A QUICK SMOKE
Ken Adams’ 1947 BSA A7 Star twin smokes from the left cylinder after starting, but soon clears. He says: “It’s not wet-sumping and I first changed the rings, then had it rebored... then new valves and guides – I even modified the inlet guides to take valve stem seals. The engine’s done 700 miles on the bored barrel and compression is 145psi both sides – but it’s still the same and I’d love to hear your thoughts!”
There’s a couple of things that strike me. First, compression – gas tightness – is not necessarily proof of oil control and rings are funny things. I once fitted a new set to a Triumph; the top ring was stepped, that’s usually a ‘wear dodger’ to clear any ridge at the top of the bore, so I fitted it step-upwards. Compression was fine, but that engine drank oil and I learned that with these particular rings the step should face down; turning them over cured the problem.
Second, smoke on starting is usually blamed on exhaust guide wear because while a running engine blows oil away, when stopped, it runs down the valve and then pools, where it awaits the next start-up. Since Ken’s guides are new, I wonder if they’re getting swamped by excess oil in the rocker box. Later models have a one-piece rocker box and all the oil simply drains down the pushrod tunnel – but the boxes are separate on the early twin and the exhaust drains through two small holes in the barrel that could be partly blocked.
BSA twins also have a tiny hole in the left conrod which encourages oil flow through the bearing. The rod should be fitted with hole pointing inward, toward the flywheel; if it was the wrong way round, maybe oil could spray up the bore, but I doubt this is Ken’s trouble as it would smoke all the time. That was my six-penn’orth – but Ken replied that he’d already discounted these suggestions.
Well, in that case all I can say is that Ken’s is a very early model – in the first 1000 by the engine number – and there’s always a risk in buying the first iteration of anything. Maybe I should say: ‘Ah, the early models all do that, sir; may I show you our latest range?’!