89 Mercury 80hp - fuel issue, stumped

[kofkorn]

Hi all, my nephew just picked up a new to him 89 Merc 80hp. The first time he took it out, he was able to get about a 1 mile jaunt in before the motor started stumbling. It runs ok while pumping the primer ball.

The boat is a ProCraft set up with a 12 gal permanent tank.

We spent the week diagnosing and working at it.

He's the list of attempts:
* Changed out the fuel lines
* Used a known good separate tank /line system directly to the fuel pump.
* Attempted to rebuild the fuel pump
* Realized it's an older 4 port style fuel pump.
" Converted to a newer 3 port by piping the oil injection into the fuel pump inlet (with a check valve) including new fuel pump.
* Completed a prime on the fuel system.

Just today, we discovered that the air is getting introduced to the system via the impulse line off of the crankcase. When the line is blocked, the air stops. See the video.

As there is another impulse port on the back side of the fuel pump, directly attached to the crankcase, what is the rubber line doing?

If not this line, where could the air be coming from?

Thanks!

 

[electricjohn]

Are you getting air before the pump? If not, then it looks like your fuel pump rebuild went wrong.

 

[Chris1956]

The crankcase should not introduce air. Air in the crankcase will decrease the fuel pump's efficiency, as well as screw up the fuel scavenging. I agree with electric john

 

[kofkorn]

Thanks for the responses. The fuel pump on the motor currently was a new from the box pump, not a rebuild. My brother has clear line on the fuel pump inlet as well, and that shows no air entering the fuel pump.

I'll have him open the fuel pump and see if there's a diaphragm tear.

Would it be possible for the crankcase pressure/vacuum to be causing a tear in the new pump?

 

[racerone]

Is it air or fuel vapor that you are seeing?

 

[kofkorn]

How can we tell the difference? And what in the pump would cause fuel to vaporize?