So i took the Blazer up to Big Bear in Dec. , and as we were up there, my 305 ran like crap. I think it was probably the elevation. But as we came down from the mountain, it ran better, but not like it did before we went up. Now the exhaust smells horrible, and it isn't running like it used to. Could it be the timing or something. Or maybe something to do with the knock sensor, cause i think it came loose.