What stops my engine starting - Razor USA Razor Dirt Quad Electric ATV

define "stopping". If it involved using the new ignition switch, then I think a return to the dealer is necessary. You know it takes 3 times for them to get it right?

not familiar to your set up but as with any system encluding motor cycle. Get a hose and syphone all that you can out and fill with the proper gas. Start it up...it might be a little harder to start but once you get it started it will clear. I once had a guy fill my car with diesel...supposedly gas and I drove the car a long way then kept stopping up with gas. worked fine....sure did clean the engine.

I assume the motor controller PCB would be the problem.

you answered your own question

Once an engine seizes, it must be replaced or overhauled. Full stop.

You might need a new thermostat, the temp switch may also have to be replaced.

check oil level, there is low oil sensor in case, when stopped all oil runs back down into case this alows it to start then it starts pumping the oil throughout and results in low oil shutting down engine (safety sensor) your probly down a liter or so

Overheating maybe, engine timing

The problem would appear to either be with the mixture or with the head. Too rich a mixture (too much fuel) will cause black plugs - when you take them out do the engine and plugs smell of petrol? You need to adjust the carburettor. Mark where the throttle screw is to start with and then slowly screw it (in - I think) to reduce the petrol flow.
The other alternative is that their is a problem in the head. Does the engine blow a lot of blue/black smoke out when it is hot. If yes it means a full engine strip down!