The spring that returns the shifter back to rest position gets weak from engine heat and use. I could %!ยค into first gear but soon as the motor warmed up it would not shift into 1st or 2nd gear. If the spring is weak it will not put the shift pawl on the pin drum in the correct position and the transmission will not shift. This part is very easy to replace, remove the shift lever cover the return spring is mounted on the segmented gear on the right. I see the part on Ebay all the time.