Faulty ignition coil????

[arokkrok]

In need of some assistance if possible....
I have a 1996 90hp 2 stroke Yamaha that seems to be dropping a cylinder after running for about 20-30mins. It will operate fine until then, getting full throttle range not running rough or anything then it drops its revs and vibrates and jumps around until I bring the revs back to just more than idle (approx. 7 kts) as my tacho doesn't work.
when it first started happening it would play up like this for a short amount of time maybe 5-10min and then come back to running normal, but as time has gone on it has gotten worse and it will only come back to running good if I shut down the engine and leave it to cool for about 30mins sometime longer.

I have changed my spark plugs and checked over the leads etc with no sign of damage or wear.

Is this possibly a ignition coil problem? Or any ideas on what it could be????

[ausnoelm]

Does the alarm work??

[arokkrok]

overheating alarm?
If so yes I went off once before a while ago before I replaced thermostat and cleaned out water galleries and spent a lot of time on the cooling side of things

[ausnoelm]

OK, next question, is it STILL working? you say it worked once before.

[99yam40]

I would use a spark tester or a inductive timing light to verify if spark is going away from one or more of the plugs when this happens.

If there is still good spark to all plugs you can move on to other things, maybe fuel problems

if a fuel pump leaks it will enrich #3 cylinder to where it will not fire properly

[arokkrok]

I haven't checked if alarm is still working? what is the best way to do this?

[arokkrok]

I have noticed a slight milky substance in #3 carby, I believe its oil/fuel/water mixture which makes me believe its this cylinder that's dropping as for why that's still got me wondering. Could be fuel pump leaking??

[ausnoelm]

Be pretty rare for a faulty fuel pump to allow water into the carb! You must have water in the fuel system, it needs to be properly cleaned out, from the tank right to the carbs, and there is no spray or miracle in a bottle to do this, it's not hard to do, also make sure the oil tank does not contain some water.

[arokkrok]

Since finding the milky substance I have discontinued using the under floor tank and have since change to a small 22l plastic tank just for testing purposes. I haven't emptied the oil reservoir as yet but last weekend when I took it out I still had the same issue after about 30min running.
My initial thoughts was contaminated fuel but after changing the tank replacing the fuel filter and putting a bit of metho in the tank and still having the problem persist its starting to make me think there is another issue. I took my mechanic mate for a quick run (car mechanic that is) and he was very quick to say he thought it was the ignition coil due to the fact it was failing when the motor had warmed up. Any further ideas?? another guy I spoke to said it could be the stator or CDI box