What wrong w my go kart?

I put a brand new 900 dollar engine and a new carburetor on it and the thing still doesnt work. It barely ever starts and when it does it only idles and if i push on the pedle it shuts down. Sometimes like 1 out of 100 times it will go and once it does go it never reaches top speed. Please help!! oh and it also backfires sometimes, PLEASE.