Need help with my Mercury Outboard

[kayak1]

I have a 1978 Mercury Outboard 115 hp which runs great. The problem is that the motor runs for a few minutes and then the primer bulb collapses. It runs with the fuel cap removed or on auxilary tank but does not stay running when the the fule source is from the original below deck tank. Any suggestions would be welcomed. Thanks,
Kayak-1
 

[IKAT]

What boat
What tank

Age of tank and lines

sounds like sucking air or clogged --pick-up tube--

If useing same bulb--would leave that out--If not could just be bulb--They Do Go Bad

Also Leak around the pick-up in tank.

Ethanol problem with hoses

Clogged anti siphon valve in overflow

[kayak1]

IKAT.... Not sure what you mean by what tank.....

[IKAT]

It runs fine on the Aux. tank --Right??

If so Ck lines and pick=up tube from on-board tank for leaks

Sorry -did mean what year as a benchmark--If not used often in the summer--Wasps and other little creatures will build nests in your overflow or vent line and cause a vacuum.

If runs on Aux. tanke--check everything from the Engine back to tank.

[kayak1]

IKAT..... Thanks for your help I will try that.
Kayak-1

[how many]

having the same problem with my boat,let me know what you find.i wont be able to work on mine till later on next week.thanks   mike

[Hotrod]

That Kayak is gunna FLY!!!... 

[Andys Delight]

Vent is clogged somewhere.  Just replace the hose and vent cap. 

[SouthPaw]

 whsclogged vent.

[Fishin Dude]

I'll second the fuel tank vent diagnosis. If it runs without the fuel cap, but dies with it on, air is not going into the tank , stopping the fuel flow.

[Bucktail]

That Kayak is gunna FLY!!!... 

That's what I was thinking!