This DIY repair manual by Haynes, covers BSA Unit Single motorcycles, first introduced in 1958 and discontinued in 1972, including the following models:

• BSA C15, 247cc
• BSA B25, 247cc
• BSA C25, 247cc
• BSA B40, 343cc
• BSA B44, 441cc
• BSA B50, 499cc

Book Excerpt: 1958-1972 BSA C15, B25, C25, B40, B44, B50 Repair Manual FUEL SYSTEM

AIR CLEANER: LOCATION, EXAMINATION AND REPLACEMENT OF THE ELEMENT

250 AND 350N STAR MODELS
1. On the earlier 250 and 350 machines the air cleaner is concealed within the fairing below the seat and is connected to the carburetor by a rubber hose. Remove the two nuts retaining the cover on the left-hand side of the machine provides access to the filter which is retained in its housing by a spring-wire clip. Remove the filter, which is a wire gauze and fabric type and wash in gasoline. Ensure the filter is dry before replacing it.
B25, C25 AND B44 MODELS
2. The air cleaner on these models is the pill-box type and screws directly onto the carburetor air intake. After unscrewing the filter can be dismantled for cleaning, by unscrewing the bolt holding the perforated outer cover together.
3. The element is made of surgical gauze and should be washed thoroughly in gasoline and allowed to dry before replacement. If the element appears unserviceable, it should be renewed.
B50 MODELS
4. The 500cc models are fitted with a paper element type filter located behind the panel on the left-hand side of the machine. The element is retained by a single center nut and washer and after removal, can be cleaned with a stiff brush or, if available, an air line. Do NOT wash in gasoline or detergent. To maintain the engine at peak performance the element should be renewed every 5000 miles.

IGNITION SYSTEM

CONDENSER: REMOVAL AND REPLACEMENT

1. A condenser is included in the contact breaker circuitry to prevent arching across the contact breaker points as they separate. It is connected in parallel with the points and if a fault develops the ignition system will not function correctly.
2. If the engine is difficult to start or misfiring occurs, it is possible that the condenser has failed. To check, separate the contact breaker points by hand while the ignition is switched on. If a spark occurs across the points and they have a blackened or burnt appearance, the condenser can be regarded as unserviceable.
3. It is not possible to check the condenser without the necessary equipment. In view of the low cost of replacement it is preferable to change the condenser and observe the effect on engine performance.
4. To remove the condenser, unscrew the terminal nut on the end and lift off the contact breaker return spring (on earlier Star? models remove the flat metal strip). Remove the retaining screw and lift out the condenser. On more recent models the condenser is housed in the electrical box beneath the fuel tank and is removed by disconnecting the terminal nut and unfastening the retaining screw.

Subject: 1958, 1959, 1960, 1961, 1962, 1963, 1964, 1965, 1966, 1967, 1968, 1969, 1970, 1971, 1972 BSA Unit Singles service, maintenance, repair. ISBN-10: 0856961272 | ISBN-13: 9780856961274 | Haynes 127