Fuel line losing pressure

[fivebucks]

Looking for ideas of what to check next.  I have a 1991 75hp Mariner outboard and I am losing pressure on my fuel line.  I have to pump the bulb first time out in the morning and then again if I fish in one spot for a while.  If I do not it will bog down on me and may or may not kill.  If I pump the bulb too much while I am bogging down I will partially flood the motor and kill it.  I then have to turn the key for several 15 second bursts or more to start the motor.

 

I have replace the bulb fuel line twice to make sure I didn't get a bad one the first time.  I also replaced the fuel line from my built in gas tank to where it goes to connect with the bulb gas line.

 

Does this sound like the fuel pump?  I want to eliminate any cheap fixes that I can before replacing the fuel pump.  I blew a cylinder a few years ago and I assume the carbs were all redone at that time by my guy.

 

I owned it for the past 20 years and I have only used non oxy fuel.

 

Any ideas?  Thanks in advance for any help.

[RebelSS]

Pinhole or small tear in diaphragm.

[leech~~]

So, losing pressure when off at night and at low speed fishing. Small leaks like Rebel said sounds about right.

If it is running fine when at higher rpm's then the fuel pump, fuel flow and carb are keeping up the pressure needed then. The pressure is slowing leaking out some where which has to be a small leak at the diaphragm, fuel line connectors, or fuel pump seals. Good luck.

[Ufatz]

Leech is absolutely on the right track.  Likely an easy remedy here.

[fivebucks]

Thanks for the input guys.  When the bulb is pumped up it does run fine at high RPM's after a little bit.  I can hear/feel it want to kick in all the way and then it goes fine.  So sometimes it hesitates just a little bit before jumping up to full RPM's.  Of course the boat is on the lift at the cabin so this weekend I will check the size of the lines under the cowl so I can buy some of those along with some other parts.  I will have to decide if I trailer it home or work on it up at the lake when I am there for a week in mid June.  Thanks again for some direction.

[RebelSS]

I've got $100 that says it's the diaphragm, like I first said. Put yer hand under the carb input are where your line and diaphragm are. Pump that bulb hard. if you get a drop or two of gas leaking out there, you DO have a hole or tiny tear. Pumping it up makes it run better at high speed and /or brings up idle is the telltale sign.

[fivebucks]

Great - even with my limited mechanical skill I think I can handle that test.  The serial number on my motor shows 2 different diaphragms - one with an Inline check valve and one with a 90% check valve.  I will find out Saturday which one I have.  I appreciate the help.

[leech~~]

24 minutes ago, fivebucks said:

Great - even with my limited mechanical skill I think I can handle that test.  The serial number on my motor shows 2 different diaphragms - one with an Inline check valve and one with a 90% check valve.  I will find out Saturday which one I have.  I appreciate the help.

 

Um, Reb your up on this one. My Merc 75hp has floats.

[Crow Hunter]

I have a 75 horse mariner with the same issue. Are you saying the diaphragm is not internal to the carbs, but outside the carbs?

[bobbymalone]

the diaphragm is not internal to the carbs.  It's part of the fuel pump which feeds the carbs.

 

At least that's how my evinrude works.  my carbs have floats.   But I had the same issue you described and I replaced the fuel pump.  It was a $30 fix.

Edited May 24, 2017 by bobbymalone

[RebelSS]

Hey, they want free advice, that's what they get. That's what I've fixed in numerousJohnson /evinrude small outboards with the same issue, as long as I've been working on outboards. Not your issue, tough noogies. Wonderful. Take it to a mechanic and quit complainin'. I could care less.

[bobbymalone]

[leech~~]

56 minutes ago, RebelSS said:

Hey, they want free advice, that's what they get. That's what I've fixed in numerousJohnson /evinrude small outboards with the same issue, as long as I've been working on outboards. Not your issue, tough noogies. Wonderful. Take it to a mechanic and quit complainin'. I could care less.

 

Op, some little Sailor got on up the starboard side this mornen!  

[swamptiger]

Reb's old girlfriend might have had a diaphragm, but you would find that one in the lower unit - not in the carburetor...

[fivebucks]

Rebel - you were almost 100% right.  It was a gasket inside the fuel pump that was torn.  Replaced all the diaphrams and gaskets while I had it apart and the boat runs as great as it ever has run.  I'm glad I didn't bet you a hundo.  Thanks for the help!

[RebelSS]

Wondermous!!  Anytime, fivebucks! Years of workin' on that stuff does count for something!!!  I wish ya WOULDA bet me!!     Glad ya got 'er done!!!