Small Engine Troubleshooting Problems? Ask a Mechanic Now.
Is the battery charging system working?
Do you have a voltmeter to test with?
Test battery voltage before start, should be above 12,5 volts, then check again with engine running. Voltage has to increase at least one volt , around 13.5 volts.
You say new engine? Maybe the voltage regulator output is not connected to the battery positive,