12.5 briggs engine on riding mower .\r\nengine starts and runs , but surges, if I I run it with the choke out just a tiny bit ,it run smoothly. Tryen adjusting the carb ,no effect I replaced the air fiter ,same result

How many adjustment screws did you find on the carb?

Just one

Should be 2, idle speed and mixture. Check the base of the barb. How to Adjust a Briggs & Stratton Carburetor | eHow.com (http://www.ehow.com/how_5650449_adjust-briggs-_amp_-stratton-carburetor.html)

Thanks 1 I'll check it out

Hi,
Newer models only have slightly effective low speed adjustment. The EPA guidelines are clear; if you can make it fail, then do. The adjustments went with that.
You have a dirty main jet circuit or passage to the low speed jetting that causes this. A carb cleaning is in order.
Peace,
Clarke

Thanks clark that's probably it as I took this carb off a old mower. Any pointers on cleaning this carb. It does just have the one ajustment.

Hi,
Link attached is the manual:
http://www.mymowerparts.com/pdf/Briggs%20&%20Stratton%20Service%20and%20Repair%20Manuals%20S pec%20Charts/270962%20Single%20Cylinder%20L-Head%20BRIGGS%20&%20STRATTON.pdf
Peace,
Clarke