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mercury 150 EFI 2 cylinders not firing

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  • mercury 150 EFI 2 cylinders not firing

    i have a 2000 mercury 150 efi saltwater series motor. the past few times i've run the boat, it would run on only 4 cylinders for the first 5 minutes or so. after the motor's good and hot(and under load, can't replicate on muffs), the other two will kick in and it will run perfect all day. it will even run fine after hours of sitting. but if i leave it over night and try in the morning, i'm down two cylinders again until it's hot and happy. i thought i had solved the problem by cleaning the ground coming off of the bottom two coils as they looked pretty green and nasty. this seemed to figure because these were the two cylinders that were not firing. after doing this, the problem seemed to be fixed and i ran the boat for about 2 hours and again experienced power loss, only this time the motor was already hot and it was dropping two different cylinders.(1&3 instead of 5&6) now i'm stumped because the problem seems to be moving around the powerhead somewhat randomly. ??? i have tried fresh plugs and seeing if the problem follows the coils which it does not. there is a spark at the cylinders that are not firing. i also checked the compression and am getting between 110 & 115 psi on all cylinders. stumped. please help!!! spark or fuel?