losing power at WOT

[tshamp1957]

I have a 1981 Mercury Marathon 135hp outboard. At WOT it runs fine for a short distance, then bogs down. If I let it sit a few seconds, I can open it up again, but then the same thing happens. Any ideas? I have rebuilt carbs, fuel pump, replaced primer bulb with OEM, and changed out all fuel lines. Still the same problem. Can someone help me please.

 

[jimmbo]

Hmm. well first of all, there was no 135 hp in 1981. Is it an inline 6 or a V6?

1970 - 71 inline6

1986 and newer V6

As for it being a 'Marathon', Mariner used that name on some of its engines.

As for your problem... Portable fuel tank, or built in tank? Have you tried running it with different tank?

 

[tshamp1957]

It is a V6. I'm sorry it is a 1986. It is basically a built in tank. I have not tried running it on another tank.

 

[jimmbo]

You replaced some fuel line components. Did you pull the fuel pickup out of the tank to inspect it for blockage? Some pickups have a filter to stop dirt/water from entering the lines.

 

[tshamp1957]

I went to remove the fuel pickup tube and found that it is actually welded to the tank.

 

[jimmbo]

You're sure that was the fuel pickup tube and not the tank vent?

 

[tshamp1957]

Yes I' sure. On the right side of the tank is the fill tube and vent tube. On the left side is the sending unit and the pickup tube.

 

[tshamp1957]

OK Jimmbo, I disconnected the fitting at the engine from the fuel line and capped it off. I pump the primer bulb (Brand new OMC) and it gets hard and then loses the pressure after about 30 seconds. I used soapy water on all fittings to check for leaks and found nothing. Any ideas, and yes I have the arrow on the bulb pointing skyward.

 

[tshamp1957]

Still hoping to find a solution to my power loss problem. Anyone have any ideas?

 

[jimmbo]

Try running the engine on a portable tank

 

[Chris1956]

Does the primer bulb collapse,when the motor starves for fuel? If so, the fuel tank vent could be partially clogged. Also, most fuel tanks have an antisiphon valve. These can get partially clogged as well.

 

[racerone]

The primer bulb loosing it prime after 30 seconds is NOT an indication of a problem or external leak.-----The bulb is a manual pump to fill the carburetors and once that is done it does nothing.

 

[tshamp1957]

Roger that racerone. So do you have any suggestions as to why it runs at WOT for a bit then bogs down. After setting a bit it goes good again then bogs down. It is as if the carbs empty out then refill. They have been rebuilt and so has the fuel pump.

 

[jimmbo]

There plenty of places in a fuel system where a restriction or an air leak can happen. Both can result is fuel starvation