Detonation is caused by either to much timing or cheap crappy low octane gas in most cases. If the belt was just done they might have put it 1 tooth off if they don't know mit's. Most of the time they line up the cam pulley with the edge of deck, there is a small mark just below that that is your timing mark.Maybe one of the boy's will post a pic of where the marks are so you can double check as I don't have the 4 cyl in mine.