Vintage times

RICK ANSWERS YOUR QUERIES

Right up my street was a query from Rick Wyatt in Minnesota concerning a Blackburneengined 1929 Motobecane he’s rebuilding for a friend. Rick asked if I could confirm the year of the engine and also how to set the valve timing, since there seem to be no timing marks. An FG engine number prefix suggests a 1928 500cc sidevalve, so that’s OK; the Surrey company had several French customers in the 1920s and ’30s. There are no timing marks because the crank pinion is a taper fit and not located by a key. Instead, Blackburne timing is set by dividing the valve overlap 2:3 over top dead centre. ‘Overlap’ is the period around top centre on the exhaust stroke of any engine where both valves are open at once – the inlet starts opening before the piston reaches the top and the exhaust doesn’t shut until on its way back down. The idea is that inertia causes the exhaust gas to keep moving even after the piston has stopped pushing, while opening the inlet valve early takes advantage of the vacuum left behind to draw the incoming charge. Clever, but obviously you can overdo it, which is why highly-tuned engines like the Clubmans BSA Gold Star with a whacking 120° overlap are rather intractable at low speed...

Rick needs to measure the overlap in degrees using a timing disc and divide the degrees by five, setting the cam so two fifths of the overlap are before TDC. I know, it took me a while to understand it, but if the overlap was 50°, then you’d set the inlet to open 20° before TDC and the exhaust would close 30° after. More modern engines tend to bias the opening toward the inlet, but for any vintage engine where no information exists, setting TDC in the mid-point is a good starting point.