I have a BRIGGS & STRATTON 20" 300 series weed eater lawn mower and it take a long time to start. Sometime it doesn't start at all or it stalls once it get started. If you could give me an answer to my problem I will be grateful.

Possibly Water In The Fuel.

A little research into my weedeater mower I found that the check valve in the carb that controls the suction does not seat. The design flaw without a background spring type to force the check valve to seat will not allow the proper suction. The first indication is the engine running out of gas. Pumping the ball will allow the engine to run for a short time but will over time completely stop suction. As the gasket (neo) has a tendency to curl a small amount without the backstop. I feel this to be a design flaw and weedeater should consider some sort of correction. There will be a time when the engine will not start because of the lack of fuel. The gasket is between the carb body and top. Impossible to really correct problem. Reinstalling will sometimes allow the restarting/running of the engine. Good Luck

When it runs does it puff black smoke out the muffler and rev up and down? If it does then you have to replace the gasket diaphram in between the gas tank and the carburetor.