Lawn Mower wont start

You need to obtain a manual for Briggs & Stratten engines. All the info you need will be in it. The most common cause for non starting in these engines is "Tight Tappets" Wether it's an OHV or the old "L" head engine, because they rev and work so hard, the valves seat themselves deeper into their seat which reduces the valve clearance. There is no adjustment on the older "L" engines, so you have to remove metal from either the valve stem, ot the lifter itself. I have in the past done this in situ with a point file, but it is a tedious job. Otherwise you must remove the head and take out the valves and grind the stems. A tricky task and probably should be done by a mechanic.