I have an 86 trx 125 that "jerks" when you ride it. I know it is a Drivetrain issue but I am new to owning and working on four wheelers. My chain kept popping off while riding so I tried to tighten by adjusting, but the chain was still too loose, I tried to test drive after tightening and now it jerks... I was afraid of tearing something up so shut it off and took left side apart. This model has the sub tranny, and found a broken gear there, after inspecting both front and rear sprockets, both have a tooth broken off.....does anyone know if this would be the only cause of my problem, or should I look deeper before I put it back together with new parts?

Also curious how difficult it is to replace the shifter spindle, some idiot welded mine cause the splines were worn off, or is there an easier fix..

Thanks for reading.