1978 inline six question

[neptuna]

I have a 1978 mercury inline six tower of power. The serial number appears to be 5316117.

The problem is the engine starts fairly easily but does not stay running after about 2 minutes unless I contiually pump the primer bulb. It runs/idles for 2 minutes and then begins to loss power unless I pump the primer bulb. I had the engine which was running good at the time professionally tuned in the spring along with some other items, such as water pump, fuel lines, fuel and water filter replaced. In addition, the shop recommended having the fuel that had been winterized pumped out which I approved.

Since that time I have attempted to use the boat once when this probelm occurred.

I have
replaced the fuel lines (3/8)
replaced the fuel pump and fuel lines that were attached
replaced the fuel tank (went from below deck 20 gl to an above deck 9 gl)
replaced the primer bulb
Fuel vent on tank/ran with cap removed too

I have not replaced the quick connect or housing for the fuel filter. I'm open to any suggestions. thank you

 

[CharlieB]

Re: 1978 inline six question

Pumping the primer to supply fuel sounds like the fuel pump is either;

Not working, may have been reassembled incorrectly

or

Sucking air into the fuel system anywhere before/between the fuel pump and primer

Aftermarket quick disconnect fuel fittings are suspected of poor O-ring sealing, they seal O.K. when pressurized yet fail under vacuum and air sucks past.

 

[neptuna]

Re: 1978 inline six question

Pumping the primer to supply fuel sounds like the fuel pump is either;

Not working, may have been reassembled incorrectly

or

Sucking air into the fuel system anywhere before/between the fuel pump and primer

Aftermarket quick disconnect fuel fittings are suspected of poor O-ring sealing, they seal O.K. when pressurized yet fail under vacuum and air sucks past.

Thank you fofr your response.
The fuel pump appears to be ok. No leaking from the diaprams. Also the problem was consistently the same before replacing the fuel pump as it is after. Is there anyway I can check the quick disconect before replacing it? Thank you

 

[Chris1956]

Re: 1978 inline six question

Bypass the quickdisconnect. Remove the cowling, remove the fuel fitting from the fuel line. Run the fuel line into the motor where the control cables exit. Clamp the fuel line diectly to the fuel input (black plastic cup) on the fuel pump. Make sure the fuel inlet has the oring to seal it or it will suck air.

 

[neptuna]

Re: 1978 inline six question

i replaced the quick disconnect this morning. same result.
ok i will connect the fuel line directly to the pump, but if this works i'm still unsure what it is telling me other than the fuel pump was assembled correctly. will this further help identify air?
oh yeah, the outboard is a 115hp, forgot to mention that.

 

[neptuna]

Re: 1978 inline six question

should i leave the primer bulb in place when i reconnect to the fuel pump?

 

[Scott Danforth]

Re: 1978 inline six question

yes

 

[Chris1956]

Re: 1978 inline six question

The only fuel filter on the motor was the fuel screen under the black plastic cup on the fuel pump. Is the nylon gasket under the screw that connects the black plastic cap to the fuel pump? Are the pulse lines that power the fuel pump in good shape and clamped securely?

If you have backdrag carbs (likely) there may be a screen under the brass carb cover fillings. These can get clogged.

 

[neptuna]

Re: 1978 inline six question

ok, connected the fuel directly from the tank to the fuel pump and same result. Not sure about the pulse lines but would willing to try.

 

[halmc]

Re: 1978 inline six question

it'd help to know how this problem developed. You say it was running well before you took it to the professionals. Did it ever run well after the professionals had at it?

I woudl be hyper-suspicious of everything they touched, and would be especially on the lookout for incorrectly connected lines. They undoubtedly test ran the engine for less than two minutes. Which is about the same as not testing it at all. To my eye, our service industry in the USA is in nearly as much trouble as manufacturing.

If there is a leak that permits air to get in the fuel supply system, thus interfering with the vacuum that brings the fuel in, it would almost inevitably turn into a fuel leak when you're squeezing the bulb, unless the leak is on the tank side of the bulb. Even still, it takes a heck of a leak to curtail the fuel supply.

Have you by chance let the engine die, then checked to see whether there is fuel in the carb bowls? If the boat sat up for a while, it's a possiblity that the idle circuits are clogged and they sometimes present the symptoms you're observing. But if that were the case, there would be plenty of fuel in the bowls when the engine dies. When the engine dies, does it pick up RPM as if it were leaning out?

 

[neptuna]

Re: 1978 inline six question

actually the boat developed the problem the first time i used it once the work was completed by the mechanic. the engine runs and sounds decent hmmmm...... other than the small problem of having to pump the primer bulb ever two minutes.

i did replace the pulse lines when i changed the fuel pump. i did not check the carb bowls, and no it does not pick up rpms it just dies like fuel starvation. it was geting dark when i transfered the fuel line to the fuel pump but i will check again tomrrow during the day to see if i can detect a fuel leak as i'm squeezing the bulb.

 

[CharlieB]

Re: 1978 inline six question

Temporarily install a clear hose between the fuel pump and carbs, run the motor and watch for air bubbles passing thru proving air is sucking into the system anywhere before the fuel pump.

Find and cure any air leaking then be SURE to remove the clear hose once finished with your repairs as clear hose is NOT approved for fuel use.

Also beware the vacuum/pressure pulse hose that operates the fuel pump is important, cheap hose will 'flex' and seriously reduce fuel pump operation.

 

[halmc]

Re: 1978 inline six question

actually the boat developed the problem the first time i used it once the work was completed by the mechanic.

That's my precise experience with professional mechanics, especially the ones who prefer to be called technicians. If they are true professionals, they should stand behind their work and restore your engine to running at least as well as it did before they worked their magic on it. On the other hand, you may prefer not to give them another shot at screwing it up worse.

The best policy, I think, is to learn your boat's systems yourself, not just because you're far more motivated than the professionals, but because your familiarity and competence could save your life one day.