STARTING AGAIN
Andy Reynolds has just started restoring an ex-army BSA B40 he’s had for 10 years and bought a new Wassells carb after being advised to ditch the troublesome standard 398 Amal fitted. Having never set up a carb from ‘out of the box’ before, he read up on the theory – but finds that although the bike will start, it only runs for a few seconds before stopping again. Says Andy: “All the blurb on the box assumes you are tuning a running engine, how on earth can you do it if it won’t run in the first place?!”
Fair comment, Andy! The original B40WD carb was a butterfly throttle type specified by the military and largely responsible for the bike’s Mod-directed 65mph top speed, so it’s worth changing. But did the bike run on the old carb? That at least would help confirm the new carb is the problem – although it’s worth noting that fuel goes off fast these days and in my experience, stale fuel can behave this way, with the bike firing but refusing to run. I would drain and refill with fresh petrol if there’s any doubt.
Two other possibilities? First, it’s easy to dislodge the slide needle when working on a Concentric. If it drops into the needle jet, the bike will only run on the pilot system, with no fuel for anything more than tickover. Secondly, there could be a problem with the carb itself. There could be a fault or even a bit of machining swarf stuck in a jet, so I would dismantle the new carb and check it out. Failing that, perhaps try a richer slide cutaway (lower number), because the slide can affect starting and weak is usually more detrimental than rich.