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Old 23 September 2021, 15:51   #1
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Mercury 8Hp 4 Stroke issues

My 2005 8HP has been playing up lately. Start's fine first time, then runs OK on the throttle / in gear, then at idle, it stalls. If I idle at slightly higher revs, it slowly dies and stalls. Starting the engine again is problematic, takes a while. Opening the throttle seems to help a bit, but not every time.

I've removed and cleaned the carb this evening - but still have the same issues.

Once I put everything back together, I noticed when I used the primer bulb in the engine, in squirted fuel out from the bottom of the carb, from the carb seal around section the diaphragm where the primer button hose feeds into.

I've now taken that section off - looks like the 2 O rings need replacing.

I have 2 questions:

1) Does anyone know the name or part number for just the 2 O rings
2) Any view on if this loss of integrity would be causing the above issues.

Obviously, it needs fixing, but wondering if its also the cause of the problems - could it it sucking in extra air and interfering with the mix, causing the engine to die?

I'm trying to upload a pic of the o rings but struggling to post that - keeps saying invalid file?
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Old 23 September 2021, 16:32   #2
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It might not be sucking air in but just meaning not enough fuel is getting in so giving the same result?

Can you fudge it into running with a bit of choke, ie cutting the air to match the cut in fuelling?
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Old 23 September 2021, 16:54   #3
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Ok - might give that a try. Thanks
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Old 23 September 2021, 17:36   #4
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This is just an example but is this your carb?

https://www.boats.net/catalog/mercur...r676746-and-up

Anyway you can work back through that website to get the correct serial number range parts diagram for yours and check the part numbers.

Then either buy a box of 100 mixed sizes fuel compatible quality o-rings and have loads left over... or go to the dealers with the part numbers you need... or at least a print of the carb diagram to point out what you need.

There is no point trying to tune or fault find if you know these o-rings are leaking until replaced.
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Old 24 September 2021, 02:01   #5
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Quote:
Originally Posted by Rupe View Post
Ok - might give that a try. Thanks
I'm trying to picture where these o rings are in the system but the issue sounds like fuel starvation but as it's not happening the moment you start the engine but only after it has been running a short while and then it takes a while to get going again then my guess would be that this is linked to the fuel in the carb pot. Ie when that is full because you've manually pumped it full from the tank or it's had time to be fed in from an internal tank the engine runs fine but you're burning the fuel quicker than it is being replaced. The symptoms sound similar/same as when we forget to open the fuel tank breather for example.

If you use an external tank then you could test this by reprising the bowl and carb after it has stalled to see if it fires up nicely again.

The o rings may be the cause as they could be causing a vacuum to fail that is required to draw fuel but it could also be caused by a dirty fuel filter, a failing or failed fuel pump, collapsed fuel line, tank blow large etc but as Fen suggests, there is no point in looking any further until you've got a good seal back on the carb and ruled out what seems likely to be the culprit of excess fuel isn't supposed to be dumped but returned to the system.
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Old 24 September 2021, 03:35   #6
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OK - found how to do it! Thanks for the replies - these are the O rings below the diaphragm.

The box of multi sized ones sounds like a good approach followed by the other steps once I know I have a good seal


20210923_195413 by Rupemca, on Flickr
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Old 24 September 2021, 13:49   #7
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They look quite surprised!
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Old 24 September 2021, 14:19   #8
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They look quite surprised!
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