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oil is blowing back up through carb.

do you mean that oil is blowing up into the air cleaner?
run a compression test as mentioned, you can pick up a tester for about $30 and if you can change plugs you can run the test. if your intake gasket leaks your screwing up the fuel/air ratio and in turn your rig will run like crap. you can seach for a intake leak by spraying around it with wd while its running. run the comp test, post the numbers and replace the intake gasket then go from there. btw, does it smoke and welcome to the wire.

98' BBJ Grand Laredo 5.2 locked and loaded,.01' Gen III XLS. Frankenmonty lives on in another.