My car has been acting up lately, under heavy load below 2,000 rpms it knocks, and above 4,200 rpms it stumbles, but only at half throttle or more. I noticed that the left rear of my car smells like gas lately, could a EVAP canister saturated with gas cause this problem?

 

I forgot to add, I had my heads off and a machine shop resurfaced them, could they of taken too much off of the heads?