A technical overview and history of the J1
The J1 was the four seat car in the range. The engine was the 847 cc unit previously seen in the C-Type with twin SU carburettors giving 36 bhp. The car cost £220 in open and £225 in Salonette form.
The J2 was the commonest car in the range and was a road going 2 seater. Early cars had cycle wings but these were replaced in 1933 by the full length type that was typical of all sports MGs up to the 1950’s TF. The top speed of a standard car was 65 mph but a specially prepared one tested by The Autocar magazine reached 82 mph. The car cost £199. A couple of serious failings of a J2, was that it only had a two bearing crank shaft which, due to metal fatigue and stress, was almost certain to break at some stage and which could, if at speed, basically destroy the engine block. Its other problem was that it was not fitted with hydraulic brakes as used in cars of today, but had Bowden cables to each drum. Naturally this is well before luxuries such as power assisted brakes so in an emergency you literally had to stand on the brake pedal which then ran the risk of breaking one or more cables which made driving and stopping something of an adventure. The non syncromesh gearbox takes some getting used to but with its short gear stick it becomes second nature to double de-clutch and rare to grind the gears.
The J3 was a racing version with the engine capacity reduced to 746 cc by shortening the stroke from 83 to 73 mm and fitted with Powerplus supercharger. The smaller engine capacity was to allow the car to compete in 750 cc class racing events. Larger brakes from the L-Type were fitted.
An overview courtesy of Wikipedia
The MG J-type is a sports car that was produced by MG from 1932 to 1934. This 2-door sports car used an updated version of the overhead camshaft, crossflow engine, used in the 1928 Morris Minor and Wolseley 10 and previously fitted in the MG M-type Midget of 1929 to 1932, driving the rear wheels through a four-speed non-synchromesh gearbox. The chassis was from the D-Type with suspension by half-elliptic springs and Hartford friction shock-absorbers all round with rigid front and rear axles. The car had a wheelbase of 86 in (2,184 mm) and a track of 42 in (1,067 mm). Most cars were open two-seaters, but a closed salonette version of the J1 was also made, and some chassis were supplied to external coachbuilders. The open cars can be distinguished from the M type by having cut-away tops to the doors.
The J1 was the four-seat car in the range. The engine was the 847 cc unit previously seen in the C-type with twin SU carburetors giving 36 bhp. The car cost £220 in open and £225 in Salonette form.
The J2, a road-going two-seater, was the commonest car in the range. Early models had cycle wings, which were replaced in 1933 by the full-length type typical of all sports MGs until the 1950s TF. The top speed of a standard car was 65 mph (105 km/h), but a specially prepared one tested by The Autocar magazine reached 82 mph (132 km/h). The car cost £199 .
The most serious of the J2's technical failings is that it has only a two-bearing crankshaft which can break if over-revved. The overhead camshaft is driven by a vertical shaft through bevel gears, which also forms the armature of the dynamo. Thus any oil leak from the cambox seal goes into the dynamo brushgear, presenting a fire hazard.
Rather than hydraulic brakes the car has Bowden cables to each drum. Although requiring no more pedal force than any other non-power-assisted drum brake if they are well maintained, the drums themselves are small, and even in-period it was a common modification to replace them with larger drums from later models.
The J3 was a racing version with the engine capacity reduced to 746 cc by shortening the stroke from 83 to 73 mm  and fitted with a Powerplus supercharger. The smaller engine capacity was to allow the car to compete in 750 cc class racing events. Larger brakes from the L-type were fitted.
The J4 was a pure racing version with lightweight body work and the J3 engine, but using more boost from the supercharger to obtain 72 bhp.