The final experiment with performance enhancement began in October 1933, when No. 1850 had its Walschaerts valve gear replaced with J.T. Marshall valve gear at Eastleigh Works. The engine was trialled on the Western section, where the gear showed promise at slower speeds, with reduced consumption of coal and water. Problems were encountered at speeds over , at which a severe "knocking" sound was reported by the footplate crew. When trialled on a Basingstoke–Waterloo semi-fast (a high-speed passenger train that stops at selected intermediate stations), the valve gear disintegrated near . After immediate withdrawal from traffic, the locomotive had its Walschaerts valve gear re-fitted and No. 1850 re-entered traffic in April 1934.