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I generally do all my own maintenance. No different this year and I changed all the oils, filters, and spark plugs on my 2007 Yamaha F250 TUR. On first launch I felt the motor was not running smooth and had a definite shudder resonance as we went through the speed range, especially rough about 2000 RPM. No misfire just rough. Idles fine (just a slight rumble) and I can't discern it is missing a cylinder until I get underway.
Once home I pulled all the plugs and found one plug that had not heated (all brand new so could easily see plug tip not discolored). I tested the coil pack by pulling the plug and grounding the plug against the engine while turning it over. No spark at cylinder, went to a "good" cylinder and had spark. Plugged in coil pack from bad into good and no spark, so I bought a new coil pack.
I'm unsure of the cylinder number assignments so the one not firing was bottom left cylinder and I am calling it #5.
I replaced the coil pack and again all new spark plugs and when I launched again hitting on 5 cylinders. Same exercise in troubleshooting, but this time the unheated spark plug was on a different cylinder. It was the bottom right cylinder and I'm calling that #6.
I'm going to go back and recheck the "bad" coil pack, I might have skimped on my first assessment and presumed it was no good.
I did call local Yamaha dealer but have to wait 2 weeks to bring it in. I trailer the boat, so at least I can get it there.
What might cause the bottom two cylinders to not fire?
Thanks in advance for your input.
Gene
Once home I pulled all the plugs and found one plug that had not heated (all brand new so could easily see plug tip not discolored). I tested the coil pack by pulling the plug and grounding the plug against the engine while turning it over. No spark at cylinder, went to a "good" cylinder and had spark. Plugged in coil pack from bad into good and no spark, so I bought a new coil pack.
I'm unsure of the cylinder number assignments so the one not firing was bottom left cylinder and I am calling it #5.
I replaced the coil pack and again all new spark plugs and when I launched again hitting on 5 cylinders. Same exercise in troubleshooting, but this time the unheated spark plug was on a different cylinder. It was the bottom right cylinder and I'm calling that #6.
I'm going to go back and recheck the "bad" coil pack, I might have skimped on my first assessment and presumed it was no good.
I did call local Yamaha dealer but have to wait 2 weeks to bring it in. I trailer the boat, so at least I can get it there.
What might cause the bottom two cylinders to not fire?
Thanks in advance for your input.
Gene