250 hp 1999 bought new out of the box 12 months ago, flawless performance until now and has had scheduled maitenance. Yesterday after 5 hours or so of trolling at 1,500-2,500 rpms I noticed it was running a little different as I accelerated to run in. Not much vibration or knocking just a little different, rough, like it was missing. I made it in no problems but in the channel I noted max throttle rpms at 4,800 when normal is 5,700. before I take it to the shop I would like some ideas, could it be the spark plugs, a lost cyllinder or the timing. Could the trolling have something to do with this as I was told at the 100 hour check that there was carbon build ups in the spark plugs and questioned as to what oil I was using which by the way is the premium Yamalube as my brother has a 55 galon drum. Thanks, Mark Sturge
If it helps anyone out there, I replaced the spark plugs today and sea tested it and there was no difference. I guess Im heading to the shop..Thankx Mark
The same thing just happend to me with my 200 HP OMC. I found a bad cylinder..after changing the plugs and running it...one plug, number 3 cylinder was not firing. I changed the coil...no change...I would suggest looking for a bad cylinder...look for a spark arch or just drop in new plugs and see if one of them look "new" with no browning and wet with fuel/oil. I would suspect a power pack or a stator if all the easy stuff dosent work. Id start witht he coil because they are cheap and easy to install...abt 25 bucks...2 bolts and they are in. Let me know how you end up...I go in on the 24 of sept for my WARRANTY (thank *** ) repair.. John
Well I should report back, it was a connector coming off the CDM unit to the electrical harness. So I was running one cyllinder less. I had checked the wires at least three times then I saw this little blue color and thought "there is no blue on the other connectors?" Plugged it back in and the engine is prime time again. I did do a de-carb of the cyllinders, replaced the plugs and the engine is smooth and screaming. Ive got 3 1/2 years left on my extended warranty and I am trying to learn as much as possible for that fatefull day when it expires.....Thankx Mark
Sounds like you lost a cylinder. I would check to see if they are all firing before I took off to the shop. Crank it up and pull the spark plug wires one at a time and see if it makes a differance in sound and rpm. If you find one that doesn't, then that cylinder is dead. You said you have already changed the plugs. If you find a dead cylinder, check to see if the plug wire is firing. J.B. has many post here on checking this, but make sure you can get about 1/2" arc back to the block with the plug wire.
Loss of RPMs in a 200 hour old mercury
Linear Mode
