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Old 03 September 2012, 16:16   #1
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Country: UK - England
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Yamaha 90hp 2 stroke 1997 - misfire low revs

Can anyone help with the below please?

I run a Yamaha 90hp 2 stroke and have had this for about 18months. Recently the engine seems to have developed a misfire at low revs when running at around 5 knots (no idea of RPM). There also seems to be a very strong exhaust smell mixed with a slight fuel smell at ow revs as if not all fuel is being burnt. It may be that this is more noticable at low revs because there is little wind to blow it away and it is in fact happening all the time!
  • Initial startup from cold, starts very easily with a some smoke for about 30 secs. Quite often there is no need to pump the bulb.
  • Runs well at full revs
  • When engine warm and running slow appears that there is a misfire and runs quite lumpy with the exhaust smell as described above
Engine serviced new plugs and fuel filter about 4 months ago, engine ran fine.
Problem with fuel/water separator about 2 months ago, changed from the bulb type to the oil filter style of separator. Not sure, but the misfire I believe may have happened around this time. Is it possible that the separator is the culprit?

New ignition coil assemblies and plug assemblies fitted about 4 weeks ago did not resolve the misfire.
All Plugs are wet and have a light fuel/oily coating after use.
Checked TDC and this appears ok.
Any advice gratefully received, would be nice to get this great engine running better.
Thanks


Mark
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Old 05 September 2012, 16:29   #2
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Split petrol pump diaphragm leaking into crank case and over fueling one cylinder? this is fairly common.
partly blocked idle jet? less likely though as from your description there's plenty of fuel getting into the dead cylinder?
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Old 05 September 2012, 17:40   #3
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That sounds exactly what is happening, shall let you know if it resolves the problem, looks like an easy fix.
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