An engine must be kept in excellent mechanical condition to prevent unbalance. A leaky valve or fuel injector, leaky compression rings, or any other mechanical difficulties will make it impossible for you to balance the load unless you secure the engine and dismantle at least a part of it.

To obtain equal load distribution between individual cylinders, the clearances, tolerances, and the general condition of all parts that affect the cycle must be maintained so that very little, if any, variation exists between individual cylinders. Unbalance will occur unless the following conditions are as nearly alike as possible for all cylinders:

1. Compression pressures

2. Fuel injection timing

3. Quantity and quality of fuel injected

4. Firing pressures

5. Valve timing and lift

When unbalance occurs, correction usually involves repair, replacement, or adjustment of the affected part or system. Before any adjustments are made to eliminate unbalance, it must be deter-mined beyond any doubt that the engine is in proper mechanical condition. When an engine is in good mechanical condition, few adjustments will be required. However, after an overhaul in which piston rings or cylinder liners have been renewed, considerable adjustment may be necessary. Until the rings become properly seated, some lubricating oil will leak past the rings into the combustion space. This excess oil will burn in the cylinder, giving an incorrect indication of fuel oil combustion. If the fuel pump is set for normal compression, and the rings have not seated properly, the engine will become overloaded. As the compression rises to normal pressures, there will be an increase in the power developed, as well as in the pressure and temperature under which the combustion takes place. Therefore, when an overhaul has been completed, the engine instruments must be carefully watched until the rings are seated, and all necessary adjustments are made. Frequent compression tests will serve as a helpful aid in making the necessary adjustments. Unless an engine is so equipped that compression can be readily varied, the engine should be operated under light load until the rings are properly seated.