well i dont know anything at all about atvs or yamaha but... i assume its a 4 stroke small engine right? 2 cylender? Did it just stop running or has it ever ran for you or did you get it already broken? Well basicly you need to make sure those spark plugs are getting spark. just unscrew one and connect the wire to it and ground it to the engine block and try to start it for a second. if it sparks that one works. you might feel a zap but it wont kill you. do that with all the spark plugs. then do the same thing again and instead of messing with the spark plug put your finger over the hole it was in when you turn it over and make sure you feel the pressure as the piston moves. if it has compression and spark then it is a fuel problem. Now that you have been cranking the engine over a bit testing the other stuff there sould be unburned fuel in the cylender. take a flash light or any bright light really and look in there and see if its dry if you cant tell see if you smell gas. if you dont take every spark plug out spray a shot of starter fluid in each cylender and put the spark plugs back in and try to start it if it starts to run for a second then you know you have a fuel problem for sure.
i assume its fuel injected since you said you dont think its getting injected... so it should have a fuel tank then fuel line to the fuel pump somewhere and then that fuel line should lead to the injectors or some sort of fuel rail on the engine. you can try removing the injectors from the fuel rail or whatever they are connected to. then try starting it for a second see if gas flows out of where it was connected. if it does the injectors are dirty or broke try cleaning them out with carb cleaner if that dosnt work... replace them. if fuel dosnt leak from the fuel rail then you have a crushed fuel line or clogged fuel filter or a bad fuel pump. Just need to narrow it down and check each step one at a time and make sure it all works.
But like i said i know nothing about atvs just basics on how engines work and i assume its basicly the same as a car/motorcycle/lawnmower.
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