Mercury 7.5 cannot suck fuel?

[Ricky_rich]

I have a 1980 Mercury 7.5 that I’m having problem with.
Engine starts up fine and runs great. But after a few minutes it’s coughing and dies. If I pump the bladder on the hose when that happens, I can keep the engine going.
So I thought the diaphragm might be bad. Had the carb apart and cleaned, put together with new diaphragm and gaskets. But the problem is still there. Engine cannot suck fuel from the tank by itself. The little hose from the crank case to the carb looks good.
Anyone have an idea ??

 

[Scott Danforth]

it is either the fuel pump or the fuel line.

if you have a leak in the fuel line, it will suck air
are the crank case pulses getting to the back-side of the diaphragm to operate the pump?

 

[WesNewell]

In the 80's I had a 115hp evinrude that had a weak fuel pump. I just installed an automotive electric fuel pump in line and wired it to the ignition. Not only did I then get max power from the engine, I never had to screw with the fuel bulb ever again. On the same engine the power pack , that was simply a 4 diode rectifier went bad. A new one was like $100 bucks or something. I replaced it with a $3 (35A iirc) rectifier from rat shack and never had another problem from it.

 

[Scott Danforth]

In the 80's I had a 115hp evinrude that had a weak fuel pump. I just installed an automotive electric fuel pump in line and wired it to the ignition. Not only did I then get max power from the engine, I never had to screw with the fuel bulb ever again. On the same engine the power pack , that was simply a 4 diode rectifier went bad. A new one was like $100 bucks or something. I replaced it with a $3 (35A iirc) rectifier from rat shack and never had another problem from it.

and how does that help the OP on a 7.5hp outboard

first, there is no electrical system. so none of your suggestions would ever work on his motor

second, the little walbro carb has a max fuel supply pressure of 2-3 psi, where any automotive pump will put out up to 7 psi, which would result in fuel simply being dumped out the of motor

third, adding an automotive fuel pump to a marine motor is dangerous as they are not J1171 compliant

 

[Ricky_rich]

Looks like we can close this case.

I found out that the pulses to the diaphragm don’t come through the little hose. It comes from a hole next to the intake.
I took the carb of today to go through everything again. Found out that someone, (me) have put the gasket between intake and carb the wrong way around.



So my little Merc runs just perfect now.

 

[Scott Danforth]

Thanks for closing the loop on this one.

 

[Texasmark]

it is either the fuel pump or the fuel line.

if you have a leak in the fuel line, it will suck air
are the crank case pulses getting to the back-side of the diaphragm to operate the pump?

I'll go for cracked fuel lines. Look your lines over carefull for cracks of any kind and fuel wet spots on the hose where they don't belong. Get on the squeeze bulb and pump it up hard. It should get hard. If not line leaks, FP diaphragm/internal valve seals, or carb float controlled needle valve not fully seating...defective, seating ring around the rubber....cracked, deormed, or floats not correctly set. With a hard/leaking down squeeze bulb, if the problem is fuel line associated, you will have fuel coming out somewhere. At that age all your rubber should be/have been replaced anyway!