OWNING A TRIUMPH T140 750CC 1973-1983
With these articles I am only interested in what makes these bikes tick and will really only discuss mechanicals.
They were first introduced in 1973 as a 724cc, this is distinguishable by the fact that these first models used a 75mm bore and 4 outside head bolts with 26tpi CEI thread. These bolts are a carryover from the previous 650 models but all other bolts in the 750 10 stud top end are UNF as are all the other frame and engine threads, either fine UNF or coarse UNC as used in the engine cases, except 2. The 4 x 3/8” CEI head bolts were changed to UNF by mid first year when the bore was increased to a full 750 with 76mm pistons. These models like both the previous first 2 years of the Oil In Frame line of twins used small pins to locate the rocker boxes to the head and the style of bolting used socket nuts to put tension on the head gasket with all ten studs . The rocker boxes were than fitted. This differs to pre Oil In Frame Triumphs ,1970 and earlier ,where head tension was pulled down through the rocker boxes and the rocker box gaskets. This means it is difficult to maintain proper tension as the rocker box gaskets tend to squash , then leak . More modern wire impregnated gaskets are available to help with this dilemma but are still not the best fix. Copper rocker box gaskets are best but one needs to be sure mating surfaces are flat. Both era ( pre-unit and unit) 650s ,the Triples and unit 500s all have this problem. So back to T140, with the T140/(TR7 single carb) models, the push in header pipe system ala BSA and others continued until 1980 when a return to a screwed in stub with a header pipe that pushed over the stub then clamped pipe, was reintroduced. The push in system was phased in as cost cutting in 1972 on the 650. The balance pipe introduced in 1969 stayed until the end in 1983. The T140 and TR7 both used Amal 30mm carbs, 2 for the Bonnie and 1 for the Tiger, of course . Basically the factory moved away from screw in inlet manifolds and introduced the flange mount block. The 73 version is a solid piece with 2 stepped flanged bolts tightening the manifold to the head and 2 small steel cups and rubber washers keeping tension on the carb ‘O’ ring. This system works fine. If you find you are stuck without the rubber washer a 5/16” spring washer will suffice. In 1979 Triumph changed to Amal MK2 type carbs which are spigot mounted via rubber hose. These work well especially after benefitting from a jet kit . The model is a T140E, E being for emission achieved by the weaker mixture and no dripping fuel from flooding like the previous MK1 carbs. Lucas electronic ignition also played a part in this ‘Emission” model. Points were fitted up to 1978. Later years post 1980 saw Bing carbs used for better economy but seemed troublesome and I have seen a few bikes from those years retro fitted with MK2s. The rocker arms didn’t differ from earlier models and the shafts used a 3/8” CEI 26tpi dome nut to hold on the rocker oil manifold until 1978 when they moved to 3/8” unf. At this time the rocker adjustment screws and locknuts , the last of the CEI went to UNF. The rocker box inspection cover and therefore the rocker box changed from 4 screw fixing to 6 screw fixing when the engine went from 724 to 750.
When the engine went from 650 to 750 the compression was lowered from 9-1 to 7.9 -1, giving a softer state of tune. The larger capacity doesn’t mean more power but gave a bit more torque and the factory upped the gearing from 19tooth on the 650 to 20tooth on the 750. This gave 65-6mph @4000rpm instead of the 650 s 60mph @4000rpm. They are quite capable of pulling 21tooth on the gearbox which gives 70-1 mph@4000rpm. It was a little softer than the 650s but in my opinion makes them nice to ride. The factory played with balance factors all through the 650 years which began in 1950 and finally settled on 74% with the 750 although only statically balanced. The RE big twins were the only factory dynamically balanced British twins I know of. I use this 74% a lot on different ridgidly mounted twin engines and get good results for medium to fast touring. 65-75% balance factor range is a good starting point on any parallel twin if unsure. The T140s have a nice heavy crank and very stout conrods. These are 6” long as the factory changed the cylinder height by ½” for the move to 750, in part to make top end work easier .The 650 had the taller cylinder of course with a 6.5” conrod which used bronze small end ( piston pin) bushes from start to finish. The move to the 750 conrod saw the factory fit the piston pin direct into the conrod. A good move as wear is non existant. I have seen a few 100,000 + mile 750 engines with a rebore or two but no small end wear. The pin will wear first unlike the bushed rods which wear both. The stroke never altered from 650 to 750 at 82mm. The major change in the oil in frame period , 1971 on ,saw the crank change to a metric bearing on the timing side, a 306 ball as per a Norton. This can be a problem as they decided against using a heavy duty ball such as a10 or 11 ball bearing and stayed with the 8 ball bearing. Some of these broke the cage pins and collected the balls together causing some noise on the timing side. It is a good move to fit the stronger bearing if inside the motor and nowdays lots of these would have been changed. Some of the high mileage motors I have seen still had the 8 ball bearing. Go figure? At about 100,000 miles the crank may need a grind and will have seen 2-3 top end rebuilds in that time. Depending on service intervals. There is a swag of modern replacement parts of much higher quality out here now than back when they were introduced and these parts such as aluminium bronze valve guides, also black stainless valves to name a couple, will effectively double top end mileage.
The 650 of 1972 got a 5 speed gearbox mid year which went through until a rethink by 1976 as a few breakages had occurred. This had been fitted to both twins and triples. The rethink included basically a whole new set of gears for the layshaft, almost the same but stronger and not really mixable with the first set. The mainshaft set stayed the same although the output sprocket shaft got an ‘O’ ring for better sealing. Breakages happened to the 1st gear dog ,a cross shaped piece, which sometimes snapped when slamming into gear with a clutch pack that had stuck or pulling wheelies . Also the high gear at the other end, which was eventually widened would break a tooth under harsh treatment. Now I have one of the early transmission sets and with 50,000+ miles and some roughish treatment which has not failed. When the 750 was introduced the factory went to a triplex row primary chain . BSA and Norton had done 10 or so years before. The 750 crank differed to the last 650, also a metric bearing crank (which coincided with the 5 speed introduction), with a longer rotor nose. The clutch was the same as the previous 650 and longer stator studs were used to mount the alternator stator and line up on that longer nose. Not much changed until 1976 when they went left shift which meant new castings , primary inner and outer gearbox castings, all because of demands from the USA.
The T140 and TR7 came out with a disc front and a conical rear 7” hub, the rear a carryover from 1971-2. The front disc is a 10” cast iron with a cast iron caliper on new Ceriani styled legs and wider triple clamps than the previous drum braked conical model. The tubes run directly in the aluminium lower legs ( no bushes). The tank was changed and the US variant had no seam down the centre. The UK and others mostly got the breadbox, a big boxy thing that grows on you. I like them both. ( chicken or plain) The badges on the yank version were the same as started in 1969 and went to ‘79. They also had a new alloy casting for the taillight and it was used from ‘73-83. The guards were chrome with the front a 6 bolt 3 stay affair then in’76 it went to 1 rear stay and a cross brace between the forks. In 1976 with the move to left shift the factory saw fit to add another disc to the rear . This uses the same master cyclinder body as the front one. The front brake is a bit weak and they were dumbed down through fear of user failings as this was the first foray into disc stoppers for Triumph. Reduced bore master cylinders are available to increase front brake power. Braided lines and aggressive pads make a difference . Larger floating discs can also be had. The rear is fine, I think. New stainless master cylinder bodies are a good upgrade.
If contemplating one of these motorcycles then think better filtration. Kits are available for the frame oil feed that sees a proper cartridge element replace the simple mesh screen, maybe an oil cooler as the Oil In Frame tank is not a lot .
The frames are a sturdy one piece twin downtube loop fully welded with no cast lugs. They are the weapon of choice for classic racers particularly in the US as they are relatively cheap and are proven good handlers. A bit of work on the front brake , maybe some better shocks and good tyres and one has a great midrange tourer or mountain fun bike. I have one made into a dirt bike and it does well. Basically they are versatile , quite reliable and relatively cheap.