Engine problems after warmup

Hi all,

I have an older Snapper lawnmower, probably 1970-1980 not sure. It has a 11hp Briggs and Stratton engine. It seems that it has developed and issue after running. It starts and runs fine for about the first 20 minutes ( I can almost set my watch to it everytime and halfway through the yard) then it decides to fizzle out and die like it has just run out of gas. If I put the throttle all the way down to the lowest idle it will start and run fine at low idle however, as I steadily start increasing the throttle, it speeds up to a point where it just dies at a certain point ( like half throttle). Sometimes if I am patient enough I can increase the throttle so slowly it will hold and get up to speed and stay there but this a slow and tedious process and doesn't always work. Any thoughts as to what would cause this. Thx

 

If the engine is that old, then it has an older adjustable carburetor. There should be a screw under the carburetor bowl. Unscrew it just 1/8 turn or so and see if it behaves better.

 

make sure the gas tank cap is vented