My son and I have rebuilt our 1986 moto 4 225. Thought it ran great till trying to get it into gear. Will never go into gear after I’ve warmed it up. I can hear the transmission. I just found out that it will go through the gears right after starting it but once it warms up the drive stops turning the wheels. I can find all sorts of troubleshooting for not shifting when it’s cold but not the other way around. Oil has been changed with the proper oil and it’s topped off. Anyone ever see this before?

I found a manual I’m going to print off but I’m not confident I’ll find this one in the trouble shooting section.

Thank you
Tim