Why is my atv tranmission in reverse when the shifter is in forward.

Service manuals show 15 mentions of relay

https://www.google.com/search?q=2000+yamaha+kodiak+400+4x4+service+manual

..
https://www.google.com/search?q=2000+Yamaha+Kodiak+400+4x4+relay+locations

..The video says it tests all 3

get a colortune tool.it shows how your firing.read instructions that come with,so simple to tune carb in with.sounds like pilot out of adjustment.could also have a leak in ur carb boot or head.take a torch without lighting open valve with motor running run torch around gaskets and front of carb boot listen for change in motor.if it gets higher rev uve found a leak.

probably needs adjustment of shift cable.

short in wiring or loose wire at a connector. check and repair any found

loose or pinched a wire under plastic

look for loose wires and check the resistance of the plug cap

I take help from online websites that have this service like helping in repair some like reliable-store have instant help.. others are a bit slow which I don't prefer... rest is up to u.. U can also get free manual works fine .

your carb needs cleaned/adjusted. your float could be set wrong. could be an air leak somewhere.

Check to see if one of the shifter rods behind the side pannel are bent.this usually happens due to lack of lube end people try to force it into gear.

as soon as your bike begins to high idle it will not fall into gear you probably will need to set your linkage on your shifte