The usual problem is the vibrator. The contacts become dirty of pitted. It can be carefully disassembled and cleaned. The other thing to check is the large 10 ohm potentiometer. They also have a tendency to get dirty and lose contact. Schematics for the Merc-o-Tronics are difficult to come by. The is an older discussion on the Smokestack antique engine forum you may find useful.
