Lawn mower will not run?

Question

I have a 20 inch Rally push mower with a 3.5 HP Briggs & Stratton engine. I havent been able to get the lawnmower to stay cranked for more than 3 seconds without it shutting off. I have changed the spark plug, changed the oil, removed the gas tank and emptied it and put fresh gas in. I have also cleaned the air filter. I have also made sure it was not clogged anywhere. I can prime the engine, pull the cord and it will crank and run for 3 seconds and shut off. Any suggestions?

Answer 1

This engine doesn't have a fuel bowl.  The carburetor is mounted directly on top of the fuel tank.  This sounds like a typical Briggs carburetor issue.  There is a diaphram between the carburetor and the fuel tank.  It get old and quits doing it's job.  For about $5.00 you can get a new diaphram and gasket.  It is not a hard job to remove the fuel tank, remove the carburetor from the fuel tank and replace the diaphram and gasket.  Check the tank for dirts while you have it off.  Make note of how the springs and rods attach to the carburetor before you take them off.

Answer 2

Locate the fuel bowl that sits between the tank and the carb (the fuel line should lead to the area).  Warning: this will make a mess.  Do this outside where gas won't hurt anything!  

Remove the bowl using a 1/2 inch wrench or socket on the nut underneath it.  The bowl can then be pulled straight down but may need a slight twisting motion to get it loose.   Fuel should be coming out of the needle valve (thingy that the bowl covers and lets gas into the bowl).  There will be a small float, that's usually brass, that you can lift up and down with your finger.  

If the float appears level then it is stuck.  Gently push it down with a finger tip and hope the fuel starts flowing.  If so, flip it up and down to work it loose.  If gas still don't come out or is really slow then the needle valve is stuck, the seat is plugged or has swelled closed.

As for the bowl, it may have water or crunchies in it.  Clean out the bowl and don't loose the 2 gaskets.  One is smaller and goes between the 1/2 inch s***w and the bowl- the other is thin and wide as the bowl and sits in the ledge of the bowl to seal it to the carb.  A new needle and seat is cheap and can be replaced by a mechanically inclined person.