I have had vacuum leaks & a low fuel pressure problem at separate times. both have caused crappy idle and back firing through the intake if I tried to rev the engine. My question is, why will a lean fuel condition cause a back fire through the intake? I understand that a rich condition will let unburned fuel ignite in the exhaust & cause an exhaust back fire. how does the intake back fire happen if your ignition & cam timing are correct? just a question out of curiosity.