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Old 13 June 2010, 17:48   #1
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Mariner 50hp 4 stroke 1998

My engine is running rough at low revs, as soon as it gets to approx 2800rpm it takes off like normal.

Any ideas what it could be?

I'm going to change the plugs and fuel filter, and i'm guessing I should clean the carbs out, how easy is this to do? Is it a case of taking off the float bowls and making sure all the jets are clear?

thanks.....
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Old 14 June 2010, 16:13   #2
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Yip definetly get those carbs taken off . Check float height , drop levels etc . Balance them up with Vacuum gauges etc . it sounds like your intermmediate jets are blocked and then over that 2000 , high speed kick in . barry
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Old 16 June 2010, 15:11   #3
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I just changed the plugs and found 1 of them to be far more oiled up/wet than the rest, this plug was in the top cylinder.

would this also point to a blocked jet?

the engine did feel like it was running on 3 cylinders until the 4th kicked in at 2800rpm
see pic below :



Thanks.....
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Old 16 June 2010, 17:39   #4
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Wet plug usually means you're losing spark on that cylinder.

I would find it odd that a 4-stroke has that much oil going into the cylinder, though. Does the plug reek of gas, or is that actually oil?

jky
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Old 16 June 2010, 18:11   #5
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not sure, didnt think to check.....doh
The engine does not lose any oil which hopefully is a good sign.
My plan is to clean out the carbs, especially the one that feeds this cylinder, then balance them (might need some advice on balancing ), take it out for a run with the new plugs to see how it behaves an then inspect the new plugs again.
I really hope its something simple like a blocked jet or even a duff plug.
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Old 16 June 2010, 18:19   #6
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I'd be interested to know why one plug is pure white as well.
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Old 17 June 2010, 05:45   #7
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The top plug on my 90 4str. also looks oily like that. Mine also misfires from 2000 to 2800 rpm. Below and above that it runs really well.

It's not too much of a problem to live with and it's been like that for a couple of years. Having tested almost everything else, I think the carbs need balancing. If I ever get my gearbox back in one piece, I'll get them done too.
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