What exactly is a 1978 Triumph Bonneville?

By Dave Porter, Technical and Customer Support at The Bonneville Shop

History of the 1978 Triumph Bonneville

As any experienced Triumph parts expert will tell you, the 1978 Triumph Bonneville model year presents some challenges when making recommendations for suitable replacement parts for the Triumph Bonneville T140 models. To help understand why this is, a historical account of events leading up to the manufacture of the 1978 Triumph Bonneville is in order.

When the 1978 T140V production commenced with VIN number HX00100, there were great expectations for US sales of the 1978 Triumph Bonneville models. After the formation of the Meriden Co-operative and the rebirth of the Triumph Bonneville T140 models in 1976, it seemed like the good times were back at Meriden, with full-scale production resumed. The Co-op was set to take over the marketing efforts from Norton Villiers Triumph in 1977, once the contract with NVT expired.

1976 Models

The new T140V models were well received in the US, which was their primary market. Disc brakes front and rear, left-side shift, and favorable magazine reviews contributed to a reasonably successful selling season for the 1976 T140V and TR7RV, although priced at $1895, the Bonnevilles were perceived as expensive for a somewhat dated design. Meanwhile, the US dealer organization, Norton-Triumph Inc. was price dumping the unsold stocks of Tridents and Commandos in an effort to clear showrooms of the 1975 models not called Bonneville.

1977 Models

The 1977 models were essentially a carryover model from the previous year, with some minor changes in the front mudguard mounting and a new optional Polychromatic Blue paint scheme. All told, the Co-op managed to produce around 20,000 units for the 1976-1977 model year run, despite unfavorable changes to dealer’s terms and the continuing cash flow problems for NTI.

The newest US subsidiary for the Meriden-built Triumph Motorcycles Ltd. was now called Triumph Motorcycles America (TMA) and was headquartered in Placentia, Ca. in July 1977. Both the manufacturing and distribution of Triumph motorcycles had endured a long state of flux from the early days of the Umberslade Hall design center, to the collapse of the BSA group, the worker’s blockade, and finally to the period of the re-emergence of the Bonneville. It is no surprise that the 1978 Triumph Bonneville T140 model followed in this fluid manner.

With most of the leftover 1977 models sold off and a newly appointed TMA president, Jack Hawthorne, (a former executive at American Honda), it was time to swing for the fences at the Co-op in Meriden. TMA established a convenient “800” toll-free phone number for dealer orders and consumer inquiries, and new marketing practices were put into place, despite the restricted budgets for advertising.

The exciting new color schemes of the otherwise largely unchanged T140V were showcased in various magazines, and under Hawthorne’s guidance there was a period of prosperity, albeit short-lived. A dock strike in October 1977 on the East Coast contributed to a delayed shipment of the new 1978 models, and a weak US dollar meant the profit margins for the Meriden Co-op were slim. Despite the market conditions, nearly 12,000 units were built, and US dealers often listed the T140s at prices above the MSRP.

There were some new specification upgrades to the 1978 T140V: an eyeletted composite cylinder head gasket, 7.9:1 compression ratio pistons, rocker adjuster screws with UNF threads, an outer gearbox cover gasket, heavier 9 gauge rear wheel spokes, and “leak-proof” fork oil seals.

Stylistically, the new side panels and wider Dunlop K81 tires (US market), in addition to the bold paint schemes, easily distinguished the new Bonneville from the previous 1976-1977 models. As a sidenote, one of the new finishes, Tawny Brown and Gold was met with limited enthusiasm in the US, and some 480 units were sent back to the UK by TMA. Today, the chocolate brown/gold finished T140s are often referred to as the “UPS” models. Production of the 1978 T140V models commenced in July 1977 and continued through December of that year.

1978 Triumph Bonneville Emerges

The models that were produced from early January 1978 were known as the T140E, dubbed the 1978.5 models. The US Environmental Protection Agency mandated new emissions guidelines that required compliance for all motorcycles imported into the US manufactured after January 1, 1978. This directive had a profound effect not only on the performance of the T140, but on the continuous financial strain of Triumph. Brian Jones and Jock Copeland designed a new parallel-port cylinder head that was to be fitted with Amal Mk2 carburetors, and a revised engine breathing system that could meet the EPA requirements.

The Mk2 carburetors eliminated the “tickler”, formerly used on the Amal Mk1 Concentrics to flood the float bowls and thus enrichen the mixture for cold starts. The Mk2s used a cold start plunger system to enrichen the mixture, which obviated the need to introduce petrol into the atmosphere by spilling it out of the tickler valve on the Mk1 carburetors. The revised crankcase breather was now routed into the air box on the T140E, rather than having the crankcase gases routed to atmosphere through a hose along the rear mudguard.

There were other revisions to the T140E models, such as a halogen headlamp bulb and new glass reflector, louder Lucas 6H horn, inverted gas-filled Girling rear shocks, and new seat upholstery. The Smith’s instruments were also replaced later in the model year with French-made Veglia clocks, due to a supply shortage with Smith’s.

The majority of the 1978 Triumph Bonneville models were supplied to the US market in the T140E condition through the end of the production run in July 1978, at engine number HX10747. There is some conflicting information that exists in David Gaylin’s “Triumph Motorcycles in America” suggesting that the last 1978 T140E was numbered JX10747. The “J” would indicate that the final iteration actually left the factory in August 1978, after the initial 1979 models were in production from HA11001, or July 1978. Could these last few 1978 models be the third version of the 1978 T140 Bonneville?

1978.75 T140E

Although I have never personally seen one of these alleged 1978.75 T140E models, there have been suggestions that these models exist, with the center-mounted ignition switch console, neutral warning light, negative-ground electrical system, Lucas Rita ignition, Lucas RM24 three-phase stator, new black Lucas 33708 and 33709 handlebar switchgear, and the revised instrument binnacles specified for the 1979 Bonneville models.

There is little to no information (that I could find) regarding the production totals of the mysterious 1978 “E” models partially built to the 1979 specifications, but the latest T140E VIN number I have confirmed with the original T140E configuration is EX07997, or May 1978, about 2750 units before the end of the 1978 model year production. We know that production of the 1978 T140 models ran through July, and perhaps, as Gaylin suggests, into August 1978, so there could have been several hundred of these mysterious 1978.75 models produced.

Sourcing Parts

So, why is this important, we all know that running changes to specifications during a given model year was hardly a secret in the realm of British motorcycle manufacture, especially during the 1970s, right? The reason for this article is to caution owners of the 1978 Bonneville models to be specific when sourcing replacement parts. The parts books are designated as follows: 1978 T140V 99-7003, 1978 T140E Supplement TMA-9, and 1979 T140E 99-7102.

Having your engine and frame numbers available when ordering spares will be of immense help in getting the right parts the first time around. One last comment is that the EPA mandates didn’t affect the 1978 TR7RV models, as they weren’t exported to the US after January 1978, due to the incompatibility of the MK2 carburetors with the cylinder head and oil-bearing frames. They remained in production but were designated home-market units. As such, they are rare in the US.

Thanks for reading,

Dave

---

1978 Triumph Bonneville FAQs