Merc 350 misfire

[millerjr]

I have a 1998 Merc.350/260 carb. sterndrive. Had the boat serviced a few months ago. Had to have manifolds and risers replaced. While in shop, had new plugs,wires, distributor and rotor replaced as well. Took boat out yesterday for first time since service. Started fine but had a "miss" at idle. When I throttled forward, boat kept backfiring, would not go up on plane. Had to idle back to dock. I put in fresh gas as well.. The marina did a compression test, all was good. Also had oil,filter, water seperator and impeller replaced at time of service. Anybody have any idea's what could be causing this? I'm thinking electical but not sure where to start..Thanks, Joe.

 

[Bt Doctur]

Re: Merc 350 misfire

Crossed plug wires or water is still getting in or if you had the manifolds and risers replaced because of water it may have stuck a valve.

 

[Don S]

Re: Merc 350 misfire

If it didn't have the miss before all the work, crossed plug wires would be my first thought. If by chance they put Champion spark plugs in, I would replace them with some AC or NGK plugs, have seen the Champions cause misses many times.

 

[chiefalen]

Re: Merc 350 misfire

All plug wires are seated all the way?

My thinkin backfiring classic sign of the timing is off.

They did a tuneup but did not time it?

Thats a half a tuneup, sorta like being half pregnant.

 

[jy118lfd]

Re: Merc 350 misfire

Its probably plug wires 5 and 7 crossed. I've done it and it will backfire under a load. It could also be one of those two wires are off and the motor will cross fire in the cap or where the wires touch. Mostly due to the firing order of a small block of 18436572. 5 fires right before 7 and if it is connected to 7 it will fire on the intake stroke.