emu
BSA Motorcycles
The Mystery | Cylinder Head History | Racing Success | What The Public Got | What were BSA Up To | Links & Credits

The BSA Twin Carb Mystery


Racing Success


Twin carb A7s were prepared by the factory in 1955 for use in the Daytona beach races in the USA and scored impressive results in both the 100-mile and 200 mile races competing mostly against Harleys, Camshaft Nortons and BSA entered Gold Stars.

 

  wpe2.jpg (7766 bytes)  
 

Factory racing A7, those raced at Daytona had rigid rear frames

 

Afterwards these and others made their way into the hands of people like Al Gunther and Dick Mann who raced them in smaller events for a few years.

Can A7’s and A10 go fast anyway?

Returning to my original wish, to establish BSA pre unit twins as machines capable of a performance to match Triumph twin performance – can it be done?

In the early 1950’s the BSA factory must have thought so. In 1950 Fred Rist achieved 140mph in the UK running an A10 on dope. That he did so during sand races, sliding the bike round corners by laying it down on a footrest in a huge controlled power slide adds awe. In 1951 the US West Coat Distributor Hap Alzina organised an attempt at the one-mile record at the Bonneville Salt Flats using two machines – a 500cc Star Twin and a 650cc Gold Flash. Both machines had been tuned, the A7 to comply with AMA class ‘C’ rules, i.e. it should remain outwardly the same as the machines sold to the public and must run on pump petrol. As such it had higher compression, larger valves, polished ports, a high lift camshaft and a 1 1/16" TT carb and produced 40bhp. Ridden by Gene Thiessen it achieved 123.69 mph over the flying mile. The A10 was prepared to comply with class ‘A’ rules which allowed more radical tuning and the use of dope. Fitted with twin 1" TT carbs, running a 13.5 to 1 compression ratio on 90/10% methanol-benzole fuel it produced 60bhp and achieved 143.54mph over the mile.

For the 1954 Daytona 200-mile classic, BSA entered 3 Gold Stars and 3 Shooting Stars. The Shooting Stars took 1st and 2nd with an average speed of 94.24 mph, with Gold Stars taking the next three positions and a private Gold Star in eighth position.

Year Machine Rider Position
1952 A7 Al Gunther 5th
  A7 Gene Thiessen 9th
1954 A7 Bobby Hill 1st
  A7 Dick Klamforth 2nd
  500 Gold Star Tommy McDermott 3rd
       

The third Shooting Star crashed out at the first corner.

Roland Pike

Roland Pike was a development engineer who worked with BSA in the early 1950’s and his contribution to the development of the Gold Star is well known. Less well known is his involvement with the development of the A7 twins and their contribution to BSA’s racing effort in the Clubman TT and races at Daytona Beach.

In a feature on Roland’s work published in the February 1993 edition of ‘Classic Bike’ Roland described his part in the development of racing A7’s at the request of Bert Hopwood. Bert Hopwood wanted to create a Gold Star ‘killer’ out of a tuned A7 in much the same way as he tried to make a Manx Norton ‘killer’ with the Domiracer at Norton several years later. In the article, Roland describes the work done to make the racing twins reliable, then fast.

Of relevance here, Roland confirms that the early racing twins all had twin carburettors but felt that subsequent experience showed that a large single carburettor seemed to perform just as well and was less work to set-up properly. However, this didn’t stop twin-carb A7s gaining 1st, 2nd and fourth places in the 1954 Daytona 200 mile ‘classic’ race, beating the factory Gold Stars.

When it was decided to make a production version of the twin with an alloy head for 1954, the factory dithered between single and twin carburettors, as there seemed little improvement except with open pipes. This perhaps explains the bolt-on ‘Y’ shaped manifold used on the 1954-1956 A7 cylinder head (67-1101) - the factory was simply keeping their options open for the possibility of a twin-carb model for the public.  But did the public ever get them?  Find out in What The Public Got...



  emu Sheldon's EMU Copyright © 2004-2018