in the float chamber and fuel filter. All cleaned up and engine seems to fire up nicely, no pun intended.

Doing a full check over the electrics, making sure no bare wires / exposed areas just to be on the safe side. It's looking like the float has been jammed on, the fuel has been connected, and has filled up the carbs etc with extra fuel. Hence the start up has managed to cause a backfire and ignited the fumes.

Giving the engines a good few test runs on dry land and a good going over so fingers crossed il get it in the water and it will progress further than the foreshore!

Thanks again!
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Old 23 July 2015, 11:27   #13
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You did well not to ruin it with the extinguisher powder (especially if there's no air filter). They don't half make a mess!
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Old 24 July 2015, 01:43   #14
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In future it might make sense to remove the cowling when you arrive at the slip (after towing) to allow any vapours / fuel under there to evaporate before starting her up for the first time? Once she's running pop the cowling back on.
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Old 24 July 2015, 14:09   #15
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In future it might make sense to remove the cowling when you arrive at the slip (after towing) to allow any vapours / fuel under there to evaporate before starting her up for the first time? Once she's running pop the cowling back on.
Adding air doesn't seem a good plan. Venting does if you don't trust the engine, but I'd put the hood back on before starting if it was me...
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