You’ll probably have heard pub experts discussing ‘pinking’ in relation to ignition timing on a classic. This problem occurs when the mixture in the cylinders ignites too soon, hammering the piston and con-rod into the crankshaft as the crank web is still pushing the piston up the cylinder. This is heard as a light rattle of the piston in the bore (pinking) or the heavy coming-together of the crank and its bearings (knocking).
The most obvious cause is ignition timing that’s too advanced. Setting the timing to the manufacturer’s specification is the first step. In addition to this ‘static’ timing, the ignition angle is advanced ‘on the move’ by centrifugal weights in the distributor and a vacuum advance mechanism.
This can be checked with a timing gun. Disconnect and plug the vacuum advance pipe, then point the gun at the crankshaft pulley timing marks (pick them out with a dab of white paint). Take the engine up to mid-revs and read the maximum ignition advance. Maximum advance usually comes in between 1500 and 3000rpm. If you read a much greater advance, then the car probably has the wrong distributor fitted, as this maximum is not usually adjustable. Don’t be tempted to fiddle with the advance springs. Reconnect the vacuum advance and repeat. Check the percentage increase rating in your workshop manual and, again, suspect the wrong mechanism if the readings vary significantly.
If the timing looks OK, check the fuel supply. A low fuel level in the carburettor will cause weak running, which also leads to pinking. It raises the combustion temperature and causes hot-spots which pre-ignite the mixture before the spark has arrived. Check the float level in the carburettor and make sure the float is not sitting cock-eyed on its pivot.
Try adjusting the mixture to give a smooth and steady tick-over. This should give a correct setting across the rev and load range. If there’s still a problem, check that the needle is correctly installed, the jet is not blocked and, finally that the correct needle and jet are fitted. These should have markings on them, but they are wearing items and it would do no harm to change them anyway. There’s a huge choice of needles and jets that will fit – but only one pair will be right for your engine, so buy from a specialist supplier.
If the carburettor is in order, suspect a fuel pump that isn’t delivering enough petrol at times of high demand. This may be due to an incorrect or malfunctioning pump, air leaks between the tank and the pump or blocked fuel filters.
If you still have no joy, look at the more obscure: air leaks at the inlet manifold or brake servo hose, or the wrong grade of spark plug. An exhaust valve that gets hot because it’s not seating properly will also ignite the mixture prematurely. Check valve clearances and, if OK, test compressions on all cylinders. A cylinder with low compression may have a burnt valve.