Yamaha 9.9 Problem

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36041
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Yamaha 9.9 Problem

Postby 36041 » Tue Oct 28, 2014 2:29 pm

All,

Greetings from your friends in the Bahamas. Dream Catcher is having a problem with one of her (2007) 9.9 Yamahas and I wondered if someone might have any suggestions.

The engine will start up and idle, even accelerate in neutral, but under any kind of load (in gear), it starts running rough and backfiring before dying out after twenty or thirty seconds. I pulled the second engine out to help troubleshoot and I now know that the carb is fine on the engine in question. I have swapped the carbs unit for unit and the problem stays with the same engine.

The engine is 7 years old, but has only 733 hours of total operation time. The oil was just changed and the level is correct and both the plugs and the fuel are new. I am about to set up the bad engine to run on a self contained fuel source via a different feeder hose with a separate fuel filtration system to verify it's not an issue with the fuel coming into the engine from the shared on board tank.

After that I may swap fuel pumps just to be certain they are both working, but after that I am running out of ideas and may have to resort to beating the engine to death with a large hammer and see if that makes any difference.

Any ideas or comments that might lead to this engines survival are greatly appreciated.

Regards,
Tom
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Dan White
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Re: Yamaha 9.9 Problem

Postby Dan White » Tue Oct 28, 2014 7:06 pm

Since you say the plugs are new, is the gap correct?

Also, I had very similar symptoms and it turned out the oil had leaked out and the engine was prevented from operating above an idle by a safety. But you say the oil level is correct, so that is probably not it.
Note that the green light under the key is an indicator of correct oil pressure. Is it lit?

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Re: Yamaha 9.9 Problem

Postby 36041 » Wed Oct 29, 2014 12:55 pm

Thanks for your input Dan,

I changed the oil and disabled the pressure sensor, but to no avail. Next I swapped out the fuel pump from known good engine #2; still not right. I then bypassed the entire fuel system on the bad engine going with a gravity feed/fuel line hand pump direct to the carb; same problem.

At this point I believe the fuel delivery to not be the problem. I pulled and checked the plugs and noticed the slightest about of dampness on cylinder #2 (the bottom one), but when I cleaned them up and verified the gap, nothing changed. On to the electronic side of the equation.

I started by checking the spark with a spark tester. Cylinder #1 looked white hot, #2 however looked yellow. Hmmm, a clue perhaps?

I borrowed the ignition coil pack from engine #2 and installed it. The engine caught immediately and the difference in the sound at idle was noticeable. Sure enough, when put in gear the engine pulled hard and clean. Just to be safe I swapped back in the original ignition coil and the problem returned.

I ran the engine for 10 minutes in gear pulling back against our anchor and the motor never skipped a beat. I'm guessing we've found our problem.

Regards,
Tom
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