The J-type's engine traced a lineage through the MG C-type back to the overhead-camshaft unit first used in the 1928 Morris Minor and Wolseley 10, later refined for the MG M-type Midget from 1929. Drive was taken through a four-speed non-synchromesh gearbox. The chassis came from the D-type, with suspension by half-elliptic springs and Hartford friction shock absorbers at all four corners, with rigid front and rear axles. Wheelbase was 86 inches (2,184 mm) and track 42 inches (1,067 mm). Open two-seater bodywork was standard for most variants, distinguishable from the earlier M-type by cut-away door tops; a closed salonette version of the J1 was also produced, and some rolling chassis were supplied to external coachbuilders.
The J1 was the four-seat variant in the range, using the 847 cc unit previously seen in the C-type, fitted with twin SU carburettors producing 36 bhp. The open four-seater was priced at £220, with the closed Salonette version at £225.
The J2 was the commonest car in the range — a road-going two-seater priced at £199. Early J2s were fitted with cycle wings; from 1933 these were replaced by the full-length wings that would characterize all sporting MGs through to the 1950s TF. A standard car achieved a top speed of 65 mph (105 km/h), though a specially prepared example tested by The Autocar reached 82 mph (132 km/h).
The J2 carried two notable technical weaknesses. Its two-bearing crankshaft was liable to fracture if the engine was over-revved. The overhead camshaft was driven by a vertical shaft through bevel gears, and this shaft also formed the armature of the dynamo; any oil leak from the cambox seal therefore entered the dynamo brushgear, creating a fire hazard. Braking was by Bowden cable to each drum rather than hydraulic actuation, and the drums themselves were considered small even by contemporary standards, making it a common period modification to fit larger drums from later MG models.
The J3 was a racing derivative developed specifically for 750 cc class competition. Engine capacity was reduced from 847 cc to 746 cc by shortening the stroke from 83 mm to 73 mm. A Powerplus supercharger was fitted, and the larger brakes from the L-type replaced the standard drums.
The J4 was a pure racing car using the J3 engine specification but with higher supercharger boost, raising output to 72 bhp (54 kW). Lightweight bodywork reduced overall weight relative to the road cars. The J4 was built expressly for circuit racing rather than road use.
The J-type occupied MG's competition and sporting catalogue during the critical early 1930s period when small-capacity British sports car racing was developing rapidly. The supercharged J3 and J4 variants allowed MG to contest the 750 cc racing class with purpose-built machinery rather than modified road cars. The line was succeeded by the later P-type and other Midget derivatives as MG continued to develop its competition programme through the decade.