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Firing Order Holden V8 and 6:
When setting the ignition timing on any Holden engine it is important that Cylinder number 1 be at the timing mark on the firing stroke and both the inlet and exhaust valves for Cylinder number 1 are closed. This position is slightly before Top Dead Centre (TDC). Note there is a second and unwanted timing position which occurs at the end of the exhaust stroke. When this position is present both the inlet and exhaust valves will be "on the rock" i.e the exhaust valve will be closing and the inlet valve opening. "On the rock" is also known as "Valve Overlap" because both valves are partly open at the same time or overlapping. Setting the ignition timing to the Valve Overlap position will cause backfiring out of the carburettor and the engine will fail to start. This is because the plug will spark while the valves are open instead of them both being shut.
The cylinder positions are sometimes written on the inlet manifold.