I have a 1996 kx 250 that will not select 4th gear going up or down the gearbox but selects other gears fine what could be the problem

I am sorry to tell that the gearbox need to have some major repair. It is also not advisable to continue driving as the gearbox can breakdown completely leaving you stranded. At least one the cluster drive gears in the gearbox is damaged (gear tooth stripped off) that will result in that only some of the gears being able to be selected. The broken off tooth are still in the gearbox and due to the oil movement in the gearbox it can be "washed up " in between to engaged gears destroying those as well. This will happen up to the point where there are no gear selection at all.

This button only makes the gearbox change from 3rd to 4th or 4th to 3rd, will not down shift in any other gear, make sure it is in 4th gear, usually above 60km, than press o/d off it should go into 3rd gear, if it does not than see a automatic specialist, it is usually a sensor, if you go to Toyota they will charge you a fair bit.

a gear box place should know more than me but this can also be caused bye a little ball bearing and spring .... these 2 things push on the selecter shafts and fit in to a groove that holds the shaft in the right spot .. that is what u feel when u change gear and it stays in position ....... the spring - springs can looz tention .. brake or the ball can wear or the groov it sits in to can do the same making it not hold in position ... 2nd and 4th are prob on the same shaft and this problem useully happens under load or taking off quick ........ i think they call them retainer springs and ball

sounds like syncro gear is out of it.

pull your clutch in on the left handlebar push the shifter lever down and let you clutch out slowly while giveing the bike a lil bit of gas. But if you don't know how to ride i wouldn't be jumping on a kx 250

it should be the oil check level first most likely is that

syncronizer for 4 gear total damaged it happens you need to open gearbox and change the syncro bearing for 4 th not very expensive dont change the gearbox

You will need to check the gear linkage hasits free movement and the gearbox mounting is secure. This happen on a Polo recently and the securing bolt to the gearbox mounting had fallen off leaving the gearbox hanging. I hope this helps you.

Regards

Ray