- Problems with fuel flow
- Problems with the spark
- Air/vacuum leaks
- Problems with the carburetor
- Problems with valve clearance
Fuel flow problems can be caused by a clogged fuel filter, a clogged fuel line or a faulty fuel supply valve (petcock). One way to bypass all these components is to rig up a temporary gravity fed fuel flow directly to the carburetor. If you connect a fuel supply directly to the carburetor from a container held above the level of the carburetor and the bike revs freely, you've identified the problem and you need to check (or replace) each component of the fuel supply to make sure it's working OK.
If the bike starts, it must be getting some sort of spark, but the spark may be weak or there may be a problem with the ignition timing. The first thing to do is remove the spark plugs and take a look at them. If there are heavy deposits on the tip (black or white), or if it looks worn or damaged or if the gap isn't what it should be, it should be replaced. You could clean it and regap it but plugs are cheap ($5 each or less), so you should probably replace them anyway and see if the engine runs better. If there's no difference, at least you now have a pair of spare plugs if the ones in the engine ever fail!
Today there are very few engines on which you can adjust the ignition timing. All timing functions are done via the CDI (Capacitor Discharge Ignition) module. If it's bad, all you can really do is replace it. They're pretty reliable though, so that might be something to try if all else fails to find a problem. The only way the timing can go wrong is if the camshaft chain jumps a tooth and that's not likely to happen to a running engine. If you've just rebuilt the engine then it's possible you didn't get the timing right.
Leaks in the air or vacuum lines can cause problems too. Usually it's leaks in the vacuum lines which control several functions related to the carburetor operation and fuel supply. First check all the vacuum line connection and the lines themselves for splits and cracks. If they look OK you can spray water on them while the engine is running and see if there is any change in engine speed. If there's a leak, the liquid will temporarily seal it and the engine will run better. Some recommend spraying WD40 or carburetor cleaning fluid on the lines, but if you do there can be a risk of fire, so be careful. Intake manifold leaks (i.e. leaks in or around the rubber boots which connect the carbs to the engine on one side and the airbox on the other) can result in unstable idling. Again you can srpray water or WD40 around them. If the idel speed changes when you do so, you have a leak.
Carburetor problems are the most likely source of poor running. First check the idle speed and pilot mixture screws. The mixture screws come sealed from Kawasaki so you have to remove the carbs and drill out the caps before you can adjust them. If the adjustment is OK, or you can't get the engine to idle well, then comes the next step. Over time (especially if the bike is stored for long periods of time with gas in the carburetor), "gummy" deposits can build up. These are the residues which are left behind when gas evaporates. They can clog internals valves, jets and passages inside the carburetor which can affect fuel flow. The best way to deal with them is to remove the carburetor from the bike, take it apart and soak it in carburetor cleaning fluid, which will dissolve the deposits. Make sure the jets are clear too by passing a very fine wire through them.
Valve clearance ("valve lash", "tappet clearance") can affect the way the engine runs. This is the clearance between the camshaft and the valve stems. If it is too large, the valve won't open all the way (and you'll get a loud "clattering" noise) and if it is too small (or non-existent), the valve may not properly close. Either way is a problem. You either won't get fuel/air into the cylinder properly or you won't get good compression and/or exhaust and the valve may overheat and suffer damage.
The carbs also need to be balanced, which is the process of setting them so that they both open by exactly the same amount. This is normally done at idle speed. However if you are going to do a valve adjustment, do that first since it can affect the carb balance.