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In principle
Warm up engine, turn it off.
Remove a plug, replace it with compression guage.
Disable Ignition system so engine can't start.
Crank engine over on starter with accelerator pedal held to floor until guage reading stabilises ( about 5 seconds should be enough).
Note reading
Squirt a but of engine oil into the cylinder (about half a teaspoon) and repeat the test
Note this oily reading.
Repeat for each cylinder.
Don't worry too much about the actual readings, it's more important that they're all within 10% of each other (without oil in the cylinder).
A cylinder whose compression increases when you add oil may have worn piston rings, a broken land or a scored bore.
Two adjacent cylinders with low compression might mean the head gasket is gone between them.
A cylinder with lower compression than the others which doesn't come up when you add oil might have worn valve seals or valve guides.
Guage readings are usually roughly in the 100psi to 200psi range. Someone with a manual for the 1.5 might be able to tell what the standard compression should be.
It might recommend the best way of disabling the ignition aswell.
On older carb equipped cars you can just pull off the low tension coil cables, newer cars with big brother ECU's can need a bit more care. You may have to pull all the plugs out at once and earth them securely on the block or remove the Ignition fuse or put the ECU into test mode.
I'm not familiar with the 1.5 so I can't say for certain, my old BG 1.6 SOHC had a carb (How quaint!).
Hope I haven't bored you to tears with all this,
Ed
PS Always take any conclusions from a compression test with a grain of salt.
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