My 110cc Chinese quad will only crank and start if I jump the starter and I changed everything but the electric start.

 

My Kawasaki will only start if I jump it from starter too and I just changed the solenoid and the battery.

 

Starter circuits usually have a lock-out so you can't start them in gear and/or without the brakes on. The most common problem is the start button not connecting properly, next a faulty solenoid, but new owners not knowing to have the bike in neutral with the brakes on is another common problem. To check the solenoid if it is not clicking when you press the button, you get an incandescent 12v test lamp and put it across the two thin wires going to solenoid. Switch on, lock-outs on, press start button and, if lamp lights, you must have a faulty solenoid. If it doesn't, one wire should be pos and the other neg with start button pressed, so test for which has no power to it and trace that one back, to find the fault.