1986 Mercruiser 3.0L

[jandmreece]

Engine fires and purrs. Approximately 3 minutes in it starts running rough and finally dies. What I have done:
New Carb
New fuel supply lines (added primer bulb)
New fuel pump
Checked my vent line and blew air through it…all good.
The boat was doing this before I changed all this.
Any help would be greatly appreciated.
When she first fires it runs perfect

 

[Scott Danforth]

Primer bulb is not allowed on an I/O

Stop throwing parts at boat. Not sure why you changed the carb

Look at the ignition system. Have you serviced the points?

 

[jandmreece]

Not throwing parts at the boat. Carb was changed a year ago for a different reason. Just letting the thread know it wasn’t the issue. It’s a 1986 the lines we’re old anyway. Yes the points are fine, it’s not ignition system

 

[jandmreece]

Not throwing parts at the boat. Carb was changed a year ago for a different reason. Just letting the thread know it wasn’t the issue. It’s a 1986 the lines we’re old anyway. Yes the points are fine, it’s not ignition system

It’s a fuel issue.

 

[Scott06]

It’s a fuel issue.

A couple thoughts

take,out the primer bulb see if it goes away.
test fuel pressure when it dies, verify pumping throttle that fuel shoots out of accelerator pump nozzles
verify good hot spark when it dies

 

[Grub54891]

Check the anti-syphon valve at the tank/fuel pick up line. Sometimes they get debries in them restricting flow.

 

[jandmreece]

Check the anti-syphon valve at the tank/fuel pick up line. Sometimes they get debries in them restricting flow.

Took it off yesterday, the ball moves freely with no obstructions.

 

[jandmreece]

A couple thoughts

take,out the primer bulb see if it goes away.
test fuel pressure when it dies, verify pumping throttle that fuel shoots out of accelerator pump nozzles
verify good hot spark when it dies

Yea I am going to remove bulb today and not have so much slack in line. If that doesnt work I am gonna throw the supply line in an external tank and see what happens.

 

[Scott06]

Yea I am going to remove bulb today and not have so much slack in line. If that doesnt work I am gonna throw the supply line in an external tank and see what happens.

Might be worth checking the filter in the bottom of the fuel pump and carb inlet if there is one there unless you verified the fuel system was clean

 

[jandmreece]

Even though I am removing the primer bulb, I had the supply line in a bucket and had a solid flow of clean fuel when I was pumping it through, the fuel pump is new with filter. My carb doesn’t have a filter that I know of

 

[jandmreece]

It runs great when first fired up, makes me curious if either my fuel lines aren’t collapsing or if my impeller was bad would that immediately shut the engine down? I would think it would slowly considering fuel would still be in lines? My impeller is due to be replaced

 

[Scott06]

It runs great when first fired up, makes me curious if either my fuel lines aren’t collapsing or if my impeller was bad would that immediately shut the engine down? I would think it would slowly considering fuel would still be in lines? My impeller is due to be replaced

no impeller being bad would only lead to over temop buzzer going off wont shut engine down.

 

[Scott06]

Even though I am removing the primer bulb, I had the supply line in a bucket and had a solid flow of clean fuel when I was pumping it through, the fuel pump is new with filter. My carb doesn’t have a filter that I know of

I bet there is a screen where the fuel line goes in. May be a non issue just figuring you had replaced the carb beacuse it was gummed up possible fuel contamination

 

[ltklein]

Sounds like you're running on the choke.

At 3 minutes it should start backing off of the choke, leaning out your mixture. I'd say either your mixture is too lean, or the carb has picked up some junk in one of the orifices/jets.

Keep track of your mixture screw position and then give it a 1/2 turn out, and maybe even a full turn and see if it acts any differently.

Assuming the new carb was changed for an identical carb and not a carb swap?

 

[Scott Danforth]

measure fuel pressure. lines running from pump to carb are steel, so you will need the test fitting. you should have 3-5 psi from the pump.

fuel pressure will tell you if there is an obstruction

 

[jandmreece]

I will attempt all these this evening and give update, I replaced carb a year ago due to one of the screw holes for the linkiage was stripped located in the body of the old. The carb was exactly the same just a newer PN. I’m gonna get rid of the bulb and line slack first.