Merc 40Hp Problem

[Kosta1]

Hey guys, today I took my boat for spin after about 6weeks . Everything was going fine we lauched at foreshore boat ramp and was half was to the oilwharf we the motor started serging. I checked the fuel vent and made sure it was getting fuel. I then decided to go back to the boat ramp,when I got back I checked the sparkplugs, I closed them a tiny bit as per wat the book states. Went back out for another spin, and the same thing. This time I just ran it to the oilwharf as I thought it may just be a blockage that hopefully will clear. On the way back it was fine but as soon as we arrived at the boat ramp it started again. The motor is a merc 40hp 4stroke carbi 2000 model. It seems to me as if one cylinder is not firing at idle and is cutting in and out at about 4000rpm. I'll put new plugs in next week but other than that does anyone have any suggestions. Any help who be greatly appreciated.

Cheers

Edited October 10, 2010 by Kosta1

[Fester]

Did you fill up with fresh fuel? sounds like oil has settled in carby from fuel left in it, or fuel has gone stale in tank.

[Kosta1]

Yeah I did put fresh fuel but it probly had 8 liters of old fuel still in it. Is there a way that I can drain the carbies. Btw it's a four stroke, so could there still be oil in the carbi. Thanks

[Geoff]

It sounds like fuel. You will need to check the system for air leaks (sucking air) or blockages.

The easiest way to do this is s to run a hose direct from the tank to the motor , ie by passing the existing system.

If the motor runs OK then the problem is somewhere between the tank & motor.

If the problem persists then the next place worth checking is the fuel pump.

On 2 strokes they use a diaphram system ( not sure about 4S) & over a period of time the diaphram can crack allowing fuel to be pumped directly into the cylinder which floods causing a miss.

If there is a filter just before the pump check that it is clean

Geoff

[locodave]

Well on my 2stroke mercury I had a similar problem. What happened was the engine dropped in revs and then picked back up again.

What happened was that the fuel hose was falling apart from the inside. Motor is 2001. So I have a few hose bits stuck around the fuel pump diaphram area.

I replaced all my fuel lines and a new diaphram and now it's all good