Open circuit (no load) voltage of a fully charged 6V battery should be around 6.3 volts, 2.1V per cell. Try monitoring the voltage at the battery posts while someone else pushes the starter pedal. If it drops below about 4.5 volts under load, you've either got a discharged battery or one that's got an open circuit inside one of the cells and must be replaced. A good 6V battery should be able to crank the engine at normal starting speed for at least 15 seconds without the terminal voltage dropping below 4.5.
Good luck!
Jerry

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