Holy cow did you win the lotto? lol If you put in a new valve you have to cut the seat because it won't be at the same angle as the new valve. That's the mistake I made over n over before buying the cutter.

it sounds like a carb issue. Clean it really good and make sure to clean the jets with a fine wire like out of an old brake cable. and check the fuel. make sure it isnt bad and make sure it flows out of the petcock. i am on my second big bear and it will run forever if properly maintained. And i traded the first one for a camper trailer when i was homeless that was worth triple what i paid for the quad. I missed it and when the opportunity arose to get the current one i jumped on it as fast as i could oh yeah check and clean the spark plug as well. and let us know what it looks like. For example is it gray and dry or black and wet or clean as a whistle or wet and clean. these are just a few examples of the description you can give.