Johnson 15HP Outboard

[TMonter]

Shoot - I would slap the lid back on it and run it for the next 20 years, but thats me.

My bad tank acts the same, but at an idle. At an idle it can't pull enough vacuum to over come the air leak. Priming the bulb does. If you question the tank, start there. It is possible that the fuel pump could have issues, but a simple o-ring is an easy thing to eliminate.
And from experience I can tell you that even a small air leak in the fuel line can and will cause some weird behavior.

I have two tanks and it does the same thing on both tanks so it's definitely not a tank issue. The other indicator is that it only seems to do this when the motor is hot. I replaced the connector to the motor yesterday so we'll see how it goes later this week.

 

[Jags]

I don't remember if that is a vacuum operated pump or mechanical. If vacuum - make sure you don't have any other leaks to deal with.

 

[TMonter]

I don't remember if that is a vacuum operated pump or mechanical. If vacuum - make sure you don't have any other leaks to deal with.

It's a vacuum operated diaphragm pump operated off of crankcase vacuum. I sort of wish it were mechanical but this design looks simple enough. A whole new pump is about 40 bucks it looks like:

http://www.amazon.com/gp/product/B000N9LCH8/ref=ox_sc_act_title_4?ie=UTF8&psc=1&smid=ATVPDKIKX0DER

Or I can rebuild it for about 1/3 the cost:

http://www.amazon.com/gp/product/B0009TOBTI/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=ATVPDKIKX0DER

I'll likely go for the rebuild but as I understand it they are sort of a PITA to rebuild.

 

[TMonter]

It's a vacuum operated diaphragm pump operated off of crankcase vacuum. I sort of wish it were mechanical but this design looks simple enough. A whole new pump is about 40 bucks it looks like:

http://www.amazon.com/gp/product/B000N9LCH8/ref=ox_sc_act_title_4?ie=UTF8&psc=1&smid=ATVPDKIKX0DER

Or I can rebuild it for about 1/3 the cost:

http://www.amazon.com/gp/product/B0009TOBTI/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=ATVPDKIKX0DER

I'll likely go for the rebuild but as I understand it they are sort of a PITA to rebuild.

Well I finally solved the problem this last week and it turned out to be a old diaphragm in the pump that had gotten soft and wouldn't suck hard enough when it got air in the line. I bought a rebuild kit online for 12 bucks, got it running good and then promply sold it after I bought a mid-90's 4-stroke mercury on craigslist.

The bonus is I sold it for a couple hundred more than I bought it for. (Bought for $350, sold for $550)