hi looking for help, cant get 4th or 5th gear on my sherpa 91 model, will put into neutral go down to first ok back up 2nd and 3rd ok but lever wont get back any more sometimes goes into neutral where 4th should be really stumped also can put into 1st and 2nd not using clutch, changed gear oil no help, thinking may have to take casing off will i need any special tools and what to look for when casing off, also finding neutral is a nightmare kind regards in advance