Compression is a term used to describe the volumetric ratio of the uncompressed volume of the cylinder to the volume 'compressed' between the piston crown and cylinder head.

The use of a compression tester or compression gauge can be useful in determining the amount of compression your moped is getting. Without a sufficient amount of compression, your moped will not run. Typically (in the United States), compression is measured in PSI, or Pounds per Square Inch. Different models of mopeds require different minimum amounts of compression to run.

When your moped does not have enough compression, it will not run. When it has enough to run, but not enough to run optimally, it will have poor acceleration.

The more compression you have, the more low end acceleration you will have. However, if you have too much compression, it can cause your moped to 'knock' or 'ping' which means you are pre-igniting. Pre-ignition is bad, and it will ruin your moped. One solution is high octane gas. Another option is to retard your timing. Yet another option is to add a head gasket to reduce compression.

High compression will also often make it difficult for you to pedal your moped to start it. One solution has been for some moped manufacturers to place a 'decompression valve' on the cylinder head, which allows for easier starting. Having extremely high compression will never make it impossible for you to pedal start your moped, or 'kick it over.' If your moped won't turn over at all when you pedal, you probably have a different problem.

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