Yamaha c75 runs out of puel prime on long runs

[Angler17]

1994 Yamaha c75 . seems the fuel pump cannot overcome anti siphon valve on inboard tank. Had the same issue with an older evenrude 70hp. Boat had 3/8 fuel line on it when u bought it. I upgraded old lines go new when I put in new fuel tank. Problem existed prior to as well as after fuel tank replacement. Been running this way for several yrs. Just have to reprime about every 15-20 min. Hve eliminated water separator in favor of glass inline filter. I can actually see the filter run dry. My thought is that 3/8 line and antisiphon valve is to large for this hp. Fuel pump is small and has tiny fuel lines to carb. Am I asking this pump to do more work than its capable of? Any thoughts greatly appreciated!!

 

[99yam40]

Check fuel pump pressure and vacuum.
There is a good post on this in frequently asked questions section of the forum.
Hard to believe size of line would have anything to do with it.
Sucking air, tank vent plugged, or a restriction in line maybe.

But if restriction was from the bulb back to tank the bulb would collapse flat I would think.

I know that if I do not open the vent on tank my C40 will suck in the plastic tank, so the pump will draw a good vacuum if every thing is sealed up to let it

 

[Angler17]

Thanks, been battling this for ling time, replaced all lines and clamps. Just can't seem to pinpoint issue. Pump is new as well. Compression good as I replaced rings and pistons during rebuild. I'll keep digging thanks.

 

[99yam40]

test as I said to makes sure it is doing what it should be doing.

There are seals between crankcase sections and the outside of crankcase
There are also check valves in the crankcase the can go bad also.

Anything that can vent the crankcase section that provides the pulses for the fuel pump can affect the pump

 

[Angler17]

Thanks I will check crankcase. I was so concerned with fuel lines never thought to go there. Thanks

 

[99yam40]

Did you find and read the testing of fuel system in FAQ section I mentioned?
It will tell you if fuel system is forming the proper vacuum/pressures

Has the fuel pump been replaced? 1994 is old