MAG 350 MPI misfiring

[Bobajob]

I’ve got a running issue with a mercruiser MAG 350 MPI and could do with some advice.

Its starts and runs fine and will run in the harbour at 4 knots without issue. Once out of the harbour it will accelerate up to 2500rpm / 25 knots and runs well for about 10 minutes before starting to misfire getting progressively worse. If I throttle back to 1000rpm the engine runs fine indefinitely. If I increase speed a gain it will run for a few minutes and then start misfiring again.

It feels to me like a fuel issue to me and I have changed the fuel filter, but the symptoms remain, I also tried opening the fuel filler thinking it could be a blocked breather. Engine was serviced and winterised at the end of last season and has 170 hours on it.

You’ve probably gathered from my description of the fault that I’m a novice boater but do have a fair bit of experience working on cars. I’m after some advice on where to go next before I end up loading the parts cannon!

 

[jointventureII]

I'm not sure how exactly, but could it be temperature related? Happening after 10 mins up at cruising revs (ie once fully up to temp), then stopping when at lower revs (cools down), then reoccurring at cruising revs but after less time (already running fairly hot)?

 

[Bobajob]

I'm not sure how exactly, but could it be temperature related? Happening after 10 mins up at cruising revs (ie once fully up to temp), then stopping when at lower revs (cools down), then reoccurring at cruising revs but after less time (already running fairly hot)?

Could be, I had a similar issue this weekend with my old VW T25 van due to a faulty coil over heating.