I have a Yamaha bear tracker 250. It has a new carb, stator, and battery on it. What it is doing is in any gear but neutral it will start sputtering, lose power, and slow down. We cannot figure it out. It starts and idles great and even sounds good in neutral but when it is under a load it has this problem. We’ve adjusted the valves and tried doing small things to fix it but it will not run right. Can anyone help?