Well I can't answer that. I have no way of knowing without pulling the cam out. I also personally haven't had the distributor out but that dosen't mean that it hasn't been out. How can I check either of those. I checked the firing order once before but I will check it again. I've also noticed that the vacuum diagram under the hood dosen't match any of the diagrams that claim to be for may year truck. Not even the ones in Chilton's. I've been going by the one under the hood. I'm going to re check my mixture setting also. I was wondering if a worn cam could cause this to happen. Is there some way that the exhaust valve could be opening before the spark plug fires? Or perhaps the plug isn't fireing at the correct time. I don't know what would cause that other than the distributor. I would think that a bad cam would show up on the compression test?