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Leak down Test and Compression Test

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Leak down Test and Compression Test (as they apply to the 4stroke DRZ)

These two diagnostic procedures are often confused, misused, and why faulty data results.

Many motorcycle mechanics prefer a leak down test to a compression test for a verity of reasons. Personally, I feel I have more control over the testing procedures, and a more accurate ability to identify where the mechanical fault is from the overall diagnostic procedure then with a compression test.

A leak down test uses an instrument to determine the condition of the engine by introducing compressed air into the cylinder and measuring the rate at which it leaks out.

Dual dual gage leak down tester.

snap on leak down.jpg

A common dual gauge tester allows you to set the pressurization of the cylinder. Set the feed gauge to 100 psi, make the connection to the spark plug adapter, and then re adjust the feed pressure to 100 psi. That pressurizes the cylinder though a fixed size orifice. The second gauge indicates the pressure in the cylinder. The difference between the two is the percentage of leakage.

Compression testing is a basic form of leak down testing which also includes effects due to compression ratio, starter/battery condition; automatic decompressor built in to the OEM and some aftermarket cams. Other variables are introduced by the user such as length of cranking period.

Leak down testing confines the results to cylinder leakage alone. Because it is done with the engine which is not running (Static), leak down testing removes many of the data skewing variables found with a compression test.

Compression testing has an additional issue of testing many components at once, all while the engine is cranking over. Making it difficult for the mechanic to isolate an issue with any one engine component.

Leak down testing is done on a static engine with the cylinder at top dead center. In most all cases, the tester will need to hold the piston and crank at Top Dead Center (TDC) through mechanical means. On the DRZ that can be done with a flywheel holding tool for the S, SM and E models using an OEM flywheel. For the K model, or those engines using an aftermarket flywheel without the cast in HEX. You can hold the engine by placing it in gear, and using that to hold the crank in position. It is much more difficult, to keep the crank at TDC as required this way. And not recommended as a standard procedure. There are other ways to hold the engine at TDC, from adapters that both feed compressed air into the cylinder and act as a piston stop, to holding the crank in place by the primary gear side. On one DRZ engine with a TrailTech flywheel I was working on, we built a simple tool with a length of drive chain.. that held the crank in position via the countershaft sprocket and transmission. The engine was bolted to my stand, as was the adjustable chain tool. Engine placed just before TDC, tool was attached to the table and sprocket, adjusted till the crank just rotated to TDC. Way more work than it was worth.

Once the crank is locked in place at TDC, Pressure is fed into a cylinder via an adapter and spark plug hole. The flow, which represents leakage from the cylinder, is measured converted to a “percentages”.

Leakage is given in completely arbitrary percentages. Simple based on pressure in, and pressure leakage. These percentages do not relate to any actual quantity or real dimension. The relative meaning of the percentages is only relative to other tests done with the same design and make of tester. This cannot be over stated.. Like a Dyno reading.. It is variable based on equipment.

Leak down readings of up to 10% are usually acceptable in a DRZ type engine. But a 1- 5% range where I like my engines to be for top performance.

Advantages to a Leak down test Vs a compression test.

As the leak down tester pressurizes the cylinder; the mechanic can listen to various parts to determine where any leak may originate. In example, a head gasket would likely cause cause bubbling in the cooling system. Or a leaking exhaust valve will cause a hissing noise in the exhaust pipe. Leaking intake valve will result in hissing noise at the carb.

Most leak down testers use 100 psi as the standard input pressure. But most any pressure from about 20psi and up can be used for measuring the percentage of leakage. Using a higher pressure like 100psi will make the sounds of leakage louder and easier to identify. As mentioned above, engines pressurized to 100psi must be locked at exactly top dead center or it will rotate under the pressure. There is a limit to more is better when it comes to pressure.

Pressure above 100 psi will yield no better results, and only place more stress on the engine, the holding device. And in some regards increase the safety risk should something go wrong.

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