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Old 18 August 2014, 15:05   #1
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Useless Donkey!

My 1998 2 cylinder 2 stoke 8HP Mercury donkey engine doesn't seem to be running very well and is a pig to start. Last time I tried it all was fine, now its hopeless.

I removed and cleaned the carb - all clean, I changed the plugs and checked spark - fine on both, tried a different fuel line and fresh fuel supply - no change, compression is fine.

I can only make it start and run if I physically hold the throttle butterfly valve open on the carb. When I removed the new plugs and went to re-fit the old ones I noticed that the top plug was spotless, I don't think fuel is getting to the top cylinder.

Anyone got any ideas?

Thanks
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Old 18 August 2014, 15:13   #2
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My 1998 2 cylinder 2 stoke 8HP Mercury donkey engine doesn't seem to be running very well and is a pig to start. Last time I tried it all was fine, now its hopeless.

I removed and cleaned the carb - all clean, I changed the plugs and checked spark - fine on both, tried a different fuel line and fresh fuel supply - no change, compression is fine.

I can only make it start and run if I physically hold the throttle butterfly valve open on the carb. When I removed the new plugs and went to re-fit the old ones I noticed that the top plug was spotless, I don't think fuel is getting to the top cylinder.

Anyone got any ideas?

hanks
Sounds to me like it could be your crankcase seals, instead of drawing air through the carb it is drawing air through the seals, hence you still get compression, and the engine will run holding the butterfly open.

HTH
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Old 18 August 2014, 15:16   #3
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Interesting, I wonder if they've perished from lack of use? I barely ever start the engine and I wouldn't think its don't more than 10 hours from new...
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Old 18 August 2014, 15:27   #4
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Interesting, I wonder if they've perished from lack of use? I barely ever start the engine and I wouldn't think its don't more than 10 hours from new...
To be honest it is the only thing I can think of that would cause the problem you are having, with the butterfly closed it is drawing air in through the least line of resistance ie. crancase seals, with the butterfly open that then becomes the least line of resistance and will draw fuel through.

But does seem odd on an engine with such low hours, perhaps they have dried out, it will be the top seals more than likely, on my Mercury 9.8 2 stroke it has 2 seals at the top, installed opposed to each other, or back to back.

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Old 18 August 2014, 16:03   #5
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Just to add that it is a fairly straight forward job if you are handy with the spanners, if I were to do mine again and I had the seals to hand I could have it finished in an hour.

When I did mine I took the seals down to the local bearing supplier to get replcements, alot cheaper than from Mercury and he had them in stock.

HTH
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Old 19 August 2014, 10:26   #6
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Changed upper crankcase seal today was no hassle. Bought a new seal for 2 from local bearing supplier as you said. Hasn't made any difference. Still wont run properly.

I see there are two small rubber crankcase breather? type hoses that T together and then run into the base of the carb. The upper one is clear when you blow through it, but the lower one seems blocked and I cannot clear it. Could that be an issue?
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Old 19 August 2014, 12:30   #7
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Sorry about the dead end.

The pipe going to the bottom of the crankcase is a scavenge pipe that removes excess fuel that gathers there. It could have a non return valve inline so will not be able to blow down but probably could suck?

The only thing that I can think of (going off what you say) is that the idle jet is blocked, and by holding the throttle butterfly open you are drawing fuel from the main jet.

As you can appreciate it is difficult to diagnose without being able to see or hear what is going on.

I will keep thinking about it and if I come up with any more bright ideas
I will let you know.
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Old 19 August 2014, 13:06   #8
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Try some propane injection, but as it is a two stroke don't rev it much as it won't be getting enough oil. Or if you have some starting fluid with top end lube in it that may well be superior since it will provide some lubrication.
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Old 20 August 2014, 08:52   #9
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Have you checked the Fuel diaphragm and butterflies ? is there full fuel supply to the carb ? also are the reed valves free in movement ?
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