Ex-Meriden man, Hughie Hancox explains
In the austere postwar years, the 1949 Triumph Thunderbird was a breath of fresh air as far as the Americans were concerned. With its Nacelle, 650cc engine and sprung hub rear suspension, it really was the first 'superbike' with a parallel twin engine.
After its French tour-de-force over 24 hours, with one of the riding team being my old mentor Alec Scobie, it quickly became the affordable bike to have with the British working masses. It was the big brother to the Speed Twin that had carved itself a niche and was widely accepted by most of the police forces throughout the country. Indeed the Speed Twin had led the victory parade in London, ridden by Metropolitan Police riders as it passed in revue before the King and Queen.
The Thunderbird however, was something else, more 'cubes' as the Americans say, more power and pretty smooth too! The only 'fly in the ointment', according to the Americans, was the 'God awful colour'. The flat blue-grey smacked of RAF issue machines from the war years. The Americans wanted something more snazzy, so at the end of the 1950 build in October 1951, the colour changed to the polychromatic blue so readily recognised as the Thunderbird colour. The other important modifications were to the gearbox, now having the speedo drive off the inner cover and the rear sprung hub.
In the meantime Edward Turner was thinking of his statement to the motorcycle press where he had said something along the lines "The Thunderbird is a long distance cruiser, giving 70 miles to the gallon at 70 miles per hour."
The carburettor of the day was the Amal type 276, in use since well before the war and coming to the end of its life. With the Amal Monobloc not due until 1954, the only other instrument currently available was the SU constant velocity carburettor, widely used on the majority of four-wheeled vehicles and of course, the smaller MC2 version on the Ariel Square Four.
Talks between Triumph and SU went ahead with one of their main development men being camped in our experimental department, until an acceptable unit was fitted to a 6T Thunderbird for mileage test. A new inlet manifold had to be made to take the carburettor, but unfortunately, due to its size, the unit wouldn't actually fit and allow a rubber connector from it to the existing air cleaner to be fitted, due to a too severe angle. This meant a frame modification was required in the form of a cast loop, let into the saddle down tube, at the end of 1951, ready for the 1952 season. This allowed a straight connector hose to fit onto the inlet of the carburettor, pass through the loop and fit onto the new D-shaped Vokes filter seated snugly behind the down tube and in front of the rear mudguard lower front position.
The carburettors arrived from SU in boxes, already set on the mixture (bottom jet nut), with only the minimal adjustment required by the road testers when the models were up to temperature. In the Repair Department we did get a few machines back from the dealerships with problems they couldn't solve, and some even had carburation or mixture difficulties that neither we at Meriden nor the dealership could rectify. Enter Ginger Woods, SU technician par excellence.
When we had two or three machines with such problems, it would be 'send up Ginger' time, and he'd duly appear with his little attache case, the SU emblem emblazoned in gold on its lid. Usually 'Ginger' cleared up the troublesome models within the day, being supplied with regular mugs of tea made by Arthur Ashley, the repair shop labourer.
After an instrument had been checked, reassembled and roughly set, it was refitted and the machine started. He would take it out for a ride and once back in the repair shop he would finely tune the job, with the engine ticking over at about 700rpm, he would roll the twist grip between the palms of his hands. Under this treatment, a machine equipped with the Amal 276 would simply have choked and the engine would have stopped. With a Ginger tuned SU however, the engine revved freely up the range without a hint of hesitation. It was well documented that ridden sensibly, the SU equipped Thunderbird, on its 7:1 CR could easily return in excess of 80mpg. Quite an achievement for a 650cc twin.
I saw 'Ginger' a good few times up until going into the army, and of course by the time demob came around the SU was on its way out due to being too expensive, especially as the Amal 'Monobloc' was being used on the two other 650 models we made, the Tiger 110 and the TR6 trophy.
With the demise of the 1958 'Burnished Gold' Thunderbird, the last model to be equipped with the lovely old SU, a lot of customers requested that their 1959 Monobloc fitted Thunderbirds be changed back to the SU. As history shows, this however wasn't to be.
burlen fuel systems
The ever-helpful Burlen Fuel Systems, down in Wiltshire, now manufacture the complete range of Amal motorcycle carburettors, plus Solex and SU. However, the MC2, which has a smaller body size than those found on cars, is not cost effective to manufacture because of the obvious low demand. However, Burlen hold a good number of spare parts for these carburettors and, should you or your club be able to rustle up an order for 100 units, they will in fact tool up and make them. As it stands, they are in the process of picking up old units for both Ariel and Triumph and refurbishing them, to essentially offer a service exchange arrangement. Alternatively, they will refurbish your carburettor for you. Burlen -aka The SU Carburettor Company, are at Spitfire House, Castle Road, Salisbury, Wilts SP1 3SB. Tel 01722 412500. www.sucarb.co.uk
So tell 1tl6> what is an SU carb anyway?
SU (the full name is Skinners Union) made carbs controlled by an air regulation device. Similar in principle to the constant velocity carbs found on most 1980s/90s Japanese bikes, but being British and 50 years older, they are obviously much more beautiful in appearance and operation.
Put simply, air flow through an SU carb is regulated by a piston and venturi rather than directly by the throttle cable and carb slide (as in a traditional carb). So once properly set up, an SU item is much more efficient, hence the smoother performance and better mpg.
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