Mercruiser 350 MAG question

[sk16]

I have a Mercruiser 350 MAG MPI in my boat which is working well except for one problem I have not been able to pin down.

When cruising around 3500 revs she goes well but every now and then there is a noticeable decrease in cruising speed and a slight smell of petrol around the boat.

Current solution is to reduce the revs to around 2000-2500 for 5-10 seconds then back to 3500 and she goes again (until the next time this happens).

This does not happen on a regular basis (well I haven't been able to link any event to this occurrence). Sometimes it happens once in a day, sometimes 3-4 times in a hour.

Any ideas or advise?

 

[lovezoo]

Obvious places to start looking is the fuel filter and see if the fuel vent is blocked.

If there is a smell of petrol you should also check your fuel lines are not leaking in the engine compartment.

When was the engine last serviced?

 

[sk16]

Thanks for the info SuperScuffer

The fuel lines have no leaks (I check for this before each startup). The last service was 9 months ago just before I took delivery. It appears that the smell (pertol or maybe exhaust) may be from too much fuel getting through and being burnt inefficiently (but this is just pontton talk).

Has anyone tried these fuel cleaners you can add to the petrol? Do they work?

 

[Bloater]

Hi sk16,

Do you notice any misfiring when this happens? Does the smell of petrol come from the engine bay or from the water? Does there appear to be any petrol/oil on the surface of the water?

I think your first step should be to discover if the petrol has passed through the engine or not, ie. is it unburned fuel or a fuel line/filter/pump problem before it gets to the engine?