See Figure 1
A noticeable lack of engine power, excessive oil consumption and/or poor fuel mileage measured over an extended period are all indicators of internal engine wear. Worn piston rings, scored or worn cylinder bores, blown head gaskets, sticking or burnt valves and worn valve seats are all possible culprits here. A check of each cylinder's compression will help you locate the problems.
As mentioned in the tools and equipment of portion General Information & Maintenance , a screw-in type compression gauge is more accurate than the type you simply hold against the spark plug hole, although it takes slightly longer to use. It's worth it to obtain a more accurate reading. Follow these procedures:
- Warm up the engine to normal operating temperature.
- Remove all the spark plugs.
- Disconnect the high tension lead from the ignition coil.
- Fully open the throttle either by operating the carburetor throttle linkage by hand or by having an assistant floor the accelerator pedal.
- Screw the compression gauge into the No. 1 spark plug hole until the fitting is snug.
- Ask an assistant to fully depress the accelerator pedal. Then, while you read the compression gauge, ask the assistant to crank the engine 4 or 5 times in short bursts using the ignition switch.
- Read the compression gauge at the end of each series of cranks, and record the highest of these readings. Repeat this procedure for each of the engine's cylinders.
A cylinder's compression pressure is considered within specification if the lowest reading cylinder is within 75% of the highest. The minimum acceptable pressure for these engines is about 100 psi.
- If a cylinder is unusually low, pour a tablespoon of clean engine oil into the cylinder through the spark plug hole and repeat the compression test. If the compression comes up after adding the oil, it appears that the cylinder's piston rings or bore are damaged or worn. If the pressure remains low, the valves may not be seating properly (a valve job is needed), or the head gasket may be blown near that cylinder. If compression in any two adjacent cylinders is low, and if the addition of oil doesn't help the compression, there is probably leakage past the head gasket. Oil and coolant water in the combustion chamber can result from this problem. There may be evidence of water droplets in the oil film on the engine dipstick when a head gasket has blown.