If you have a sudden loss of compression, you probably won't be able to fix it in a parking lot or by the side of the road. What you are looking at is either head gasket failure, ring/piston failure, or valve failure, or any combination of the three. Often this is a sudden and violent event, accompanied by horrible engine noises and ominous clouds of smoke. I recommend you immediately shut off the engine to prevent further damaging the bike, and truck it home.
Here is how to check your compression and help differentiate between the possible causes of low compression:
If you have a very gradual problem of a bike running worse and worse, you may simply have valve components which are worn and in need of servicing. Here are two methods for adjusting valves:
However, don't let all this get you down. Just because you can't find the source of the problem in air, spark, or gas, doesn't mean you necessarily have a compression problem either. You may have a model-specific problem or something I have neglected to mention. However, a compression test should be amongst the first things you check.
Also, don't think that low compression necessarily means "motorcycle cancer." Anything on a bike can be fixed, it all depends on how much effort you want to put into it. Even if your motor is fried, you can always just replace it if you don't want to rebuild it. Here's how I did it on a Kawasaki ZX-9:
If you are not sure where to go from here or how to get started, contact me using the form on the home page.