The biggest Triumph story for 1978 came not from Meriden, West Midlands, but Washington, D.C., where the Environmental Protection Agency implemented regulations against the escape of fuel and oil vapors into the air. As the result, the Triumph T140V Bonneville’s air-cooled, overhead-valve 744cc parallel twin got a makeover. Engineering director Brian Jones and former road-tester extraordinaire Alistair “Jock” Copeland developed a new cast cylinder head with parallel rather than splayed ports. The intakes were fed by square-bodied 30mm Amal Concentric Mark II carburetors, which mounted on rubber. Doing away with float ticklers, a lever to enrich the mixture was located on the left, operating both carbs thanks to a clever linkage.

The primary case and oil tank also benefitted from updated breathing arrangements. When the revised bike arrived in 1978, the engine’s compression ratio was reduced to 7.9:1. Output fell to 49 horsepower at 6,200 rpm. Aggressiveness had always been a Bonneville characteristic, so the question concerned the new model’s name. While critics thought up pejoratives, Triumph dubbed it the T140E Bonneville 750 (“E” reflected the bike’s emissions-compliant status). Additional changes included the use of sealed wheel bearings and thicker, 9-gauge wheel spokes to better meet the forces generated by the front disc brakes. The side covers were new as well. Chocolate Brown and Gold (with a narrow-nose brown seat) was a surprising color scheme, but two others were offered on the 1978 Triumph T140E Bonneville. Although the Bonnie still had kick-start, many owner note that their bike fires on the first kick.