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2003 Yamaha F115. First 10 min after launching was slow speed running normally, then I increased revs to 4000, it ran normally for 60 seconds and then the revs dropped slightly and it developed a misfire. What started as a miss and loss in power at higher revs quickly became a significant loss in power and irregular idle. Traced to a problem with the ignition. At idle unplugging 1&4 made a small difference to the idle (I could hear plug lead no1 shorting to the plug as I removed/refitted it but otherwise made little difference to the running of the engine. Plug 4 was similar (although couldn't hear the arcing), disconnecting plug leads 2 or 3 made the engine stumble significantly. This appeared to identify a problem at the upper coil or upstream. Disconnecting the upper coil at idle makes the engine stumble but it continues to idle, reconnecting makes the revs pick up slightly but then settles (ICV working?), disconnecting the lower coil the engine stalls immediately. I swapped the leads and the coil connections and the problem moved to the lower coil. Based on this I concluded that the problem is upstream of the coils. I have confirmed that I have 12v on the yellow/red power wire and have disconnected the black/white and black/red switched negative wires from the CPU and undertaken a continuity test to check for a broken wire. The continuity (with a multimeter) is good. So It seems I have traced the problem back to the CPU. However... not wanting to rush into that expensive diagnosis... could there be another culprit? The Crankshaft position sensors? are there any other tests I can undertake?
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No one got any ideas on this? -
I would follow the steps in your service manual for troubleshooting the ignition system with the proper test equipment.
unplugging the plug wires can make the volage rise and jump spark where it can damage things like inside the coils themselves
use a spark tester or an inductive timing light to see if the spark is happening on the cylinders.
with this said, I am not familure with that 4 stroke motor so I refer to service manual for testing stepsComment
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