Here's what you need to investigate to determine *why* the compression is so low:
* Worn piston rings: This is the most likely culprit. Worn rings allow combustion pressure to escape past the pistons into the crankcase.
* Burned or damaged valves: A valve that's not seating properly will also cause significant compression loss. This could be a burned valve (common with overheating or lean running conditions), a bent valve, or a damaged valve seat.
* Head gasket leak: A leak in the head gasket between that cylinder and the cooling system or crankcase will also result in low compression. You might see milky oil (emulsion of oil and coolant) or white smoke from the exhaust if this is the case.
* Cracked cylinder head or block: A crack in the head or block, though less likely, is also a possibility and would require significant repair or replacement.
Next Steps:
1. Compression Test Verification: Before doing anything else, *verify* the compression test. Ensure the battery is fully charged, the throttle is fully open during the test, and the test is performed correctly. Do a wet compression test (add a small amount of oil to the cylinder) on that cylinder. If the compression increases significantly with oil added, it points to worn piston rings.
2. Leak Down Test: This test is far more definitive than a compression test. It identifies *where* the compression is leaking. A leak down test will pinpoint whether the leak is past the rings, valves, or head gasket. This is a very important step.
3. Visual Inspection: Inspect the spark plug for excessive oil or coolant. This might give a clue about internal damage.
In short: Don't waste time on further tune-up work until you address the severely low compression. The engine needs a proper diagnosis to identify the cause of that low compression before any repairs can be made. Worn piston rings, burnt valves, or a head gasket leak are strong possibilities requiring major engine work. A mechanic specialized in these types of repairs would be beneficial at this point.
