Once upon a time there was a little tinkerer that didn't bother to check the valve clearence before doing the comp test. he had low compression on one of the cylinders and took it all apart again to find..nothing. after that little tinkerer had it all together again, he did check the clearence and it turned out, one of the valves had no clearance at all and was not able to close, thus low compression. the tinkerer was me.
Since you had the head off, are you positive that the camshaft is mounted exactly right? I once had the head off and mounted the camshaft several degrees off (due to a broken dowel pin of the spark advancer ) . same symptoms as you have (backfire every now and then, will never run).
...and on another occasion I reassambled the spark advancer 180° off (I don't know if that's possible with the 550, with the 650 it is), which also gives backfire (albeit only one every two minutes...a really big one)
Sorry no logical steps, just sharing some experiences.