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2001 mercury 90 two stroke runs then reduces revs by 1000 then goes for the day

MickMercury

New member
2001 Mercury 90 ELPTO two stroke.
I have a problem of many years intermittently, where the motor runs fine for 5-10 minutes then drops speed then kicks back in and runs fine all day, going out to islands (15-20 km) and snorkelling in different locations. Originally the problem could sometimes be driven through, then you had to stop and idle or shut down completely, start again and off we’d go.
3 mechanics have checked it out, it’s been serviced more than once with the issue in mind.
Fuel has been drained and replaced, hoses from filter to engine replaced, new plugs new filters, alternate fuel source used, carbies striped and cleaned more than once. New coils, trigger changed.
Rimfire plugs have been changed to normal plugs but there has been no difference.
The problem is more regular now since a concerted effort to solve it. You can take off do over 5000rpm flat out (normally I run at 4100) and it will drop back to 4000 rpm (reduces by 1000rpm). It farts like it’s missing but it’s actually firing and trying to go. Then it will free itself after 5-10 mins and fire fully on all 3 cylinders. It seems like a blockage has cleared then you are all set to go for the day.
Mercury service person who has serviced it said as its an older in age motor and it would cost a lot to pull apart it’s probably not worth it. He seems to think it’s a problem inside the motor like a corrosion or a blown crankcase gasket.
Compression test was apparently OK.
The motor has 475 hours. It has not been run on ethanol fuel.
Thanks for any suggestions as to what to look for.
 
I had a 2002 90 and one of my OEM CDMs had a temperature related problem. It wouldn't fire till the engine had warmed up somewhat. Yours could have the opposite problem. When it warms up it cuts out. Neighbor had a 90 of that era and also had a CDM problem. Hard to catch that problem at a repair shop. Would have to have a test prop installed and the engine in a tank of water to run it under load at max rpms.....saw that happen to Merc Mark 55, back in the late '50's. Very impressive.

Suppose you take a spark plug socket wrench with you the next time you go out and leave the cowl off. Let it do it's thing and give it a few minutes before you stop. Pull the plugs and see which (if any) is wetter than the others.

If you find such, swap the CDM for that cylinder (not the plugs) with another cylinder. Get back up to where you were running and run there for awhile assuming the 4k problem will still be with you. Stop again and pull the plugs on the 2 cylinders where you swapped the CDM. If the other cylinder's plug is now wet, your problem is that CDM has a thermal problem.

If you find them all the same, you haven't lost anything but a little time and minor inconvenience. Otherwise looks like you covered most everything. I have no experience with the rose pedal reed valves on these engines and never had a problem with them. so I don't know how they act up. Seems a reed would be a constant problem anyway, not something intermittent. Faztbullet probably knows. That engine doesn't have an over rev limiter module so it's not that.
 
I had a 2002 90 and one of my OEM CDMs had a temperature related problem. It wouldn't fire till the engine had warmed up somewhat. Yours could have the opposite problem. When it warms up it cuts out. Neighbor had a 90 of that era and also had a CDM problem. Hard to catch that problem at a repair shop. Would have to have a test prop installed and the engine in a tank of water to run it under load at max rpms.....saw that happen to Merc Mark 55, back in the late '50's. Very impressive.

Suppose you take a spark plug socket wrench with you the next time you go out and leave the cowl off. Let it do it's thing and give it a few minutes before you stop. Pull the plugs and see which (if any) is wetter than the others.

If you find such, swap the CDM for that cylinder (not the plugs) with another cylinder. Get back up to where you were running and run there for awhile assuming the 4k problem will still be with you. Stop again and pull the plugs on the 2 cylinders where you swapped the CDM. If the other cylinder's plug is now wet, your problem is that CDM has a thermal problem.

If you find them all the same, you haven't lost anything but a little time and minor inconvenience. Otherwise looks like you covered most everything. I have no experience with the rose pedal reed valves on these engines and never had a problem with them. so I don't know how they act up. Seems a reed would be a constant problem anyway, not something intermittent. Faztbullet probably knows. That engine doesn't have an over rev limiter module so it's not that.

Thanks for that info much apprecited
 
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