I bought this for cheap off craigslist. If I drive it (160lbs), it barely moves at all -- with a slight downhill I can crawl forward. If my kid drives it (40lbs), it moves OK, still kinda slow (bit better than walking pace?) and nothing like the youtube videos.
I measured the batteries at 23v when under full pedal, though they're probably original. What should I buy, upgrade, or replace to give this thing more oomphf?
With the battery pack Voltage being 23 Volts at full throttle that indicates that the battery pack is dropping too much Voltage while under load. A good battery pack will be at around 26 Volts with no load and only drop down to 25 or 24 Volts under load.
The controller will go into Low Voltage Cutoff (LVC) mode and shut down the motor when the battery pack reaches a Voltage level of 21.5 to 22 Volts, so when the battery pack is at 23 Volts the battery pack is almost completely discharged and the motor will run slowly and without very much power.
As long as the charging system is working properly which can be determined by observing the battery charger and seeing that its light is red at the beginning of a charge cycle and then turns green at the end of the charging cycle then the battery pack can be presumed to be old, worn out, or faulty if its Voltage drop to 23 Volts under load and replacing the battery pack will restore the speed and power that the Crazy Cart had when it was new.
We carry battery packs along with many other parts for the Crazy Cart on our Razor Crazy Cart Parts Page.