MP340- Running on seven cylinders/rough at 1400 rpm

[paulct]

I've had this problem for a while, and the dealer was unable to find anything "wrong". But I get a pretty good vibration/roughness at around 1400-1500 rpm (in neutral and underway). From my old car days, it sure seems like a misfire/missing cylinder.

So I plan to pull the plugs, check/change plug wires etc. However the last time I did this stuff it was on a 68 GTO, when things were simpler!

Is anti-seize compound required on these new motors? Not sure what might be aluminum or cast iron these days. Its a MP 340 in a 2007 20V.

Any other things I should look for?

Thanks!

[chpthril]

Unless you are the 2nd owner, this is under warranty and should be addressed by a dealer. Take the dealer's Tech with you on a sea trial to show him exactly what you are feeling. He should also take the ECM interface as well. If he knows what he's looking for, and can interpret the data, it will give him some direction to go

[dogbert]

Unless you are the 2nd owner, this is under warranty and should be addressed by a dealer. Take the dealer's Tech with you on a sea trial to show him exactly what you are feeling. He should also take the ECM interface as well. If he knows what he's looking for, and can interpret the data, it will give him some direction to go

Wasn't there another person who had the same problem with their boat and MP 340 on here? I think they were headed to Powell or Mead for an extended trip and were bummed about having to get their motor rebuilt.

[R&T Babich]

Our boat had a cracked spark plug and similar symptoms as yours. A faulty plug wire would be the same. Might try another cap/rotor and stow it away as a spare if that's not the problem. After taking out the side panels changing the plugs on ours was a snap. The heads on ours are cast iron, but I don't know if the newer engines have gone to aluminum. I prefer cast iron in a boat.

[chpthril]

Wasn't there another person who had the same problem with their boat and MP 340 on here? I think they were headed to Powell or Mead for an extended trip and were bummed about having to get their motor rebuilt.

Wasn't is Talltigeguy's partner

[FIC]

We have seen some come in with cracked spark plugs , with the side covers out and somebody driving you can sit next to the engine and here an arching sound and in some cases see it arch to the exhaust manifold.

[da.bell]

Wasn't is Talltigeguy's partner

[dogbert]

Don't go there, girlfriend

[paulct]

"Unless you are the 2nd owner, this is under warranty and should be addressed by a dealer. Take the dealer's Tech with you on a sea trial to show him exactly what you are feeling. He should also take the ECM interface as well. If he knows what he's looking for, and can interpret the data, it will give him some direction to go"

Tried the dealer as I stated, like most very subtle problems, they were not helpful, that is why I am doing basic stuff myself. If I can't fix it, THEN I will force them to look again. Frankly, it takes longer to work with them then to take 45 minutes to check plugs/wires.