I have a BMX 150cc ATV with a Lifan engine. The carburator says it's a Sheng Wey. I am looking to upgrade the carb to something better, but I don't know how to tell what size carb I have now. Also, what would be a good upgrade option for this size engine? I'm looking for better throttle response and some extra power.

 

a new carb could run a few hundred bucks.
Have you modified the air filter any yet? If not do that first and rejet.. ..

I don't know what the stock air filter is but I am betting that it's a very retrictive airbox.. If you can take some pictures of it...