So, you have a motorcycle that won’t run. The first thing you probably want to know is how much is it going to cost me to fix it and can I do it myself and this technique will answer that. For this procedure you will need only two simple tools- a spark plug socket, ratchet to fit and an extension if necessary and a compression tester you can purchase at any auto parts store, Sears or Walmart. You don’t need anything special in a compression tester but make sure it is the type with the screw in to the spark plug hole adapter. This is a good tool to invest in because along with a spark plug wrench it is all you need for any diagnosis. Also the dial should read foot-pounds to 150 at least.
Remember this- Spark, Compression and Fuel. That is all there is to any running engine. It must have all three. So begin with spark. Pull all of your spark plugs and note which hole each came out of. Also look at the tips. Oily, black indicates oil leaking past rings or valves: black sooty and dry indicates too rich a fuel mixture: white is too lean and tan to dark tan is what you want ideally. Next put the plugs back in the wires and turn the engine over watching for them to spark. This is the spark test. If one or more does not spark consider wires, plug itself, ignition last (points or electronic). Just the spark test can often lead to the problem.
Next since you can’t test fuel flow through the carburetor easily do your compression test and if it checks out and spark is alright then you must have a fuel problem. Again refer to the tips of your plugs to confirm this. Compression must be above 90 ft-lbs for an engine to run but should be over 100 ft-lbs. Low compression indicates worn cylinders or valves or rings so when you find low compression you may be looking at a more complicated engine tear-down. Let’s say your spark on each cylinder checks out and your compression is around 120 ft-lbs for each cylinder but one is low. This would indicate a possible blown head gasket as would low compression in any cylinder but could also be mechanical problems in one cylinder such as a bent valve. This is also an end to the test because there is nothing simple you can do about low compression in one cylinder. Even if the engine will run it will lack power and just get worse very rapidly.
If you are having running problems and the spark and compression check out OK then its fuel. Either too rich or gasoline has gone to gel and plugged up the jets in the carburetor or in some cases too lean. Motorcycle carburetors have two kinds of jets which are simple brass plugs with a metered hole in them to allow for fuel flow. You can remove the carburetors and remove the jets(one main jet and one pilot jet) and clean them. Take care not to gouge the soft brass and use a small piece of copper wire to poke out the gelled gasoline using carburetor cleaner or B-12 Chem Tool.
Basically that is all there is to it. As I said remember Spark, Compression and Fuel and let them be your guide. You must have all three for an engine to run and if even one is missing the engine will not run. Use this test anytime you have an engine that won’t run.