What to do when your motorcycle won't start

When your motorcycle refuses to start, what do you do? You start troubleshooting, and your process should begin by remembering that in order to run, an internal combustion engine needs three things: compression, fuel, and spark.

Editor's note: A version of this article originally appeared in the first issue of Common Tread Journal, a print magazine that is free to RPM members. Learn more about the rewards, discounts, and other benefits of Riders Plus Membership.

Compression

If your bike isn’t even turning over, then you definitely won’t have compression. A non-cranking condition could be due to a dead battery, blown starter-motor fuse, faulty starter-motor relay, a safety override (like the kickstand being down while the bike is in gear), or perhaps even something as basic as the kill switch being in the off position. Hey, we all have brain misfires sometimes.

As always, check the simple stuff first. That means instead of tearing into the starter motor to inspect the commutators, use a multimeter to verify that the battery is showing at least 12.0 volts. If it reads lower than that, put it on a charger and come back later. Once you know that that battery is sufficiently charged, you can use your multimeter to see where things are getting held up. Is it a fuse? The relay? The ignition or starter switch? As a matter of course, pluck at the lowest-hanging fruit to conserve time and get as many possibilities crossed off the list as quickly as possible.

With the bike cranking, you can now remove a spark plug and screw in a compression tester, which will indicate how effectively the piston is squeezing the intake charge. Most engines create about 150 to 200 psi of pressure, but that's once they’re good and warm. Our problem bike isn't running, however, so an accurate hot compression test isn't possible. Suffice to say you should get at least 100 psi from a cold engine; anything less and you’ve got a problem. It could be worn or broken piston rings, or perhaps it's just choking on a majorly dirty air filter. Again, check the simple stuff first.

Spark

Spark is the match that lights the fire, and you can verify its presence by unscrewing a spark plug, laying it against the cylinder head to ground it and cranking the engine. In a darkened garage, you should see a sharp whitish-blue spark jump across the gap. If there's no spark, or if the spark is orange and feathery, some aspect of the ignition system isn't working as intended.

Try replacing the spark plug, and check the plug boot and wire for looseness or obvious damage, like a tear in the insulation. Beyond that, ignition-system troubleshooting is best accomplished with the help of a multimeter and a model-specific service manual.

Fuel

Compression and spark aren't much good if they're not acting on a mixture of fuel and air. Before you concern yourself with air-fuel ratios, though, remember our golden rule and, you guessed it, check the simple stuff first.

That means giving the tank a shove to see if there's actually fuel sloshing around in there. Then pop the cap and waft some air toward your nose. The gas should smell fresh and sharp, not stale and sweet. Old gas doesn't combust particularly well, and untreated fuel can go bad in as little as 30 days.

With modern EFI bikes, the only thing a home mechanic can easily check for is fuel-pump operation. With the key on, you should hear a hum emanating from under the tank for a few seconds when you flip the kill switch on. That's the fuel pump pressurizing the system. If you don't hear it, check the fuse, sidestand switch, and tip-over sensor.

On carbureted bikes, verify fuel flow at the carburetor(s) and petcock. Pull the fuel line off the output side of the petcock or inline fuel filter, and with the valve in the “on” or “Pri” position (that stands for prime, which bypasses the vacuum-operated valve used on many newer carbureted bikes) it should flow freely. Likewise, opening the drains on the carbs confirms that fuel is getting at least that far.

Then there’s the question of whether that fuel is being delivered in the correct quantity, which is much harder to confirm. Float-bowl height, clogged jets, dirty air filters, and air leaks will all throw the air-fuel ratio off. Implementing our golden rule, I'd suggest inspecting the air filter first. Beyond that, experience suggests that the pilot jet is the most likely culprit within the carb circuitry, but carburetor troubleshooting could be an article in and of itself.

Always check the easy stuff first

While a systematic approach is always a good idea, when I'm troubleshooting a non-starting bike I usually work out of order and group the easiest checks first. So I'll check the fuel tank for an appropriate amount of (good) fuel, then test for spark by laying a plug on the head and cranking the engine. At that point it's easy to spin the compression tester in and see if I've got workable numbers or if there are bigger problems within the engine. With a strong spark and compression figures, I'll then turn my attention to fuel delivery, which nine times out of 10 is where the problem lies (at least with carbureted bikes). On modern EFI bikes, fuel-delivery issues are much harder to diagnose.

Understanding the essentials and following a step-by-step approach is the best way to uncover the problem that's keeping you from doing what you really want to do, which is ride your motorcycle.