Refurbishing a boat. We took it out last weekend and rode for 2-3 hours. Now when I try to start it in the water, it starts, but shuts off when put into gear. Runs in gear in the front yard, but spits, sputters and backfires, then winds out on its own. Changed spark plugs, cleaned carb, cleaned fuel filter. Using 50/1 ratio gas to oil. Sounds like timing is off. I know it's 24 degrees TDC, but don't know how to adjust. Any ideas?