A pump of an injector will fail when the plunger spring breaks and fails to return the plunger after injection has occurred. Factors which contribute to broken plunger springs are failure to inspect the springs thoroughly and careless handling. Broken plunger springs must be replaced. Also they should be replaced when there is evidence of cracking, chipping, nicking, weakening of the spring, excessive wear, or when the condition of the spring is doubtful.

Jammed Fuel Control Rack

If an engine is to operate satisfactorily, the fuel control rack must be completely free to move. Since the rack controls the quantity of fuel injected per stroke, any resistance to motion will result in governing difficulties. When this occurs, the engine speed may fluctuate (decreasing as the engine is loaded; racing as the load is removed), or the engine may hunt continuously or only when the load is changed. If the fuel control rack becomes jammed, it may become impossible to control the engine speed with the throttle. The engine may even resist securing efforts under such conditions. Since a sticking fuel control rack can cause serious difficulty, especially in an emer-gency, every effort should be made to prevent its occurrence. The best way to check for a sticking fuel control rack is to disconnect the linkage to the governor and attempt to move the rack by hand. There should be no resistance to movement of the rack when all springs and linkages are disconnected.

A fuel control rack may stick or jam as a result of a stuck plunger, dirt or paint in the rack mechanism, a damaged rack or gear, or improper assembly. When this jamming or sticking occurs, it is necessary to determine the cause of binding. If it is due to damage, the damaged parts must be replaced; if the stickiness is due to the presence of dirt, a thorough cleaning of all parts will prob-ably correct the trouble. Avoid errors in reassembly and adjustment by carefully studying the instructions.

Backlash in the Control Rack

Backlash, looseness, or play in the fuel con-trol rack, like sticking or binding of the rack, will influence governing of the engine. Proper governing is based on the theory that for every change in speed of the engine, there will be a cor-responding change in the quantity of fuel injected.

This is impossible if backlash, looseness, or play exists in the control system. Continuous or inter-mittent movement of the rack may indicate ex-cessive looseness. Engine speed variations are also indicative of this problem. Note that even though these symptoms are characteristic of a loose rack, a governor which is dirty or out of adjustment will present similar symptoms.

Backlash in a fuel control system is generally due to a wornout gear, rack, or control sleeve. When you disassemble a pump or injector for overhaul be sure to inspect all parts of the con-trol system for signs of excessive wear. If the rack may be moved more than a prescribed amount without moving the plunger, find the parts that are worn, and replace them.