Issue with classic mini 1275 carb automatic

[ashmatee]

Hi I have a 1275 carb automatic classic mini engine. My gearbox is struggling to change gears smoothly when driving. When accelerating and it’s ready to change gear I have to come off the accelerator for it to then slip into gear. Where as before it use to change gear automatically and smoothly. I feel it will need the gearbox reconditioned. Any ideas if it’s sounds like is needed?

Thanks
Ash

[ashmatee]

Also did oil and filter change 3 months ago with Castrol 15043F Power 1 Engine Oil 10W-40 4T

And has made it worse