1986 mercury 75 running out of fuel

[nhboat]

hi, 1986 mercury 75hp 4 cylinder seems like its running out of fuel. she will idle fine and comes out of the water no problem and runs at 5000-5500 rpm for about 30-45 seconds and then loses power and will stall out. when it starts to happen if i back the throttle off she will stay on and idle. i can cruise all day at 4000 rpm and under though. no problems there. so compression is good across all 4. carbs were rebuilt by previous owner, gas tank and all lines and filter are new, trigger is brand new, rebuilt triangle fuel pump. by passed the screw-on fuel filter and still same thing happens. when it starts to die if you prime the bulb she will stay running so its definitely a fuel issue.
sounds like floats are to low to me so i double checked them and the line is about parallel to the carb body but i just adjusted them to allow more fuel in. ill see how that goes tomorrow night but im just scratching my head over here now.

 

[ondarvr]

An improper float height won't respond to squeezing the primer bulb, an air leak will though, so it's most likely that, or back to the fuel pump.

 

[nhboat]

fuel pump just got new gaskets, check valves, and diaphram today. so if the bowls are running low on fuel, if you pump the bulb to force more gas in there, that wouldnt be the issue? the motor runs great all around except when it runs out of fuel at full throttle. then i have to let off the gas and almost come to an idle for a minute and then i can take off again at full throttle for another 30ish seconds and repeat. or i cruise at 4000 rpm and have no problems.
does any one know about the plugs and water jackets? ive been reading and one thing about thoes caught my eye. i put new plugs in, and after my first short trip out, all 4 plugs still looked brand new and shiny but were wet with gas/oil when i pulled them all. what is this water jacket leak stuff ive heard of? could this cause an issue?

 

[MH Hawker]

its the fuel pump

 

[jimmbo]

Those last 1000 to 1500 rpm take a much greater amount of Fuel. So any air leak or restriction will show up very fast.

 

[nhboat]

I just rebuilt the fuel pump with a new diaphragm and check valves and cleaned the screen and all looked good. Didn't change how it runs.
And the engine runs great taking off and anywhere up to 4000 rpm. It also runs great at full throttle but only for 30 seconds and then runs out of fuel. I have to come back to idle and let the bowls fill up with fuel for a minute and I can do it all over again. Wouldn't an air leak cause to to idle up or run faster?

 

[jimmbo]

Are the O-rings on the fuel connectors on the tank and the engine end of the fuel line sealing? Have you tried running on a different tank or hose? With the engine not running, does the primer bulb get firm?

 

[nhboat]

I will take a closer look at the tank. It's a newer plastic tank with a newer bulb. All was out in by the previous owner at one time. I don't have another tank to try. I'm new to outboards so I know I pump the bulb and it gets fairly firm. But I did notice if you wait 10 mins after stopping it, it gets soft again. Not sure if that's normal or not? When it starts to lose power if you pump the bulb she stays running or if I throttle down she also stays running. If I stay full power she dies. Give her a minute to fill the bowls back up and then I can take off again.

 

[jimmbo]

Either it has a restriction in the fuel system, air being sucked into the line, bad fuel pump, vacuum/pulse line or gasket between fuel pump and block leaking.
Is the gas tank vent open enough to let air into tank?

 

[kopeck]

I saw your post in my thread. Sure sounds like it's running out of fuel to me. My uncle had a 850 Merc (pretty much the same as our units) that had a similar issue. It turned out one of the fuel lines was cracked, I think it was down by the bulb.

Do you have the triangle style fuel pump? Check the pump cover, mine was plastic and would leak. I don't think it ever cause any running issues but when you went to pump the bulb it would squirt out. Two new o-rings and still no joy. Turns out the OE plastic cover had been replaced by a new metal one. Mercury sold the conversion as a kit, cover, filter and o-ring. I say sold because it's NLA but there's still some out there. Costly bugger, like $75.00. It's never leaked on me again. Just some thing to look at.

K

 

[nhboat]

UPDATE: FIXED!
so fuel tank checked out to be good. there was some gray fuel like from the tank that was attached to the attwood ball and upon inspecting that line, i found it has a plastic inside coating that was coming apart and creating a restriction in the line. so i replaced all fuel lines. while i was at it i also bypassed the quick disconnect attached to the outboard. it seemed to be to easy to take on and off and i believe fuel was leaking from that spot. so fixed these 2 issues and took her out and she ran great now. thanks for all the help!

 

[ondarvr]

That grey fuel line has caused a huge number of problems.