When changing your gearbox oil drain the overdrive unit and clean the magnetic filter element under the rectangular sump cover. Check the fine wire mesh filter for drainage and fit a new gasket cover before replacing. Unscrew the square cover and withdraw the solenoid and check the ‘O’ rings are in good condition. Check the wiring for corrosion especially the connections.
Problem with a delayed up shift to 3rd gear?
First check that the oil level is correct.
Without getting too technical on the functions within the valve block assembly, the delay of the second to third up shift could merely be rectified by re-adjustment of the downshift cable. The correct method of adjustment is by using pressure test equipment, but without this it can still be achieved by a small manual adjustment each time to the cable to check the change speeds.
The cable is connected to the throttle linkage, with the other end connected to the downshift valve in the valve block via mechanical cam, and the position of the cam will alter the speed at which the gearbox changes gear, ie 1-2 up shift 2 –3 up shift with ‘D’ selected. The table below gives the correct change speeds that should be achieved, and can be adjusted by means of the downshift cable.
You can either check the change speeds with ‘light’ throttle or ‘full’ throttle, it depends how brave you are.
Source: MGB Workshop Manual/Service School Notes