Checking with a vacuum gauge
Connect the meter to the intake manifold. Warm up the engine and let it idle.
If the engine is in good condition, the gauge should read 17-22 inches and the needle should twitch evenly.
If the meter readings are lower than required, the following are likely causes: a leaking gasket between the intake manifold and the carburetor or throttle body, a leaking vacuum hose, or an out-of-adjustment of the ignition timing or camshaft rotation angle.
If the meter readings are 8 inches below normal, the intake manifold gasket is leaking or the injector is faulty.
If the needle drops 2 inches below the normal mark at regular intervals, the valves are leaking.
If the arrow jerks downwards very unevenly, the valve is stuck or the spark plugs are faulty.
Four inch needle vibration and exhaust fumes - valve guides are worn.
Rapid vibration when increasing engine speed - a leaking intake manifold or head gasket, worn valve springs, burnt valves, or faulty spark plugs.
A slight twitching of the arrow near the normal 1 inch mark indicates problems with the ignition.
Strong twitching of the needle - the cylinder head gasket is damaged.
If the needle slowly moves over large distances, the fuel is not mixed correctly, or the intake manifold or throttle body gasket is leaking.
Increase the RPM to 2500 and turn off the engine, the needle should drop to almost zero, then rise 5 inches above the normal mark and return to the normal mark.
If this does not happen, then the piston rings are worn out.
If there is a long delay, the exhaust system is clogged.
Compression check
1. Disable the ignition system by disconnecting the ignition high tension wire from the distributor cap and grounding it to the cylinder block.
2. Connect a compression tester to the spark plug hole of cylinder N1.
3. Open the throttle valve fully and crank the engine crankshaft with the starter. After one or two revolutions, the compression pressure should rise to the maximum mark and then stabilize. Write down the result obtained.
4. Carry out a similar check with the remaining cylinders and record the readings obtained.
5. The pressure in all cylinders should be the same. A difference of more than 2 bar between any two cylinders indicates a malfunction. In a healthy engine, pressure should build up quickly. Low pressure in the first stroke, gradually increasing in subsequent strokes, indicates that the piston rings are worn out. Low first stroke pressure that does not increase with subsequent strokes indicates a valve leak or a blown head gasket. Deposits on the bottom of the valves can also cause low compression.
6. If you have any doubts during the inspection, please contact experienced specialists.
7. If the pressure in any cylinder is too low, perform the following check to determine the cause. Pour a teaspoon of clean motor oil into the cylinder through the spark plug hole and repeat the pressure check.
8. If the compression pressure increases after adding oil, this means that the piston or cylinder is worn out. If the pressure does not increase, then the cause is worn or burnt valves or a blown cylinder head gasket.
9. Low pressure in two adjacent cylinders almost always means that the head gasket between them is blown.
10. If one of the cylinders is approximately 20% lower in pressure than the others and the engine is not idling properly, this means that the cause may be a worn camshaft cam.
11. If the pressure is too high, then most likely the combustion chamber walls are covered with carbon deposits.
12. Once the check is complete, screw the spark plugs back into place and connect the ignition system.