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Mariner 40hp Surging at WOT and dying off at high RPMS

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PatrickMonahan

Member
I have a new problem with y Mariner 40hp that it is surging at WOT and slowly dies down in the revs until it stops, I checked the plugs and they are wet fouled, also the compression seems to be 110psi on both cylinders, i dont know what is causing it to die down in revs completely until it stops, it is very intermittent, the carbs were took apart and seem to be fine so its not that, its a fuel starvation problems, any suggestions?
 
Sounds like it could be a seal somewhere, and there is a few of them.

Do you have any "oily goop" anywhere on engine block or lower pan (cowl). And what I am referring to is something that looks like maybe you gave it a spray with WD40 or fogging oil. If one of the downstream seals (somewhat after the carbs) is shot it allows gas/oil to "blow out" and when the gas evaporates it leaves behind the oily mess.

That can sometimes help pinpoint which (if any) seal(s) are shot.

Also, you mention the carbs were done, were the carb gaskets replaced (between carb and reed block or reed block and the manifold) - that could be a source of "extra air".

There were different 2 stroke Mariner 40's, one was the Merc model (in 2, 3 and 4 cylinder configurations), the other was the Yamaha (built for Merc) model and they are different with potentially different points of failure. There may even be a 4 stroke model out there, so to go any further you really need to post up "what you have".
 
I had this exact same problem last year. I replaced fuel pump, all lines from tank to carb and rebuilt the carbs and nothing fixed it. It ended up being the pickup in the tank came loose from the fitting that the fuel line hooks up to on the tank. It is was a hard plastic tube and looks like it was glued on the nipple in the tank. The glue came loose and the pickup was held in place because the bottom of the tank wouldn't let it drop enough to fall off.

It was sucking air around the nipple at WOT. I put a piece of fuel line on it, long enough to go to the bottom of the tank and curve a bit and I have been fine ever since.

Is your fuel bulb soft after it surges? If so, I bet you are sucking air somewhere in the tank.
 
Got a mechanic to check the fuel pump today and he found a pin hole on the diaphragm inside the fuel pump, hopefully this is the problem, i was also told that you leave out a plate and spring because if you put it back in, it will damage the new modified diaphragm
 
After replacing the fuel pump diaphrams and gaskets on the fuel pump, took the boat out today and it rain at full throttle nice for 15-20 seconds and then it started to stutter at high revs, about two minutes later the engine completely cut out on me but started easily again and failed, it never turned off before i installed a thermostat in in that works because the previous one was broke open and therefore the engine was always cool, could the coils be breaking down, it is a dual coil motor with a mitsubishi logo on the coil and is about 35 years old, would the coil heat up from the engine and break down, the fuel pump and carboretor have been completely ruled out so it must be something else now but what i dont know.... does anyone have any suggestions that they could add thanks
 
I've seen a bunch of old coils fail like that. Worth trying at least a used one or two and see if it helps.

Jeff
 
Does anyone know what the sound of grinding is when the engine stops in the river after uncontrollably doing down in the revs and stopping, it sounds like a grinding sound and slowly comes to a stop, it happened yesterday after possible coil failure, another thing I noticed is that the spark plug is wet fouling
 
..."Does anyone know what the sound of grinding is when the engine stops in the river after uncontrollably doing down in the revs and stopping, it sounds like a grinding sound and slowly comes to a stop,"

That's the sound of the prop dogs slipping over each other since the water is rotating it assbackwards. Put her in F and turn it the wrong way and you'll hear it.

Coils contain wiles of wire coated with a plastic substance to prevent shorting out. Over the years of heat, age and vibration, this coating breaks down causing coil failure--especially at speed.

Jeff
 
Anyone know why my engine will only idle at 1200rpm, at its lowest when the carbs are balenced? I saw it do 900rpm before I put thermostat in and now it's doing 1200rpm warm, I have no idea why it's doing this dies about 850 at its lowest and then stalls but now won't go below 1150-1150-1200 thanks
 
I seem to be not able to get any reading from the secondary circuit, would the engine run badly on the primary circuit alone and stutter at high rpmz, also I got a shock from the top spark plug lead on the top cylinder, would this have a negative effect on the spark?
 
Well I have some fantastic news, the boat is fixed and flying!!
The old coil was causing the stuttering which was got rid of when we got the new coil, but the engine mysteriously stopping was the a bad valve in the primer bulb cutting off the flow to the pump and carburetor's, never went as smooth and fast and good and reliable today, I though it would never be right or fixed, very powerful engine now, in delighted now, changed the primer bulb and it never stopped once, new coil stopped the stuttering at high rpms so 2 problems at least, thanks very much to everyone who gave me suggestions and help!
 
Same feeling I had. Spent all kinds of time on the engine and the problem turned out to be in the fuel system before it even got to the motor. Annoying but a relief at the same time.
 
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