1978 Triumph Bonneville

1978 Triumph Bonneville Background & History

The 1978 Triumph Bonneville got a late-year update by Triumph. After several years of little or no change, new U.S. emissions legislations affecting motorcycles was forcing the struggling Meriden Co-op to finally make some changes, although not the changes they would have liked to make, to be sure. The 1978 model year started with Engine #HX00100, with the standard T140V 1978 Triumph Bonneville line continuing into 1978 pretty much as they had in 1977. However, on January 1, 1978, production began on its replacement: The T140E. The “E” stood for “Environmental” and meant that the once-mighty fire-breathing Triumph Bonneville was becoming a “smog dog”.

1978 Triumph Bonneville T140E Modifications

All engine changes seemed to be aimed at meeting ever more stringent US emissions guidelines, not performance anymore.  In fact performance suffered considerably.  But so did the engine’s road manners.  They didn’t produce smooth power, they stumbled off the lines sometimes, and when cruising you could get into a funny place in the RPM range where it got weak (mostly due to the Amal Mk II’s). The compression ratio had been dropped to 7.9:1 in the US to match the UK and export markets, which helped emissions and standardized production.  Milder cams and the new Amal Mk II carbs simply added to the problems.  At 49 horsepower the T140V was already underpowered compared to the competition (now all from Japan).  Now with these added power-robbing modifications, the T140E was becoming downright wimpy.

Note how the carbs run straight back, in parallel, rather than splayed out at aggressive angles as before.  This is the mark of a T140E.

New Head for the 1978 Triumph Bonneville T140E

While the rest of the world was going metric, Triumph was still converting the engine threads over from Whitworth to American standard UNF threads.  And the ancient twin finally got a composite head gasket in place of the prehistoric copper one. But the biggest change was just north of it. A new cylinder head was cast which now placed the intake ports in parallel paths aiming straight back, ending forever that time-honored Triumph Bonneville tradition of splayed carburetors. The new head also had revised combustion chambers and reworked valve guides and seats.  All of this was meant to clean up tail pipe emissions, but all of it hurt performance.

1978 Triumph Bonneville gets Amal MkII's

Again, emissions-driven, a new set of carburetors was installed, the 30mm Amal Concentric MkII, replacing the old standard Amal Concentric the Mk I. These new carbs were boxy-looking on the outside, but besides the elimination of the tickler valve, very little was different inside. An enrichening lever was located on the left carb that operated both units via a common linkage. The whole point was satisfying the new EPA rules forbidding the escape of any fuel or oil vapor to the outside air. The old round-bodied Amal Concentrics (now called Mk I) vented their float chambers to the atmosphere via the tickler valves, which themselves were messy and had to go.  What a shame.  I used to love tickling the carbs before kicking over my Triumph.

Other Changes to the 1978 Triumph Bonneville

Further, the crankcase and oil tank venting had to be rearranged, as they were ultimately venting out the back via a hose running along the back fender. The T140E addressed this by venting the oil tank to the rocker box, and the engine, breathging through the primary case, was vented to the airbox.

Sealed wheel bearings replaced the exposed ones that kept failing, and spokes were upgraded to 9-gauge (apparently spokes had begun braking after the introduction of disk brakes with their increased stopping power). New improved fork seals solved the wear problem and could be retrofitted to earlier Oil-in-Frame twins.

This is the UK home market version of the 1978 Triumph Bonneville.  The main differences from the US-spec bikes were the fuel tank and the low handlebars.

1978 Triumph Bonneville Paint & Trim

The Jubilee seat from 1977 had become the standard Triumph motorcycle seat, sans upholstery in wild colors. It could now be finished in all black or even brown, dependent upon paint colors. The top surface’s upholstered cross-ribbing now extended partway down the sides of the seat. The nose was narrow for use with slimline gas tanks used in the US.

US machines retained their chromed fenders front and rear, but UK machines had painted fenders. The sidecovers were new & completely reshaped. They were painted to match the tank on UK bikes, but were black on US versions.

Colors for 1978 were more varied than ever before. US bikes could be ordered with 3 different color combos: Tawny Brown over Gold (pictured here); Astral Blue over Gold; and Black over Crimson. UK models got 2 choices: Tawny Brown over Gold; and Aquamarine over Silver.

1978 Triumph Bonneville Specifications

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1978 Triumph Bonneville Books

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More Triumph Bonneville Pages

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1960 T120 Bonneville

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1965 T120 Bonneville

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1969 T120 Bonneville

1970 T120 Bonneville

1971 T120 Bonneville

1972 T120 Bonneville

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1975 T140 Bonneville

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1977 T140J Silver Jubilee

1978 T140 Bonneville

1979 T140 Bonneville

1980 T140 Bonneville

1981 T140 Bonneville

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1983 TSX Bonneville

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