Shifting problems

[mrdirtbikerider]

My son has a 83 honda cr 80 and he said the shifting isnt smooth any more. Also will not go into fourth gear. What could cause this? :(

 

[QKENUF4U]

have ya changed the oil lately ?
is it rough shifting in all gears ?
if its not going into gear then that would indicate a problem inside the trans (shift fork,etc.)