The term “tune up” has been used to mean a lot of different things over the years. For our purposes, we will include the following:

* Checking engine mechanical condition (compression or leak down testing)

* Adjusting valves if required

* Installing new ignition parts as needed

* Checking and adjusting ignition and fuel system to factory specifications

* Inspecting and servicing manifold heat control valve if equipped

* Checking and adjusting charging voltage and checking battery condition

* Checking drive belt or belts and adjusting or replacing as required

* Inspection of the cooling system, and servicing it as required. Perform a visual   inspection for leaks or other hazards like a belt rubbing on a hose, bracket or wire.

Always remember that an engine that is not in good mechanical condition cannot be tuned. If the combustion chambers don’t seal properly, that is, if the valves and rings and gaskets do not seal properly, the engine will not perform correctly. That is true for all engines, new and old. All too often an engine with a leaking head gasket or a burned valve is tuned in an effort to restore good performance, only to disappoint the owner with the same old problem after the effort has been made, and new parts installed. Any time a tune up is anticipated, especially if the engine is running poorly, the compression should be checked to determine if the work will be satisfactory.

After the engine is found to be in good mechanical condition, it is advised that a thorough tuning be performed to bring the engine back to its original power and economy levels. If the compression test

reveals that there is a deficiency in the engine’s internal sealing, it is often necessary to do a cylinder leakage test. This test will tell where the leakage is located. That is, it will tell if there is a bad valve, a bad head gasket or worn piston rings; and which ones are bad.

Basically, the way it works is that a fitting is installed in the spark plug hole which will allow for the shop air compressor to apply air pressure into the suspected cylinder while the piston is at top dead center with both valves closed. The controlled air pressure, being applied through a pressure regulator, will leak out wherever the failure exists. The instrument will measure the percent of leakage. Normal leakage should be uniform through all cylinders, and it should be under fifteen percent. If there is a significant leakage in one cylinder, it is possible to determine where observing and listening as the air is being leaked out of the engine locates the failure.

If you hear a hissing noise over the carburetor or at the air inlet to the fuel injection unit, it will indicate a leaking intake valve. If you hear a hissing noise at the exhaust pipe, it is likely an exhaust valve, and if you hear bubbling in the cooling system, it could be a leaking head gasket or a cracked cylinder head or engine block. If you hear air leaking out of the oil filler tube, or blow by tube, it may be defective piston rings, or a crack in the piston itself.

The cylinder leak down test is a very useful way to determine what the problem is, where it is, and, therefore how to go about fixing it.

Another valuable test to know about is the CYLINDER BALANCE test. This test is used to find the failing cylinder when an engine starts “missing”. (Running on less than all cylinders)

Expensive shop testing equipment uses cylinder balance testing to determine which cylinder is failing. It is a very simple process: The engine speed is set at one thousand RPM. (Simply turning the idle speed screw does this) As the engine is running, each cylinder is shorted out, one by one, causing the engine to loose RPM. The theory is that each cylinder is contributing to the set speed of the engine, and therefore, as you disable each cylinder, it should cause the speed to fall. When you come to the troubled cylinder, the set speed stays the same when that cylinder is shorted, indicating that it isn’t doing its part. This is a very old but valuable method of determining which cylinder is bad, and it can be done using simple shorting probes that anyone can make up in his or her own shop.

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