Good morning my polaris sport 400 l of 1999 fails in progress and after accelerating it recovers, I ask for help, which could be, thank you

 

Probably needle and seat worn,dumping too much fuel into the carb bowl. Can cause stumbling and hesitation until the rpms rise enough to handle the fuel. Common problem on these older ones. Check the float arm for any wear. Shouldn't be wobbly and worn. If it is replace the float arm and float pin. These two parts do not come in carb kits.CARB REBUILD REPAIR KIT POLARIS 1998 - 1999 SPORT 400 2WD 2X4 98 99 | eBay Here's a couple videos on this style carb.

 

Good morning first of all thank you, I already changed the needle and the carburetor is clean the reaction of the motorcycle is this: I am in progress regardless of the speed or rotation it fails and sometimes it accelerates and it recovers and sometimes not until it goes below . Is there a grill next to the air filter that is cracked will it get too much air? Once again thank you

 

Possible it can be an electrical problem more than fuel or carb related. Pull the Black wire from the reverse limiter module under the front panel and see if that helps.They can fail a limit in forward range also. Worth a try.Item #30 on the parts break down.