Wont continue to run after prime and start

I had what sounds like a similar problem.
My issue ended up being bad gas (as in, there were little tiny flies in the gas tank).
they were small enough to get past the filter and pump.
I pulled the inlet hose to the carburator off and ran the starter and watched as 3 or 4 little flies were pushed out with the gas. ran fine after that.

I'm not sure of the model number of your mower or engine to know the fuel and carburetor set up. I do have one easy thing to try though as it sounds as if a vacume is building up in the tank. Try putting a half of tank of gas in it, leave the gas cap off, and see if it continues as it was or if it runs ok. If this stops the issue your cap vent is pluged up. I hope this helps

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blow air back threw the line towards the tank you have something clogging it or you forgot to turn the fuel on

You need to prime the motor with the fuel in order to start the lawn mower. Here are two links for you that will tell you how to do that.
http://www.associatedcontent.com/video/16313/how_to_start_a_lawnmower.html http://www.homegardenguides.com/garden-forum/home-repairs-forum/71141-priming-lawn-mower-engine-what-does-pushing-bulb-actually-do.html

I would take the bowl off the bottom of the carb.. Probably you have clogged up holes that let the gas into the bowl.Or either the needle is stuck up not allowing the gas to flow...That's why pouring gas in carbs would let it run for s few seconds....Also try fresh gas..Gas doesn't have led in it anymore..It don't last year from year...After grass season is over..Buy some from Lowe's or home depot..Pour it into your tank and let the motor run to get it done into the carburetor...Then the next year it will start

You could have a bad pump motor in your fuel pump. You will hear the "hum" of the pump motor but it won't continuously feed enough pressue to the system.
When you prime it the engine will draw enough to run for a bit then quit. They usually dry out if not kept full and sunfire fuel pumps are noted for this type of failure.
Try a used one for about 75 bucks. My kids Sunfire did exactly the same thing.

High speed jet is plugged. You'll have to disassemble the carb

did you pour all of the old gas out before you started to use it. Sometimes condensation builds up in gas tank when stored. You need to dump old gas an put in new gas.

Sound as if you have too much oil in the machine. Anytime you see white or lite gray smoke it is a sign of too much oil. Before you go through the routine of cleaning the oil out of every nook and cranny, try unscrewing the spark plug and clean it with a soft rag. It would have oil on it preventing the machine from starting. If this does work and there isn't a lot of oil in the gas tank now, just continue running the engine until the smoke clears. It won't hurt anything. But if cleaning the plug doesn't help you will have to empty all gas and oil from the gas tank, refill with new gas, squirt a little gum out carb cleaner in your carb to remove any oil there (this can be done when you try to restart the engine). Remember, when you are mowing grass if you are mowing on a hill) of any knd, make sure to turn the mower so the gas tank is on a higher level than the oil.

Start with emptying the gas tank totally. Refill with fresh gas/oil mix. Todays gasoline ain't that great.
Also after putting in fresh gas and priming if it doesn't fire at all pull the spark plug and if it is dry, pour a about a tbsp. of mix gas in the plug hole. put the spark plug back in and see if it will fire.

Phil