G'day folks,
The factory Briggs Repair Manuals are also free to download for ODK subscribing members; they're located
HEREWith a sidevalve Briggs that displays the symptoms that one does, first item to check after the choke, is the valve tappet clearances. Very common on these, for the exhaust valve to lose clearance due to wear.
If you don't have access to a valve grinding machine, it's quite possible to remove metal from the valve's stem end by careful hand lapping.