Grasping for the last straw here: got this '08 head off eBay that had very little carbon buildup and seller said it has 34 hours on it or so. Put that on my bike and it doesn't start. Backfires once in a while like it has low compression or timing is off and that's about it. Before I send the head off for inspection/rebuild, anything that would prevent it from working? I'm pretty damn sure timing marks are aligned right and it's shimmed right, so either it's a POS head or maybe timing marks are incompatible? Thanks!