1978 Triumph T140V Bonneville
With an experienced team and a lot of data.
Protect your 1978 Triumph T140V Bonneville from the unexpected.
To most Americans, the Triumph Bonneville is one of the very first British bikes to come to mind thanks in large part to Triumph’s record-setting runs on the Utah Salt Flats as well as its embrace by Hollywood actors. At Bonneville in 1956, Joe Allen took a Triumph to 214 mph. Joe Dudk then upped it to 224.57 mph in 1962, and Bob Leppans did 245.57 in 1966 with the Gyronaut X1 powered by two 650 cc Triumph motors. Movie rebels like Marlon Brando in “The Wild One”, Warren Beatty in “Shampoo” and James Dean also all rode Triumphs.
The Bonneville’s success in the U.S. was actually a bit of a mystery to the folks at Triumph. Vincents, Velocettes and Royal Enfields were faster, and Nortons handled and stopped better. Its tangerine and gray colors at its launch in 1959, meanwhile, were more suburban kitchen than rebel bike gang.
To the Triumph’s credit, however, the Bonneville weigh one third less than a Harley, offered reasonable reliability and sold in sufficient numbers that spares and service were easy to come by. The Bonneville’s bright minimalist style was instantly recognizable, as was its crackling exhaust.
The first Bonneville model was dubbed the T120, which looked quite similar to the 1938 Speed Twin it was derived from. It was powered by a 649 cc twin making 46 bhp, and featured the twin Amal monobloc carburetors from the Tiger T110 as standard. The 1959 model was based around an older frame design that could induce wobbling as the bike approached its top speed of 115 mph, but it was replaced by an improved duplex frame in 1960 for a more stable platform.
From 1963 the engine and transmission were combined into one unit, at the expense of more vibration transmitted to the rider. 12-volt electrics arrived in 1964 and further frame improvements followed in 1968, along with 30 mm Amal concentric carburetors and a new ignition system.
In, 1971 the Bonneville got an oil-filled frame instead of a separate tank. These frames have advantages, but can prove problematic if an engine failure scatters pieces throughout it. By the time the last 649 cc T120 was sold in 1972, production had reached 250,000 units.
The second Bonneville model, the 744 cc T140 debuted in 1973, but these bikes were plagued by quality issues immediately. Triumph’s financial difficulties didn’t help matters, nor did an 18-month sit-in at the Meriden factory. T140s were exported steadily to the U.S. until 1983, and while they’re not as desirable as the earlier T120, they are easier to find in sound and original condition.
The T140 was also equipped with a 5-speed gearbox and came with disc brakes from 1976, the same year the gearshift moved to the left side to comply with US regulations. Electric start was available from 1980 and the final TSS model boasted a rubber-mounted 60 bhp engine with alloy wheels as an option. Production ended in 1983, along with the rest of Triumph’s motorcycle production.
Originality is key with these old Bonnevilles. Ratty, running examples can be had for cheap, but restoring one correctly is difficult. Piston slap is acceptable at idle, but there should be no smoke, and it’s wise to check the valve guides by opening the throttle at 50 mph and watching out for smoke. This is especially important on T140s, which suffer premature top-end wear. The rear disc brake on 1970s T140s is also susceptible to road grunge, but it was relocated in 1980 and significantly improved the problem.
More than 65 percent of Bonneville production was sold in the U.S., so today collectors in England are actually willing to pay big money for correct examples – particularly the once unappreciated tangerine and pearl gray 1959 model.
Triumph released a new version in 2001, and U.S. buyers have welcomed John Bloor’s re-imagined Bonneville with open arms. Though the new twins are about 100 lbs heavier than the originals, the recognizably iconic profile has kept the name alive.