+1 on the battery charged. Most of the bike batteries you have to trickle charge them overnight before they have a full charge. If you didnt do this youd have no lights etc. Kinda ruling out all for you not trying to be ignorant. Dont be fooled as well. Ive bought several batteries and have atleast 2 out of 9 that were bad. Granted from advance and autozone however the other ones bought there Im going on 4 years now on couple of them.
Make sure your positive and negative leads to battery are ok and not corroded up. Even tho you hooked them up they still cannot be making a good contact. Wire can also corrode up the wire under the insulation creating a barrier between the copper and end for battery. The end could look just fine too. Take a good look at them. I had this on my quad and it drove me nuts for about a month. Tore everything apart tons only to find later I wasnt even making contact to battery on positive lead. Once I changed the ends started right up.
Look at your wiring harness and see if you have any fuzes inline. I have one on my positive that went on me a couple of times. Pinched wire between plastic and frame. I carry a couple spare in a baggie taped and ziptied to almost where the fuze is.
Once you eliminate these steps and if still not moving I would look at the key switch itself. See if you cant get a wiring diagram for the quad and test the leads. You can pick up a cheap tester to see if you got juice going through the switch. Could have a bad switch. With the diagram you would be able to bypass the switch.
I dont think killswitch plays a factor here unless pushed in or damaged but you still should be able to atleast turn key on and have headlight. Killswitch on like mine just stalls the bike but my headlight stays on. Cant remember for bayou. Been a long while lol.
Most quads have similar setups and this is just a general looksee but should be able to get you atleast with power.
If your unaware if bottom of carb was cleaned and whatnot just dont run the bike and pop the float bowl off and clean er out before trying to crank er up. Atleast this way you can check the float and whatnot while your doing this put a little gas in tank and let it run through and out carb (grab a jar to catch gas). Check fuel filter if any and make sure you have gas flowing and you can gently manipulate the float up and down and it shuts off gas and lets it flow. Be careful of the gasket could try to reuse however Im sure you can order one or find one. Hopefully the bike was ran empty.
I have confidence youll get it going if nothing major is wrong in engine. Really just sounds like you have something small goofing with ya. Take the plastics off and make sure you have good connections and grounds. A little time saves a huge headaches. Trust me I rewired my bike only to find the corroded wire and end. I did look good at it just not good enough. Taking the plastics off just makes everything a helluva lot easier and if you get er running you can go over everything with ease just cant ride it till ya get the seat back on.
This is a general quad fixer but most quads are same in concept just a little different in design.
If its the switch I guess Ill have to show you how to hotwire a quad. In my case a toggle switch hahahahaa
Did the bike sit outside for some time?