The small block attached to the side of the injection pump, is the lift pump. See below in the red line:
The lift pump has a piston inside, which is powered by the camshaft of the pump. Piston moves up and down and the two valves in the fuel line result in sucking fuel from the tank (piston moves down) and pushing fuel towards the fuel filter and the fuel pump (piston moves up). The hand pump does the same, only it is powered by hand. If you have an old style hand pump (with a screw top), you might want to replace it with a new black top one, because the old hand pumps often fail.
The lift pump always pumps too much fuel, so that the fuel pressure inside the injection pump is high enough for the pressure cylinders (those pump the fuel into the engine at high pressure) to fill. Pressure inside the injection pump is regulated by a relief valve at the back of the injection pump. Even if the relief valve should fail, it would not cause high idle.
The high idle is caused by something else. First: the injection pump has a so-called pneumatic governor. The governor is responsible for the amount of diesel injected into the engine, that determines (together with the load on the engine) the speed of the engine. It is very easy for diesel engines to overspeed, that is why there is a governor. The governor makes it possible for the engine to idle at a constant low speed and not to overspeed (more than 4400-5400 rpm).
The pneumatic governor works by means of a vacuum created by a venturi in the intake manifold. A butterfly air valve (see picture below) determines the amount of air flowing through the venturi, i.e. more of less vacuum. The valve is controlled by your foot pressing on the accelerator pedal. The vacuum is "transfered" to the vacuum chamber of governor by means of an air hose. That is why there is an air hose running from the intake manifold to your IP.
Butterfly air valve in the intake manifold with venturi at the bottom.
The amount of diesel injected into the engine (and therefore the speed of the engine) is controlled by the control rod. The control rod is connected to a diaphragm. A spring pushes against the diaphragm, so that the control rod is pushed into the maximum position. The vacuum works on the diaphragm, so that it is pulled back against the spring pressure (in fact the air pressure on the other side of the diaphragm pushes against the diaphragm). So the variable vacuum created by the venturi in the intake manifold determines the amount of diesel injected.
This system works for the higher speeds of the engine, but at lower speeds and at idle the engine will fluctuate in speed ("sawing" of the engine). Therefore an auxilliary mechanical governor is installed, so that the control rod rests against a spring stop. That mechanical governor is brought into action by a rod, running from the intake manifold, over the valve cover and to the side of the IP. The rod turns a poppet cam into position. When the accelerator pedal is pressed deep enough, the poppet cam moves away and the governor purely works on vacuum.
Picture of a IP, red dot is the poppet cam axle, blue dot the air hose, green dot where the diaphragm is (on the inside)
Diagram of the governor
8 = control rod
10 = diaphragm
13 = spring
15a 15b = poppet cam
If there is an air leakage, for instance a loose air hose, torn diaphragm, leaking axle of the poppet cam, the idle will be high, because there will not be enough vacuum to pull back the diaphragm, and because the air valve will be closed, there will not be enough air to burn all the diesel, so that the engine smokes.
So you must determine where the air leak is. If you read this topic:
you can find out how the system works and how to diagnose it. After that you only have to find the parts...