"The average rider of the late
Triumph T140 Bonneville
1979 Triumph T140 Bonneville Specifications
Type: Air-cooled, OHV, twin
Capacity: 744cc (750cc)
Bore & Stroke: 76mm x 82mm
BHP: 44 @ 7000rpm (variously quoted
Compression ratio: 7.9:1
Transmission: 5-speed, multi-plate
Brakes: 10-inch discs front and rear
Electrics: 12-volt, electronic ignition
Front suspension: Telescopic,
Rear suspension: Swinging arm with
Girling shock absorbers/dampers
Wheels: 4.00 x 19-inch front,
Weight: 410lbs (dry)
Maximum speed: 105mph
Triumph T140 & Triumph TR7 models and variants
T140V Bonneville 750cc
T140E Bonneville 750cc
(Police model) 750cc
The unit 750cc T140 engine as
drawn by the late Lawrie Watts.
Triumph's 8-valve T140W TSS.
Click on the image above to find out
Click the image above for details of Sump's essential T140 Bonneville/TR7 Tiger buyers guide available NOW! Or press the GREEN button below to download an eBook showing sample pages.
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Click the link below and take a ride.
Top Triumph T140 Bonneville specialists & links
Triumph Owners Motor Cycle Club
Got the bike? Join the club. Branches nationwide. Knowledgeable people. Big social calendar.
Telephone 0776-555-3258 (Urgent enquiries only, please)
Telephone 01394 279929
British Cycle Supply Company
US and Canada based suppliers of Triumph, BSA and Norton spares. Lots of interesting stuff. These guys ride with pride.
US based British bike parts. Huge range of high quality, high performance precision spares for BSA, Triumph and Norton. Original equipment and custom parts.
IF YOU'RE LOOKING for an all-weather, all-purpose, everyday classic British bike, you’re probably staring straight at a 750cc T140 Triumph Bonneville. A lot of negative criticism has been hurled at these machines, notably citing problems with vibration and oil leaks.
But the fact remains that T140s have excellent handling, good braking, decent enough suspension, an acceptable turn of speed, and have better parts back up than just about any other classic British bike in the market place.
As for the vibes, these never were as bad as some suggested (although a poorly set up Bonnie with an unbalanced crank is a minor torture). And the much-mentioned oil leaks are no worse than those of other British bikes. But because T140s are used more, so the leaks are seen more often. Almost nobody trailers a prized 750cc Bonnie from show to show.
Not yet, anyway.
History and development of the T140 Bonneville
Launched in 1973, the T140 Bonnie picked up where the 650cc T120 Bonnie left off, and the extra 100cc came with a penalty. Edward Turner, designer of the 1937 Triumph Speed Twin (the grandfather of all successive Triumph twins—and the inspiration behind a lot of other twins from rival marques) has been quoted as saying that he never intended his 500cc engine to be enlarged. Certainly to no more than 650cc.
But by the late 1960s, the Americans (in particular) were clamouring for more horses for their track and desert courses, and if Triumph couldn’t provide the 750cc upgrade, then the Yanks would.
Eventually, feeling the pressure of Japanese superbikes eating up showroom floorspace and sales, and hoping to turn US aftermarket 750cc top-end kits into much needed Triumph dollars, the company acquiesced and finally built the long-time-coming 750cc T140 Bonneville and, soon after, 750cc TR7 Tiger.
Triumph oil-in-frame chassis
These new machines were always 5-speed, and were launched with a front disc brake and drum rear and inherited the controversial oil-in-frame chassis trailblazed by the 650cc T120. That frame started out wrong, but ended up right, and has lasted around forty years without significant issues.
Styling for the T140/TR7 was a pretty much orthodox British sit-up-and-beg configuration with a build quality that the Japanese bike industry still hadn't matched. Frames were hand welded. Tanks were hand striped. The overall feel was of a value-for-money, traditional machine, albeit manufactured around a design that could have been justifiably pensioned off in the mid 1960s.
The (nominally) 750cc engine was near identical to the 650cc power unit. But the first T140s were (for production reasons) actually 724cc. Within months however, this became 744cc—which remained the cubic capacity of the Bonnies and Tigers until the bitter end.
Development of the new seven-fifties was, by 1974, slowed by the now infamous industrial unrest at the Meriden factory in Warwickshire that saw production halted for around eighteen months—and, woe of woes, at a time when the Japanese were becoming increasingly aggressive and competitive; a fatal commercial combination that put a large number of nails in the coffin of what was left of the British motorcycle industry. Which, with Norton just about the hit the skids, was really just Triumph.
Rear disc brake and revised cylinder head
By the time the Meriden industrial situation/debacle was resolved, the Triumph Trident (which was in fact the true competition for the T140) was almost dead on its feet, leaving the ageing Bonnie desperately trying to claw back prestige and customers with a package that beginning to show a lot of mechanical wrinkles.
The Triumph engineers, however, were not giving up without a scrap, and the 750cc Bonnie and Tiger was, if not exactly technically brought up to date, then at least patched up enough to scrape it through the increasingly stringent noise and environmental pollution laws that were almost as big a problem as the incoming Japanese—and, come to that, the incoming Germans who, with their new range of BMW Boxer twins, were rapidly shedding their “old man” image and looking increasingly attractive to a younger aspirational biking set.
And then there was Harley Davidson knocking on the door and slowly getting its act together with a range of brutally attractive, if expensive, 1000cc and 1200cc twins.
In 1976 the T140 was finally given a rear disc brake. In 1978, the bike developed a new, parallel-port cylinder head (to replace the earlier splayed-head that had been a feature since 1959), while the TR7 Tiger, which had started with a single carb, retained it throughout).
That same year, new carbs were also introduced for the T140 (Amal Mk2s as opposed to Mk1s; the Tiger kept its Mk1), and overall, the bikes were looking more angular and modern—and a little too angular and modern for many Triumph diehards. Worse still, performance was down and vibration was up. At least, the modern generation of wannabe gonzo motorcycling journalists felt so, and said as much.
Loudly, and often.
Meriden under pressure
Unquestionably, such hostile press opinion cost Triumph some sales—but perhaps not as many as you might expect. The average rider of the late 1970s wasn’t, after all, going to be fobbed off with an old tech, leaky, pushrod British twin when, for significantly less money, he could buy an up-to-the-second Japanese multi that could thrash the Bonnie on any quarter mile length of straight.
However, on the curves it was a little different. In fact it was a lot different. At that time, Japanese bikes had notoriously bad handling, and British twins (and triples) were famed for their sure-footedness. The average Jap superbike had a huge hinge in the middle and (often) a distinct wobble when it approached anything like its theoretical top speed.
But the Japs were learning fast, and there wasn’t really any place else to go for the Bonnie; certainly not without radical revisions, for which there simply wasn’t the development cash anymore.
In 1979 the Yamaha 650 Special (twin) appeared; a devilishly handsome piece of kit that did pretty much all that the T140 and TR7 could do (except handle), and at a lower price. It was largely also oil tight, and vibrated less, and had a good electric starter. These 650cc Yams, since 1968, had built a reputation as dependable all-round machines, and they sold in huge numbers around the world.
But this particular Yam, with its cast wheels and black and gold livery, hit the right note during the right summer and had exactly the street cred required by contemporary riders looking to build or maintain a sub-hooligan wish-I could-afford-a Harley image. Whilst at the same time, the Yam Special was not so left-field that ordinary guys looking for a simple, decent, attractive everyday twin would shy away from.
Bonnie Special T140D and electric starters
Regardless, Triumph felt the pinch and rapidly responded with the Bonnie Special (Lester cast wheels, black and gold livery, two-into-one exhaust); a machine that in terms of looks and quality of materials easily shunted aside the Yam Special. But the king of the twins was on shaky ground (not least because it was 10-15% more expensive), and knew it.
The following year, the T140/TR7 gained an electric start; the first on a British motorcycle that actually worked. For a while, anyway. There were later problems with Triumph's hurriedly engineered system, but none of which can be shown to have really hit sales (except in the second hand market). And within three years of that auspicious event, having seen relatively minor (and some not so minor) revisions, the Triumph twin was dead.
Part of the problem was a collapsed US market. Part of it was business export market guarantee intrigues. Part of it was that the Bonnie was simply looking less and less relevant.
And sales had truly plummeted during those last few years despite most riders bearing a grudging respect for the T140 (some of whom who were, naturally, always meaning to buy one, or at least cadge a ride on one someday). And of course the biking press, now that the dirty deed was done, said that it was a pity that the British bike industry was over. But never mind, we’ve still got the Japs ....
It might have been kinder had someone pulled the Bonnie plug a little earlier, because at the end of production, the 1973 flood of Bonnies had turned into a pathetic trickle down the drain of history with bikes being offered in any specification and any colour, just so long as you bought something.
Harris Bonnies and the end of the T140
It was a sad end for a company that really loved its product; an end that enjoyed a short-lived resurgence in 1985 when the late Les Harris bought a short-term manufacturing licence and built another 1200 or so bikes up until 1988.
Today, T140 Bonnies are as good as they ever were (whatever that means to you), with overall reliability much higher (largely thanks to retro-fitted electronic ignitions; the standard T140 never got electronic ignition until 1979.
They’ll leak a little oil, which can be ignored without issue, or (mostly) fixed if you just take the trouble to rebuild them properly. On the plus side, a little black gold splashed around a motorcycle isn’t such as bad thing if you want to keep the lethal oxides at bay.
Overall, Bonnies and TR7s are nimble, tractable, decent stoppers, and maintenance light. And when they do go wrong, they can be sorted fairly easily. Also, they return around 50-55mpg. You won’t see much change out of 100mph, but acceleration is still pretty good, even two-up.
The seat height is fairly low at around 31-inches, and only those with extra short inside leg measurements will have to worry about getting both feet flat at the same time on opposite sides of the saddle. Not that that's too important. The 750cc Bonnie is an easy machine to throw around when stationary.
Is the T140 Bonnie or TR7 Tiger the bike for you?
Most T140 Bonnie or TR7 Tiger riders wear open-faced crash helmets. But if you really must wear a full-faced lid, this is one of the few British bikes that will indulge your obsession with your bridgework without looking too ... well, naff. So much so that Triumph was keen to use full-faced helmets in its later adverts (see main image) as if to prove to the world that what was pretty long in the tooth was actually very modern and cutting edge.
But which bike should you buy? The short answer is whichever one you most enjoy looking at—provided the price is in line with current market expectations. None of T140s have particular "issues", except perhaps the electric start models (which need a little extra care with the starter gear sprag clutch), and the T140W TSS derivation (with its eight-valve head and other under-developed revisions). If you're considering an early bike (pre-1976), you're strongly advised to think in terms of left or right side gear change.
It can make a major difference to some riders.
If you want to cut your maintenance chores a little further, and without a significant power penalty, buy a TR7 Tiger.
After a long time in the low value, low status doldrums, 750cc oil-in-frame Bonnies and Tigers are now steadily rising in value and prestige. And rightly so. For commuting, these bikes can easily take the daily rough and tumble, being nimble in traffic with enough presence to command some respect on the road. For touring, they're long legged enough to carry you for hundreds of miles a day, two up and with luggage, without blowing their cool and leaving you exhausted. And for Sunday fun and games on your favourite backroads, a T140 will do everything demanded of it by the average classic biker, with plenty in reserve.
They can be tuned and beefed up to show a genuine 115-120mph on the clock. But you'll also have to drop the bars and shift the footrests back to do it. The compression will need to be raised; bigger carbs will need to be fitted; and expect to do a lot of headwork (porting, polishing, and perhaps lighter valve springs).
Meanwhile, stripping some of the excess tin can cut the flab by around 60lbs, or more, thereby liberating a few extra bhp.
And if anyone tells you that the oil-in-frame T140s are rubbish, just fix up a time and a place and let us know. We’ll send some of the boys round to sort it out.
Triumph T140 Bonneville ownership tips
1. Use a quality 20W50 low detergent multigrade oil and change every 1500 miles maximum, or less.
2. Chrome discs make for worse braking. De-chrome, or replace. Unchromed cast iron discs are superior.
3. Check the wiring looms carefully when buying. Many looms haven't been replaced in over 30 years.
4. Use the best fuel you can get, and avoid cheap supermarket petrol.
5. Rejuvenate the braking system by replacing old hoses. And change the hydraulic fluid at least every two years, or less. DOT 4 will do nicely.
6. Rough fork action can often be fixed by loosening the forks from the spindle upward (including yokes), and retightening in the correct sequence according to the factory workshop manual.
7. Check regularly that the oil is returning. Raise the seat, unscrew the oil filler cap, and run the engine. There's a small pipe just visible at the top
8. Check the centre stand for smooth action and looseness, and check the sidestand for sloppiness.
9. Gears should change cleanly and click into place. You might get a first gear clonk. But beyond that, the gear changes should be fairly slick.
10. Clutches are fairly heavy, but should bite smoothly and progressively.
Keep a spare cable with you—or, at least, an emergency cable repair kit.
Copyright Sump Publishing 2009