My son did a rebuild (not his first) on his carbed YFZ. Attempting to start it, there is no attempt at a pop of any sort. Turns over. Compression will blow your thumb off. Cams timing rechecked. Removed plug and grounded to head, and plenty of spark. But with plug in, won't pop on gas or even a little starting fluid. Nothing. Can the ignition timing somehow be so far off that there is nothing to ignite when it fires? Seems like ignition timing is hard to not get right. Anything you can suggest to look it?