As i twist the throtle. The motor picks up rpm then at about 1/3 throtle. The motor go's flat like it's flooding out. Twist thru it it picks right up runs great. Or let off and it goes back to idle.if you hold the throtle in the flat spot it will not pick up it coughs, spits and sputers till it finley dies after about 10 seconds. New fuel new plugs runs great eather side of the bad stop. Any help please.