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I got a razor mx350 given to me by a friend said it did not work. I have changed batteries on it now since old ones where done an swollen. It still does not work when I wire 24v to battery direct to motor it works tire spins fast and fine. But when I have all harnesses connected and flip power switch on I hear a click and the throttle grip lights up shows full and 24V on display but when I engage throttle nothing happens. Could it be the speed control box that's gone. I got this for free to fix for my nephew but I don't want to spend a fortune trying to fix it. Do you have replacement parts for this I can order. serial #101250-07-0603CT01493 Thank you for your help

Since the motor runs when it is connected to the battery pack the motor is good, and because the throttle lights show full power the wiring harness, power switch, and fuse are good. If the lights on the throttle do not decrease when the power is on and throttle is engaged then chances are very high that all of the new batteries are good.
From your description the first most likely cause of the bike not running is the controller, and the second is the throttle. If the throttle has not been physically damaged then chances are that it is not the problem and the problem is the controller.
To know with certainty if the controller or throttle is faulty they could be tested using our Throttle Testing Guide.
The serial number on your MX350 indicates that it is a version 7. Here is a guide to test the throttle on version 1-7 and version 8 MX350 dirt bikes.
We sell parts for the Razor MX350 dirt bike on this page: electricscooterparts.com/razormx350.html
Please let us know if you have any questions.