You sling a leg over your bike, reach down for the key, push the starter or give it a kick and…no bueno, she does not start!
There’s only one way to know why a bike won’t start, and the process begins with knowing how an engine operates. It also doesn’t hurt to know some of the history of the machine in question.
From there, it’s all trial and error which involves running down the list of possible scenarios putting your theories to the test.
Step one in your personal diagnostic checklist is to review the history of the bike.
- How long did the bike sit idle? Was it a runner before it was parked? Did it start and run just fine yesterday and crap out today?
- Have you done any recent repairs, added some custom parts, messed with the electrical system?
- Has the bike been laid down in a parking lot recently? Taken a detour through a farmer’s field? Been dipped in a local lake?
- Did you notice any strange sounds shortly before the problem happened?
- What’s the weather like outside? Wet? Cold? Incredibly hot and humid?
- Did you fill it up out of a gas can at someone’s garage? Leave it unattended overnight in a bar parking lot while you slept on the pool table inside?
All of these could factor into your thinking, so you have to eliminate the least likely possibilities and focus on the ideas which are the most likely – it’s Occam’s Razor, and can use that kind of thinking to cut through the crap and find a solution to the problem.
If the bike sat idle for a long time, start with that in mind. It started and ran fine when you put it away, but it sure doesn’t now, and that’s telling stuff. The odds are that it doesn’t have any serious mechanical problems. Electrical issues, same deal here, unless someone took their pocket knife to the wiring, not much can go wrong with electrical systems that won’t display telltale signs. Pull the plug wires, lay the plug against the jugs, and check for a spark just to make sure. No spark? No fuego.
What’s left? Bad fuel, my friend. Bad fuel in the tank and carb is the cause of the vast majority of no-start problems. Water in the lines. Clogged fuel filter. Fuel systems have the highest likelihood of failure, and that makes the fuel system a good place to begin your investigation.
Don’t make the mistake of thinking about compression or tearing down your top end to fix a problem caused by a dirty carburetor. Since there are really only three things that make an engine fire – fuel, spark and compression – make sure all those systems are go before you look for a more complicated solution.
Occam’s Razor? It’s your pal, follow the principle.
Start by making certain the carburetor is getting fuel. Pop open the overflow at the bottom of the float bowl, and if you have a handful of gasoline, you know you have fuel making to the carb. And look for simple things like checking to see if the petcock is in the open or reserve position. No fuel, you might be facing a bad needle or a seat stuck in the closed position. Rap the side of the float bowls and it might come unstuck before you tear down the carb entirely.
If you do have fuel to the carburetor, and still no start, clear out all the fuel in the tank and lines and try to start and run the bike with the choke on. If the engine conks when you turn the choke off, you’ve got a bad pilot jet and a rebuild kit is in your future.
Pull a plug out, leave it in the cap attached to the plugwire, and ground the threads to something ferrous and metallic – aluminum parts won’t do the trick, pard. Kick the motor over or hit the switch and if you don’t see a bright blue spark, a decent 1/4″ gap, and a clean ceramic, you found your problem. If the spark plug fouled and coated with oil or gas, move on to test your coil. If the coil is giving you some output, you’re into something heavier like a pickup coil or a bad CDI box. If you think the CDI box is bad, don’t even bother to try and test it, just find a new one and install it. They don’t come cheap, so check every other possibility before you take that kind of step.
Bad switch? Could be. Test it by unplugging it and looping the connection to make it hot and check for spark again.
Pull your spark plug and hook up a compression test gauge. Open the throttle and kick the motor until the dial stops moving, and if you have anything over 100psi, you’re probably good to go here. If you have no compression or a very low PSI reading, you have problems with your piston and rings, and it’s time for a top end rebuild.
The most important thing to keep in mind is that, though modern engines are considerably more complicated than old-school twins, what makes a motor run ain’t rocket science. Fuel. Spark. Compression.
Scott McCrorie of Long Beach, California demonstrates how he kick starts his custom 1939 Harley-Davidson ULH:
Insuring your collectible or vintage motorcycle
You can spend a lot less, but if you plan to ride the bikes in your collection, the above pricing is a reasonable approximation of what you can expect to pay.