finally gave way to a single twin-choke Solex 32PAIA in the interests of serviceability. The effect of the new carburettor was to increase power to and torque to at 3,500 rpm. In October 1964, along with the rest of the light car range, the Series IV received the new Rootes all synchromesh gearbox, a change which coincided with the introduction of a new computerised chassis numbering system. The Motor road test of April 1964 gave the Series IV Rapier's maximum speed as and its 0- time as 17 seconds. When production of the Series IV ceased in 1965, 9700 units had been built. Series V Pending completion of the new Fastback Rapier, Rootes decided to have one more go at updating the Sunbeam Rapier. In September 1965 they introduced the Series V version which looked exactly like the Series IV inside and out except for badges on wings and boot which now said "1725", revealing a re-developed engine. Rootes redesigned the Rapier's four-cylinder engine to increase the capacity, with a new five main bearing crankshaft, making the unit stronger and smoother. This engine would be developed for many subsequent models. In the Series V Rapier the engine developed at 5,500 rpm. To further update the car, they changed its polarity from positive to negative earth and fitted an alternator in place of the dynamo. They also devised a new twin pipe exhaust system so that the new engine could breathe more easily. The effect of these changes was to increase the Rapier's maximum speed to and reduce its time from rest to to 14.1 seconds. However, for all its improvements, the Series V just did not sell. By the time it was discontinued in June 1967, only 3,759 units had been built, making it the rarest of all the "Series" Sunbeam Rapiers. Sunbeam Rapier Fastback coupé By 1967 Rootes' "Arrow" range was ready. As well as the Hillman Hunter, the range also included a new generation of Sunbeam Rapiers, with fastback coupé bodies and a sporty image. Like the earlier Series I to Series V models, it was a two-door pillarless hardtop. The Arrow Rapier – or Fastback, as it came to be known – launched in October 1967, was a four-seat coupé based on the chassis of the Hillman Hunter Estate. Although the Rapier used the tail lamps and rear valance from the Hunter Estate, the rest of its