Many problems can cause low compression in your scooter's motor. Some of them are internal -- like bad rings -- and can only be found by disassembling the motor. Others are visible from outside the motor -- like a loose spark plug -- and can be found by simply giving it a good physical inspection. But all of these problems leave a few telltale signs that something is wrong and that low compression may be the cause.
Motor Will Not Start
Sometimes the compression falls so low that the motor simply will not start. This symptom does not automatically mean that low compression is the culprit. The problem could be something as simple as an empty gas tank or a spark plug wire that has become disconnected. When faced with any symptom, it is best to eliminate all of the simplest possible causes first before assuming the worst. It also matters whether the motor makes any effort to start; obviously, a completely dead motor has a potentially worse problem -- such as seized parts inside the motor -- than one that tries to crank. If a compression problem is keeping your scooter from starting, it could be a very expensive repair.
Motor Runs Rough
It is also possible that the motor will start, but you have to crank the motor repeatedly and you cannot predict when it will finally start, if at all. Once the motor starts, it runs unevenly. You "give it the gas" but it does not rev smoothly. While low compression can often be an expensive problem, the fact that the motor runs is encouraging. Compression is best tested while the motor is warm and running, so a possible compression problem is likely to be less serious -- still expensive perhaps, but maybe less than a complete engine rebuild. The problem is more likely to be something simple like a gasket that has gone bad when the motor can run.
Motor Has No Power
Your scooter may start and even idle reasonably well, but once you start to accelerate, all bets are off. Sometimes it accelerates slowly, struggling to get up to speed; at other times it may start well enough but quickly lose power and slow to a crawl. Hill-climbing may become impossible; you may even have to help your scooter on mild inclines. While this is a frequent symptom of compression problems, it may also be a symptom of clutch or chain problems. Again, it is best to check these simpler (and potentially less expensive) problems before assuming you have low compression.
One of the most common traditional symptoms of low compression is the smell -- and possibly smoke -- of burning oil coming out of your exhaust. If this symptom shows up in conjunction with one of the other symptoms, a compression check is definitely in order.