mechanical
 

Salut everyone.
Can anyone give me any advice on a problem I am having with my ride-on mower. The motor starts and runs fine, seems to be revving at the same speed as ever, but over the months the mower is going slower and slower. In 4th gear it used to race around, now it almost crawls, no difference if the blades are engaged or not.
I have put a new drive belt , to no effect.

The mower is a MacCulloch 11hp, also sold as a Husqvarna and as a Jonsered, manual gearbox ( not hydrostatic ) .They are all made by Electrolux it seems, just painted different colours.

Please, any suggestions, advice, diagnoses ?

Cordialement