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Old 14 September 2019, 05:56   #1
DJS
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Mercury 75 HP 2 Stroke Conundrum

Its been a while since i have been here but lurking occasionally

Here is the conundrum, 2004 mercury 75 HP 3 cylinder two stroke not firing on cyl number 1

The fuel system and carbs are clean and the reed valves are good

Compression is 120 psi each cylinder

coils with their ignition leads and plugs were swapped from other cylinders that are know to be good e,g running well and yet no difference

cylinder 2 and 3 will run on their own (i.e ignition leads disconnected for the other cylinders) but not cylinder 1

Thoughts
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Old 14 September 2019, 08:48   #2
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Not sure about the mercury but Yamaha's have individual trigger coils for each cylinder under the flywheel, wouldnt be surprised if mercs are similar worth checking
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Old 14 September 2019, 09:16   #3
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Thanks for the suggestion and there is excellent spark (spark tester used) and also compression on that cylinder. Leading me to fuel however, carbs striped and all clean.
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Old 14 September 2019, 11:30   #4
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Thanks for the suggestion and there is excellent spark (spark tester used) and also compression on that cylinder. Leading me to fuel however, carbs striped and all clean.
Ah ok likely a blocked jet in the carb, a strip & clean of the carbs seems the likely answer sorry I jumped to the conclusion of lack of spark
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Old 14 September 2019, 13:00   #5
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Carbs all good and i am leaning towards a vacuum leek in the crankcase as i believe it is fuel related
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Old 14 September 2019, 14:17   #6
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Carbs all good and i am leaning towards a vacuum leek in the crankcase as i believe it is fuel related
Which cylinder does the fuel pump take its pulse feed off?, could be pump diaphragm or possibly reeds I assume if carbs have been off then gaskets are good .If its none of those and it is a lack of vacuum then its possibly serious surgery time
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Old 14 September 2019, 14:58   #7
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Fuel pump also all good and reeds are perfect i.e no fuel spitting out on idle.
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Old 15 September 2019, 07:21   #8
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When you run it is the spark plug in cylinder 1 wet with fuel when u take it out
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Old 15 September 2019, 08:50   #9
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Yes spark plug is wet and the next installment is as follows: if I spray mist fuel into carb one it will run on that cylinder alone i.e the same as the rest. hence I am thinking there is either a carb issue or drawing air somewhere else on cylinder 1.
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Old 15 September 2019, 10:19   #10
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Yes spark plug is wet and the next installment is as follows: if I spray mist fuel into carb one it will run on that cylinder alone i.e the same as the rest. hence I am thinking there is either a carb issue or drawing air somewhere else on cylinder 1.
But you said above the carbs were fine probably 80% of 2 stroke engine problems can be traced to carbs has the carb been stripped and blown through every oriface? Its relatively easy to do a crude vacuum test, with the engine running place something across the carb intake and see how hard it sucks. Even a non functioning cylinder should have vacuum
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Old 15 September 2019, 11:37   #11
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Yes carbs are clean and I couldn't see any blockages and yes when running if I
close it off it will draw significant fuel hence vacuum similar to the other 2. It could be drawing air on the engine side of the butterfly, that is my next test or a blockage I can't see.
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Old 15 September 2019, 14:49   #12
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Can you substitute the top carb from one of the bottom ones? It certainly sounds like a carb problem.
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Old 16 September 2019, 16:17   #13
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Take the reed block back out again and have a look at the piston skirt it sounds like your not getting crankcase compression. BUT Try this first it’s quick. Mark the flywheel at tdc for each cylinder. Run the strobe on it and check the timing on each cylinder. If somebody has tried to remove the flywheel with a hammer the magnet in the middle of flywheel which controls the trigger may be cracked which causes this problem with a double spark.
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Old 28 September 2019, 20:18   #14
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Might be worth checking the crankshaft seal, Iíve seen them cause the worst gremlins ever on two strokes with flaky behaviour. A damaged one might seal under vacuum but release most of the pressure as the crank case pressurizes. Very difficult to inspect for!
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Old 02 October 2019, 00:32   #15
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Easy test, while it is running spray carb clean into the top carb if the engine picks up itís a carb problem.
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