Compression problems can be the most daunting of repairs. I used to repair low-compression motors, now I have found that it is far easier just to replace the motor. many motors are interchangeable, and are easily obtained if you live in a metropolitan area. However, if you do end up tackling a motor rebuild, by all means go for it! Nothing like a fresh motor in a 30 year old bike!
First, remember that if you have a motor with low compression in one or more cylinders, no amount of carb cleaning or tuning or modifications is going to make it run right. There is just nothing you can do besides getting that compression up. Sorry, but there isn't.
Typically there are three causes of low compression in a motor. The first is worn or misadjusted valves. This is easily enough repaired, either its time to re-shim them, adjust them if possible, replace valve guides, etc. Usually this can be accomplished with the motor still in the bike.
The second cause is a blown head gasket. Often I will try to re-torque the head before replacing the gasket, but even that isn't all that hard, you just need a good repair manual and a quality gasket.
Finally you may have worn rings, scoured cylinder walls, damaged piston, etc. This is what is involved during a total motor rebuild.
To determine which of these is the cause of your low compression, first you must determine what your stock compression should be. That you can find in your repair manual, usually something around 140-180. Then, do a compression test on all cylinders. Compare the numbers. If you have a cylinder that is more than 15% lower than the others, or if all the cylinders are low like say around 100, you have a problem. Finally, drip about a spoonful of motor oil into the spark plug hole and re-test the compression. If the compression goes up, you likely have ring failure. If it stays the same could be head gasket/valves.
