A method for reducing unburned hydrocarbon emissions in a lean-burn internal combustion engine that is fuelled with a gaseous fuel comprises: operating the lean-burn internal combustion engine with an equivalence ratio below 1.0 over the full range of engine operating conditions; when engine load is below 50% of a maximum load: adjusting the timing for closing of an intake valve associated with a combustion chamber of the internal combustion engine as a function of engine operating conditions such that an equivalence ratio of the combustion chamber is maintained between 0.4 and 0.95, the timing for closing the intake valve being adjusted by one of: advancing timing for closing of the intake valve and closing the intake valve earlier during an intake stroke compared to when the intake valve closes during the intake stroke when the engine load is above 50% of maximum load; and retarding timing for closing of the intake valve and closing the intake valve later during a compression stroke compared to when the intake valve closes during the compression stroke when the engine load is above 50% of maximum load; wherein the volumetric efficiency of the internal combustion engine is reduced and unburned hydrocarbon emissions are maintained below a predetermined level. operating the lean-burn internal combustion engine with an equivalence ratio below 1.0 over the full range of engine operating conditions; when engine load is below 50% of a maximum load: adjusting the timing for closing of an intake valve associated with a combustion chamber of the internal combustion engine as a function of engine operating conditions such that an equivalence ratio of the combustion chamber is maintained between 0.4 and 0.95, the timing for closing the intake valve being adjusted by one of: advancing timing for closing of the intake valve and closing the intake valve earlier during an intake stroke compared to when the intake valve closes during the intake stroke when the engine load is above 50% of maximum load; and retarding timing for closing of the intake valve and closing the intake valve later during a compression stroke compared to when the intake valve closes during the compression stroke when the engine load is above 50% of maximum load; wherein the volumetric efficiency of the internal combustion engine is reduced and unburned hydrocarbon emissions are maintained below a predetermined level. adjusting the timing for closing of an intake valve associated with a combustion chamber of the internal combustion engine as a function of engine operating conditions such that an equivalence ratio of the combustion chamber is maintained between 0.4 and 0.95, the timing for closing the intake valve being adjusted by one of: advancing timing for closing of the intake valve and closing the intake valve earlier during an intake stroke compared to when the intake valve closes during the intake stroke when the engine load is above 50% of maximum load; and retarding timing for closing of the intake valve and closing the intake valve later during a compression stroke compared to when the intake valve closes during the compression stroke when the engine load is above 50% of maximum load; advancing timing for closing of the intake valve and closing the intake valve earlier during an intake stroke compared to when the intake valve closes during the intake stroke when the engine load is above 50% of maximum load; and retarding timing for closing of the intake valve and closing the intake valve later during a compression stroke compared to when the intake valve closes during the compression stroke when the engine load is above 50% of maximum load; wherein the volumetric efficiency of the internal combustion engine is reduced and unburned hydrocarbon emissions are maintained below a predetermined level.