Abstract: Improving the operation of a diesel engine at light loads in which the engine includes a plurality of cylinders, a piston in each cylinder, a fuel injector for each cylinder, an air blower for supplying air to the cylinders and an exhaust manifold. A valve is pivotally positioned in the exhaust manifold, and biasing means is connected to the valve acting to bias the valve towards a closed position. The modulating valve restricts the flow from the manifold at light loads thereby maintaining a back pressure on the exhaust at lighter loads for improving engine performance, but opens wider by exhaust flow from the manifold at higher loads. An adjustable stop may be positioned in the path of travel of the valve preventing the valve from entirely closing. The valve may be a butterfly valve pivotally supported off-center and the biasing means may include a weight connected to a lever arm connected to the pivot supporting the valve. Improved fuel injectors provide less fuel than normal injectors at lighter loads.