wont run

Hey I just bought a 300ex from my uncle for 700$ in payments I think I got a good deal idk but it only starts when you push it and start it in second it sounds like it wants to turn on with the button but doesn’t only turns on with the button when it’s warmed up any suggestions?

It’s an 2002 trx 400ex Been bored to 440. Has hmf exhaust it will crank but won’t run. U have to give it 1/4 throttle for it to run but then it’s loud and the timing is right every single time. I’ve changed ever electrical part cam new battery changed the fly wheel took the head off checked the...