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BASIC ENGINE TESTING

COMPRESSION TESTING

For an internal-combustion engine to run, three conditions must be met. The engine must have the following:

Compression Air-fuel mixture Ignition

These three "musts" have to be working together or timed if satisfactory engine operation is to be achieved.

Test I "Ground" the spark plug wire by connecting it to the engine. Check the compression by pulling on the starter rope. As the rope is being pulled, one should be able to feel the resistance as the piston comes upon compression. NOTE: Some engines have a built-in compression release which will release compression until the engine attains a proper speed. Usually the speed at which the release disengages occurs at the "end of the pull" of the rope when pulled at a normal starting rate. On engines with a compression release or one which fails the test, test the compression as outlined in Test II or III.

Test II If compression cannot be felt in Test I, make certain that the spark plug wire is grounded and spin the flywheel by hand in a direction opposite its normal rotation. This can be done by removing the air shroud.When rotated in the opposite direction, the flywheel should rebound as thepiston comes upon compression. If the rebound is quick, compression is good.

Test III A third method of testing compression involves using a compression gauge. The spark plug is removed and the gauge is held tightly in the spark plug hole. The engine is cranked normally and the gauge reading is observed. The normal pressure reading varies with the size of the engine. For the lowest compression engines, the reading should not be below 25 psi (pounds per square inch).

Most three-horsepower engines will give a 60-75 psi reading. Refer to the manufacturer's manual for the normal compression pressure of a given engine.

Low compression can result from several different faults. If there is a hissing sound white the compression is being checked there may be a leaking head gasket.

Worn or burned valves or valve seats will cause low compression readings.

Worn or improperly seated piston rings will cause compression loss into the crankcase. In extreme cases the engine will "belch" oil and smoke from the crankcase breather while it is running.

Low compression will usually necessitate engine disassembly. In some cases;a partial disassembly (head gasket and burned valves) will be all that isnecessary. In other cases it will be necessary to overhaul the engine. Major engine service is covered in the Overhaul section of this book.

SPARK TESTING

To consistently fire the plug, approximately 6.000 volts is needed at idle and up to 10.000 volts at high speed or when under load. All of the magneto components must be in good condition in order to deliver this high-voltage spark.

The flywheel magneto ignition consists or the spark plug, high-tension wire, coil, breaker points, and condenser. A magnet attached to the flywheel supplies the energy to the coil. Failure of any of these components can affect the spark. Some of the components may be cheeked separately, for example, the coil, spark plug, and the flywheel magnet.

If all the components are functioning properly, the magneto should be able to produce a three-sixteenth inch spark when cranked rapidly. Either of the following methods may be used to check the spark; the first method is a little quicker to perform and the second method is a little more precise.

QUICK CHECK METHOD

1.Carefully remove the spark plug wire from the spark plug. Pull the boot (rubber cover) back so that the metal connector is exposed.
2. Grasp the spark plug wire by the insulation and hold the spark plug wire connector one-eighth inch from the spark plug terminal. Fold a hand towe! anil place it between your hand and the engine head so that your hand can rest firmly against the engine to hold the wire steady. The cloth towel will help prevent accidental shock.
3. Crank the engine vigorously.CAUTION; THE ENGINE COULD START BE PREPARED? The spark should jump the combined gap easily and steadily. It should jump the one-eighth inch gap you are creating outside the engine and the gap of the spark plug. Failure to pass this test could be the fault of the spark plug. If the spark will not jump the one-eighth inch gap or is unsteady, perform the next test.

SPARK TESTER METHOD

A number of spark testers are available but a very satisfactory one can be made by using a new spark plug. Be sure to select a spark plug which is not the resistor or booster gap type. Adjust the tester electrodes to make a three-sixteenth inch gap.

Connect the spark plug wire to the tester and hold the tester firmly against the engine. Make sure that the tester is making contact with the bare metal of the engine because paint may insulate the tester from the engine.

Spin the engine rapidly. A steady spark across the gap indicates that the ignition system is functioning satisfactorily, in this method the spark is jumping only one gap and the results are more reliable. Also, the engine will spin much easier and there is no chance of the engine's starting with the plug removed. If the plug is removed, see the section on Spark Plug Service.

BASIC CARBURETOR ADJUSTMENTS

Three basic adjustments are necessary on most carburetors. These adjustments are made in the order listed below. The needle valve seats are made of soft brass and can be easily damaged. When turning the needle valve in, take care not to damage the soft brass seat. Loosening the packing nut will allow the screw to turn easier. If the fuel leaks around the adjusting screw, tighten the packing nut (see illustration).

THE LOAD-SPEED AIR FUEL MIXTURE

Start the engine and allow it to run at least three minutes to warm up before adjusting. If the engine will not start, carefully turn the load-speed needle valve itt (clockwise) until it closes. Do not force it! Back the screw out approximately one and one-half turns. This is not a correct adjustment, but it is rich enough to start most engines.

if the engine surges or runs roughly and emits black smoke from the exhaust pipe during warm up, the mixture is too rich. Turn the needle valve in (clockwise) slowly until the engine smooths out. If the engine surges and runs roughly without emitting black smoke, turn the mixture screw out (counterclockwise) to smooth out engine performance. Ignore blue smoke coming from the engine exhaust pipe at this time. It is the result of oil burning in the engine and has no effect on the carburetor adjustment unless it is excessive.

After the engine is warmed up, bring the engine to normal load-speed -near wide open throttle. Caution: Do not operate the engine at full throttle when it is not under normal load because the excessive speed will damage the engine. Turn the needle valve in until the engine begins to surge or run unevenly. This is now a lean adjustment. Turn the needle valve out until maximum smoothness and top speed are reached. Turn the needle valve slowly back and forth to find top speed without moving the throttle. If the engine will be pulling a heavy load, for example, a tiller, turn the needle counterclockwise up to one-eighth turn to slightly enrich the mixture for maximum power.

THE IDLE-SPEED AIR-FUEL MIXTURE

If the engine has an idle-fuel mixture adjustment, it should be adjusted after the load-speed mixture has been set and with the engine at near correct idle speed (fast idle). Again, turn this mixture screw in (clockwise) until the engine begins to run roughly and then back out to maximum speed. Move the adjustment screw back and forth slowly until maximum speed and smoothest operation are obtained.

THE IDLE-SPEED ADJUSTMENT

The idle-speed adjustment is a stop screw on the throttle shaft which prevents the throttle from closing farther. Single cylinder, air-cooled engines idle much faster than multicylinder engines. Normal idle speed for engines of less than ten horsepower is 1750 to 1800. If the engine will not accelerate properly from idle after the idle speed is set as described above, the idle speed may be too slow. An engine that idles too slowly will overheat because the air flow through the cooling fins will not be sufficient to cool the engine.

Adapted from:

SMALL GAS ENGINES

James A. Gray and Richard W. Barrow

School of Technology Indiana State Univetsitv

Prentice-Hall, Inc., Englewood Cliffs, New Jersey

Drawings by Robert F. MacFarlane

Retrieved from the CD3WD project.
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