We have all been there before. The bike ran before you put it away the last time, and now it won’t start, or it runs very poorly. Motorcycles (unless electric) use combustion engines which work on very basic principles. They need air and fuel and a way to ignite it after it becomes compressed. Let’s start with the simple stuff.
If your bike has electric start and it will not turn over, check the charge on your battery with a voltmeter. If you know it’s low, you might want to charge the battery first. If the battery won’t hold a charge, you should replace it. If you’re sure that the battery is charged and the electric starter just clicks, then the motor might be frozen. This could be due to a more serious problem than we will cover in this post.
OK, so now the motor will turn over either with the electric starter or with the kick starter, but it still won’t fire. Fuel in almost all cases on motorcycles consists of gasoline. Most bikes will run on pump gas, there are a few bikes which will run on race gasoline. We’ll assume you know what you have. If you are running pump gas, and the bike has been standing longer than a few months, you may want to replace it with fresh gas. Modern pump gas has a relatively short shelf life and will attract moisture causing it to be less effective. So as a rule of thumb, if it’s older than a couple of months, drain the fuel and replace it with fresh gasoline.
Next, if you have a carburetor and not a fuel injection system, you will want to make sure that you drain any bad fuel from the carburetor. Make sure the fuel switch on the tank is in the off position. Most carburetors will have a small screw at the bottom of the float bowl to drain the fuel, some carburetors only have a largish plug/screw at the bottom of the float bowl. Place some kind of pan under the bike to catch the fuel and then open the drain screw/plug and let the fuel drain out. Close up the drain and turn the fuel on and try starting your bike again.
If it didn’t start, you might want to open the drain screw/plug again to make sure fresh fuel has made it into the carburetor when you tried to start it. If here is no fresh fuel in the float bowl, then then you might have a clogged fuel line, or petcock, or fuel filter. Many street legal bikes have a vacuum operated petcock. This petcock will only open when there is engine manifold vacuum, like when it runs or you are trying to start it. Sometimes the vacuum line can be leaky or get clogged. Check and possibly replace te vacuum line. The diaphram in the petcock can be torn, in which case it will not operate. Some of these petcocks have a “Prime” or “Resrve” position that by-passes the vacuum operated function. Try that position. If the petcock will not operate, it needs to be replaced.
We’re sure the fuel is getting into the carburetor, but we’re not sure it’s making it into the cylinder. Take out the spark plug and see if it as some fuel on it, or it smells like fuel. If it’s dry, then the carburetor is not atomizing the fuel into the air. Old fuel that has water in it, will produce by-products that can clog up small jets and passages in the carburetor. Most likely one or more of the jets and/or passages is clogged and you will need to remove the carburetor and have it cleaned/rebuild by someone who has experience in this. You can try doing this yourself, but you should obtain instructions and/or a parts diagram of the carburetor. Many bike die because someone inexperienced will have tried to rebuild/clean a carburetor and not reassemble it correctly, lose parts, etc… Older carburetors tend to be simpler and your success rate will be higher. Modern carburetors can very complex.
If you have fuel injection, you will have skipped the instructions for draining the fuel bowl, and checking the fuel lines/petcock. You should listen for the fuel pump when you turn on the power. If you do not hear the fuel pump, check the fuse(s) for the fuel pump and the ECU. Then remove the spark plug and if it’s dry, fuel is not getting into the cylinder. If you are adventurous, you can obtain the service manual for your bike and find out how to read out diagnostic codes from the ECU and look up what they mean. Often times, it will narrow down the problem to a sensor that can be replaced.
OK, if your plug is wet, we need to check if your ignition is working. NOTE: DO NOT CHECK IF YOUR SPARK PLUG IS FIRING WITH THE SPARK PLUG REMOVED FROM THE ENGINE. Spark plugs ignite fuel/air mixtures and when the spark plug is removed, fuel/air mixture is pushed out the plug hole and the fumes can be ignited by the spark plug and catch your bike or yourself on fire. Get a new spark plug and install it. If this doesn’t fix it use the old spark plug, plug it into the ignition system and ground the outside of the spark plug by holding it against the engine. You should see a mild spark when you kick over / start the engine. If you see no spark, then try another spark plug, before replacing the spark plug cap, wire and coil in that order.
If you made it this far, the likely cause is a bad connection in your wiring harness/connectors causing the CDI not to function or the ignition coil. It’s unlikely that it’s the CDI unit itself, but they can fail as well. Some bikes use a separate ignition coil in the stator from the battery circuit to power the CDI. Using a DVM you should check the resistance of this coil and compare it the value that the service manual calls out. It should be more than zero Ohms and less than open, usually in the mid tens to hundreds of Ohm value. Replace coil, if necessary.
So, you have fuel and spark, now, there is one more thing to check. Compression is needed by a combustion engine to function properly. Use a compression tester to measure how much compression you have. These testers are be rented at a auto part store and comes with some adapters that are used to screw the tester into the spark plug hole. Follow the directions for your specific motorcycle to test the compression. If the compression is low, then you can check the valve clearance. If the clearance (or valve lash) is too tight, the valves may not close all the way and fuel/air mixture can leak out.
At this point, you have exhausted all the simple things that could be wrong. This is a good place to step back and take a break. Perhaps, get a buddy and go through all the checks again; maybe you missed something the first time through. When you get here, the most likely problem lies within the motor, things like rings or worn valves maybe the cause and require a partial or total rebuild of your motor. You may need to ask yourself whether it makes sense to rebuild the motor, financially. I.e. it may cost more than the bike is worth, especially if the bike is older, but not yet of vintage or collector status.