Briggs vtwin 21 hp - Only running on one cyclinder. I think- started up let run for 10 minutes one valve cover warm the other nothing still cool. mower has half the power as it did a year ago- ? help

rebuild the carb it has a dirty fuel jet

Wrong head gasket or head gasket installed incorrectly and oil ports are blocked off.

Replace coil

remove fuel line blow air threw to the fuel tank something is clogging it

probably the valves have unseated the hotter it run the worser it get warranty defect take to dealer tell what it doing tell him you think its un seated valves and cover under warranty

Check the valve clearance .. intake should be .006. - .008 ex .008 - .010 check for loose valve seats . ....

my vtwin starts but runs fast and slow like it is running out of gas whatcauses this

Sounds like you're getting enough fuel (black smoke) Could be running on only one cylinder. Either stuck valve or loose valve adjustment or one bad magneto. You could try starting with one spark plug disconnected. If it doesn't start, reconnect that one and disconnect the other. IF it runs like it was, then you're running only one cylinder, and go to work on finding out why.

I THINK THE CARBY NEEDS A KIT AND SOME ADJUSTMENT

Sounds like the tank is not venting. Try leaving the cap a little loose and see if it stays running. If it does, either the vent for the tank or a vented gas cap is plugged