Gearbox bearings? Check obvious first! As that is a big job, well I think it is, I don't own a mini auto so I don't know. Workshop manual (SP manual) says that incorrect gear change speeds whic could desribe whats happening, as it's shifting into 4th are far too low speed. It metions 'governor control rod adjustment' so check that out. Erratic gear changes mean that you would need to replace the governor thing. So have a look into that and seek professional and experienced mini help.
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1964 Morris 850, 1330 Supercharged - 81.8hp atws.
1975 Leyland Mini S 1100S powered - Nice and reliable.
1977 Leyland Mini LS - Project LS-T
