[Fuely]

Hope someone can give me some advice on this Buick. Have been working on this car for some time now with no results. 3800 engine has a misfire and if the engine is revved to around 2500-2800 there is a constant popping in the exhaust manifolds. Sometimes it will shoot a flame out the throttle body.I have been through the ignition. Coils/Module/Wires/Plugs Crank sensor/Cam sensor. Have checked fuel pressure and regulator all seems within spec.Even replaced all fuel injectors. Though maybe a valve or valvetrain problem. Had cylinder heads off and did find some leaking exhaust valves and had everything repaired. Still has the same exact problem. No computer codes either. Can any one give me some advice? I have no clue.