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Old 27 May 2010, 05:45   #1
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Mariner/tohatsu 50hp 2-stroke

My 2006 Mariner/tohatsu 50hp 2-stroke outboard keep fowling the bottom spark plug???
Thanks
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Old 27 May 2010, 08:23   #2
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It's either too much fuel / oil or no / intermittent spark (if there is the correct amount of fuel / oil, it isn't getting lit).


Does it happen at all speeds, or only at WOT or Idle?

How rough does it run? (and if very, is it constant, only at certain speeds, otr jsust randomly intermittent?)

When were the spark plugs last changed?
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Old 27 May 2010, 13:26   #3
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Bottom cylinder can run cooler than the upper ones on some engines causing fouling owing to cooling water flow ,
, if after going through all the routine stuff as 9d280 and you still have problems ,what i have done on some engines like that was to replace the spark plug on the lower cylinder with a different heat range plug than the others .
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Old 27 May 2010, 17:44   #4
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Mariner/Tohatsu 50hp

Engine runs fine at lower revs but as soon as you give it full revs the plug then seems to fowl up! Checked the diapharam in the fuel pump (fitted to the lower carb) and cleaned carb, The spark looks good and the plugs are only months old, Could it be a coil playing up?
Thanks
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Old 27 May 2010, 17:52   #5
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have you swapped the plugs around to see if theres any difference ,,
i had problems with a brand new set of plugs last year ,,engine would start ok then when at about half revs one cylinder would cut out ,,when at tick over it would run fine again ,,turned out it was a broken ceramic on the electrode , the plug even looked ok when inspected,, when it got up speed the ceramic slid down the electrode and cut out the spark ,,as it would then spark or short out up inside the plug body until it slid back and it then sparked as normal ,,,, when back at low revs it vibrated back in place and started running again , looking at it first glance it looked ok and gave out a good spark ,
another brand new new plug and it cured it .
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Old 27 May 2010, 18:01   #6
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Thanks but I have! Whatever plug I fit in the lowest (bottom) cylinder it fowls when I give the engine full revs
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Old 27 May 2010, 18:06   #7
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suppose you could try swapping the coils over see if it makes any difference ,
on some engines the lower cylinder plugcaps ,h/tleads can be prone to getting saltspray up from the drain holes which can cause problems with shorting out too .
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Old 27 May 2010, 18:20   #8
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Have you had the carbs off recently?If you have it could be overfuelling on one carb. If so, they'll need balancing.
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Old 27 May 2010, 18:50   #9
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The bottom carb on the 50 has a richer jet as standard- The mixture screw should be about 2 - 2 1/4 turns out on all carbs. When i brought mine the mixtures were all over the place (bottom one was about 3 1/2 turns out iirc ) Its deffo worth swapping the coils around+ make sure that they are all earthed properly, as they suffer from corroded earth points which in turn causes a missfire/plug foul.
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Old 28 May 2010, 04:11   #10
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Thanks for all the advice,I will try eveything! How do I balance the carbs? Zapcat sale special NGK plugs for the Tohatsu 50! Worth trying? also should I check the reed valves?
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