Re: force 125 motor kicking
1987?
Does it have the two black ignition modules?
On rare occasion one of those modules would go bad causing one cylinder to 'double fire', causing it to lean sneeze, and melt that piston.
Test ignition timing at an idle with an inductive timing light, on one cylinder at a time. You should see the timing mark in only one place around the flywheel per cyl.
Think of the flywheel as a clock face, #1 timing mark as 12 o'clock. A cyl fires at 9 another at 12 and the last at 3. If the module is bad you will find one cyl firing at two places around the flywheel, causing a lean miss, melting that piston.