that it backfired out of the exhaust is just because there was some mixture in it.
what color is the smoke?
- blueish means there is oil burned
- white is moisture aka water
- brown/black means the mixture is rich
also check if there isnt any false air drawn in at, that would explain why it reves go up.
chec by spraying some burnable goody (deoderant/brakecleaner/...) across the intake rubbers (between the cilinders and carbs) to listen if the motor respond, which means a leak
ofcourse don't spray in the airfilter