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spark coils failing '07 merc 90 hp 2 cycle
Well, bought a used 2007 90 to replace a good working 05 50 hp on a large pontoon. Heard motor run, sounded just fine, rev'ed up just fine, middle of winter so not observed in the water. Helped him remove it from a large river fishing boat claiming 47 mph with a 13x21p ss ported and tuned prop.
Installed on my 2500 lb pontoon with 2 helpers who have done this before. I knew the prop was wrong, but had not yet replaced it. Hit the water, started right up, idled out of the long cove to the big lake, slowly opened it up and it was only running 1600 rpm at wot. Took it out, rechecked all connections, just the one converter plug from 8 to 14 connector plug, all looked fine.
Replaced the two dead spark coils, still ran fine if not under load, next morning took it out, ran 4800 rpm, not the 5500 owner claimed, but acceptable, prob a prop size issue. After about an hour the rpm suddenly dropped to 3600. Checked coils, one was dead. Replaced it, the replaced one was one of the new ones, back on water, 4800 and doing fine, so replaced the prop with a 14x15p, ran just fine, at 4800, top speed about 22mph, 5 more than the 05 50hp. Not as fast as I wanted but it gets out of the hole quickly and steadily, no missing nor bucking.
Ran it 50 miles up and down the big lake, ozarks, on the main channel, stopping for an hour for lunch on the lake at the 11 mile point of the trip, started right up, 4800 again, turned around at about the 25 mile point of the trip, and at the approx 40 mile point of the trip, it suddenly dropped to 3600rpms again. No warning, no sound, just 4800 to 3600. Coils are not cheap and I don't like the down time nor the inconvenience.
Even with 1 or 2 cylinders working, it runs smoothly with no bucking. If I push it up to 5000 rpm, it starts bucking and the tach drops 2-300 rpm off and on, so I just run it at the 4800 or below. At 4000 rpm, it cruises nicely, will go to 4800, but bucks at 5000rpm. When the last coil failed, I was increasing rpm from 4000 to 4800 because it was getting late and we had a 200 mile trip home, and that's when it failed, 10 miles from my dock. Cruised in just fine at 3600 however.
Please, any help, suggestions, things to look for? I would certainly appreciate it. Thanks, Ed.
Installed on my 2500 lb pontoon with 2 helpers who have done this before. I knew the prop was wrong, but had not yet replaced it. Hit the water, started right up, idled out of the long cove to the big lake, slowly opened it up and it was only running 1600 rpm at wot. Took it out, rechecked all connections, just the one converter plug from 8 to 14 connector plug, all looked fine.
Replaced the two dead spark coils, still ran fine if not under load, next morning took it out, ran 4800 rpm, not the 5500 owner claimed, but acceptable, prob a prop size issue. After about an hour the rpm suddenly dropped to 3600. Checked coils, one was dead. Replaced it, the replaced one was one of the new ones, back on water, 4800 and doing fine, so replaced the prop with a 14x15p, ran just fine, at 4800, top speed about 22mph, 5 more than the 05 50hp. Not as fast as I wanted but it gets out of the hole quickly and steadily, no missing nor bucking.
Ran it 50 miles up and down the big lake, ozarks, on the main channel, stopping for an hour for lunch on the lake at the 11 mile point of the trip, started right up, 4800 again, turned around at about the 25 mile point of the trip, and at the approx 40 mile point of the trip, it suddenly dropped to 3600rpms again. No warning, no sound, just 4800 to 3600. Coils are not cheap and I don't like the down time nor the inconvenience.
Even with 1 or 2 cylinders working, it runs smoothly with no bucking. If I push it up to 5000 rpm, it starts bucking and the tach drops 2-300 rpm off and on, so I just run it at the 4800 or below. At 4000 rpm, it cruises nicely, will go to 4800, but bucks at 5000rpm. When the last coil failed, I was increasing rpm from 4000 to 4800 because it was getting late and we had a 200 mile trip home, and that's when it failed, 10 miles from my dock. Cruised in just fine at 3600 however.
Please, any help, suggestions, things to look for? I would certainly appreciate it. Thanks, Ed.
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