Spark present but engine still missing

QMy Austin Maestro 1.3 seems to be ‘missing’ on cylinder number two. If I remove the HT lead, it makes no difference at all to the idling. I’ve changed the spark plugs, with no effect. If I remove and refit the lead with the engine running, there’s a strong spark jumping to the plug. What can I try next? Rusty Davis, Leominster

ABegin by checking the engine’s valve clearances. They should be 0.35mm on both the inlet and exhaust valves with the engine cold. If this doesn’t rectify the problem, carry out a compression test. Run the engine up to temperature, then turn it off and remove the spark plugs. Screw the tester into each plug hole in turn and crank the engine through ten or so compression strokes with the throttle wide open.

We suspect you’ll find the compression is much lower on cylinder two than all the others. This could be due to a burned or broken valve (generally the exhaust valve) or a problem with the piston or rings. To find out which is to blame, squirt oil down the spark plug hole and repeat the test. If the compression substantially improves, the problem is with the piston, bore or rings. If it doesn’t, it’s likely to be the valve(s).