Briggs intek backfires at full throttle and surge at low.

[Wazza65]

Also I can take off the left side park plug wire and it runs really rough like on the verge of dying but it stays running. If I take off the right nothing changes other than the back fire seems to disappear.

You have a valve problem on the right side. If you feel comfortable doing try setting the valve clearances but do both sides and see what that does. Also check the plug on the right cylinder.