When troubleshooting a motorcycle, first start with the basics. These simple tests can work for any small engine. All engines need three basic things to run fuel, spark and air. These simple things can cause your motorbike to not start up or run poorly. First start off with the fuel remove the hose off the tank and make sure the fuel is flowing properly. Some bikes will have to turn the fuel selector to prime, not on, or fuel will not flow. Take a flashlight and look down into the tank, is the fuel varnished or is there dirt in the tank? Older motorbikes had a problem with tanks rusting inside and would plug the screen up inside the tank. Fuel delivery may still be a problem but we will come back to that later.
Next remove the spark plugs, you may need to remove the tank to reach them. Inspect every spark plug as you take them out. All the spark plugs should look the same. If a plug looks white colored, that cylinder may be (lean) and not getting enough fuel. If you have a plug that looks wet and black, that cylinder may getting too much fuel (rich) or not sparking good. Remove all the spark plugs and snap the plugs back into their wires. Set the spark plugs on the engine so that they can ground themselves. Crank the bike over and watch the plugs for sparks, preferably somewhere dark. Make sure you look at all the plugs to make sure they are all sparking. If one spark looks weak check the wire and plug, if old or worn replace them. If their is no spark the coil may be bad.
If you have any test equipment such as an ohm meter, you can find out what the resistance reading should be for your bikes coils. This way you will know for sure if the coils are o.k. If the ignition system looks fine move on to the carburetors. First, if the bike has not been maintained recently (a problem in itself) make sure the carbs are synchronized. To do this you need a vacuum gauge made for this. Remove the small rubber plug located between the carbs and the cylinder. Attach the gauge and take the readings from each cylinder. Replace the caps when not measuring or it won't run right. Adjust the throttle screw or linkage for that carb until they all read close to one another.
Refer back to inspecting the spark plugs. If one cylinder looked lean or rich check all hoses for cracks and air leaks. If the motorcycle has not been run in awhile the carbs may be (gummed up). Remove the float bowl off the bottom of the carb. Look in the bowl for dark varnish looking gas. If the fuel looks dark you will probably have to remove the pilot and main jets and carefully run a wire through the center hole to clean them out. Be careful to not bend the float as the height needs to be correct to run well also. Check that the small needle attached to the float between the pivot point, is moving up and down. If it doesn't move smoothly the carb will not fill up with gas or will run low as your driving down the road.
Make sure you check the battery also, if they become weak the ignition system won't have enough power to keep the bike running smoothly. If you still haven't found anything make sure you valve lash has been checked at the correct mileage intervals. If not this can cause many problems with the way it runs. Also you can screw a compression gauge into the spark plug holes and check that they are within 5-10% of each other. If one is way down you have a serious problem either with the pistons, bore or valves. This should be a good start to finding a general problem with your motorbike.