Mercury 75 HP 2 Stroke Conundrum

[DJS]

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

[beamishken]

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

[DJS]

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.

[beamishken]

Originally Posted by DJS

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

[DJS]

Carbs all good and i am leaning towards a vacuum leek in the crankcase as i believe it is fuel related

[beamishken]

Originally Posted by DJS

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

[DJS]

Fuel pump also all good and reeds are perfect i.e no fuel spitting out on idle.

[neil.mccrirrick]

When you run it is the spark plug in cylinder 1 wet with fuel when u take it out

[DJS]

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.

[beamishken]

Originally Posted by DJS

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

[DJS]

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.

[beamishken]

Can you substitute the top carb from one of the bottom ones? It certainly sounds like a carb problem.

[Davie]

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.

[Daibheid]

Might be worth checking the crankshaft seal, I’ve 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!

[Turbodiesel]

Easy test, while it is running spray carb clean into the top carb if the engine picks up it’s a carb problem.