Small Engines (Lawn Mowers, etc.): Briggs & Stratton 12.5. H.P. small engine, briggs stratton, speed needle

Question
I have a John Deere 111 Lawn tractor and replaced the engine. It had a Briggs & Stratton 11 H.P. that was starting to burn oil so I replaced it with a Briggs & Stratton 12.5 H.P. This is a newer engine  that runs good. But after it has run for a while at about 1/2 throttle it seems to be running rich and wants to cut out. I looked for a high speed needle valve but cant find one. What do you suggest I do to take care of this problem?  

Answer
Hi  Joe,

Sorry for my delay.  It was a busy holiday here.

There should be an adjustment.  If not, there might be a replaceable jet and if so, you can get a smaller one.  Make sure that the air filter is not dirty, which would richen up the mixture.  Lastly, the carb may need rebuilding.   

I am swamped with questions and can't give follow up answers until next week.  Sorry.

Good Luck,
Michael