Have a 2000 polaris scrambler 500 and only backfires and pops in reverse. drive has full powrer and starts great

gee is it a
2001 Polaris Scrambler 500 4x4 atv real if yes ask maker for that Polaris.
here
USA site, here, so if not USA change to your secret country

https://atv.polaris.com/en-us/sportsman-xp-1000-s/

5 years standing is a long time. Carburetor float will be stuck. Pull the plugs and see if they are wet and also if you have a spark.

Strip and clean carb and jets.
Check fuel tap and fuel delivery.
Drain any fuel and replace with new.
Replace the spark plug.
No lights is odd. Check if there are fuses?

Manual

https://www.google.com/search?q=2000+polaris+scrambler+500+service+manual+pdf

Parts and Parts Schematics
https://www.polarispartshouse.com/oemparts/l/pol/50343730f870023468a741cc/2000-scrambler-500-a00bg50aa-parts

..

prob have to take it off.

see if you can turn it over by hand
could be seized

If it is running but backfiring out the carb, I'd be looking at a burned intake valve.
If it's not nunning but backfires out the carb when you try to start it, I'd be looking at bad ignition or valve timing. I'd suggest that you review any recent maintenance to see if that could be the source of the problem.

restricted water flow around the engine flush it out with a hose pipe

carb needs tuning

If a tune up doesn't help, rebuild the carb. the ethanol in gas ruins the gaskets.

if your four wheeler is back firing you may have water in your gas or your gap on the plug