Why won't my bike start!?
You've said it. You've heard it. And it usually comes shortly after realizing the bike is dead. It won't start and you've got no clue as to why. You can kick all you want even with the ferocity of the Incredible Hulk but she's asleep and unless you address the problem your riding day is done.
Making matters worse is the number of different reasons why. Something as simple as an empty gas tank (yep that happens and sometimes after an hour or two of tweaking, a lightbulb pops on over the rider's head) to something more sinister and complicated like compression issues.
This is why routine maintenance is so crucial. If you're keeping up on the pre- and post-ride visual checks and necessary fluid changes you're less likely to arrive at the track or trail, get your gear on, get amped up for a day of riding only to have your bubble burst while you try and figure out what's wrong as everyone else blasts around the track.
First off, if you've been riding and your bike won't start after a stall or hitting the dirt then read What is a Hot Start. In this case, it's likely you've flooded the engine so rule that out then read on here. Most of the issues addressed below occur on a cold start and that's usually the most common occurrence of a bike start fail.
Bad spark plug? You're lucky if this is the issue because it's merely a matter of replacing the bad spark plug which takes about a minute. Your stock of supplies should always include a new spark plug specifically for these types of incidents.
However, the bad spark plug is not that common especially in a 4-stroke. Besides, your routine maintenance should catch this problem before it becomes a problem but even the best of us overlook the obvious sometimes. Additionally, the spark plug might be fine but the connections could be loose and faulty. Check those.
It could also be the stator, the coil pack or problems with the kill switch. The stator acts much like a generator/alternator and works with the coil pack to ignite the spark plug. A faulty kill switch can prevent a bike from starting (it can also prevent it from shutting off too).
Did you put the gas on? You won't believe how often this simple remedy is overlooked, especially just before gate drop. Did you fuel up? An empty gas tank is second to the bad spark plug in terms of forgetting the obvious but once you've ruled these three oops! out it gets a bit more complicated.
If you're riding a carbureted dirt bike the fuel/air ration might be off. Too much air or too much fuel prevents the engine from firing. This goes back to stalling your bike after it's been running and needing a Hot Start. Yes, it's possible to flood a cold engine simply by transporting your bike. Perhaps you rolled the throttle unknowingly, flooding the engine with gas. If your bike has a hot start, it's worth a try but don't count on it.
A stuck float can also cause you fits so look for fuel dribbling out the overflow hose on the carburetor. Shake your bike really hard to resolve this issue! Actually, that is a solution so is tipping the bike on its side. The easier method is tapping on the carburetor at the float bowl. No not with your knuckles. Use a 3/8-inch socket extension or something similar. If the fuel stops flowing you've done the trick. If the problem persists check and adjust the floats.
A clogged jet is another culprit preventing carbureted bikes from starting, and likewise, clogged injectors on fuel injected bikes. Unfortunately you can't pop a bottle of STP in the gas tank. The best solution is using a fuel injection cleaner device for the injectors and for clogged jets you can use carb cleaner and compressed air or soak the jets overnight in penetrating fluid or just replace them. If this is the problem, use better fuel next time rather than gas from the town pump. If it's not the problem then it could be an electrical issue with the injector. Told you this could get hairy!
A faulty fuel pump or clogged fuel filter can prevent a dirt bike from starting and an elevation change can screw with the bike if it's improperly jetted for that specific altitude.
Compression/Valve Train Related
You might blame the rings but often when it comes to compression it's the valves. Therefore, start with adjusting the valves. This is the most common issue if it is valve train related. What happens is heat and general wear and tear eventually stretch the valves and if the valve lash (valve to cam clearance) is out of spec and other components, like the lifter shims and rocker arms, come into play the valves can either stay open or not open all the way affecting the compression.
Or you might just need new piston rings.
As you can see, sometimes it's a crapshoot. Hopefully you like wrenching and problem solving because finding a solution often requires a mental checklist of items and checking them off as you work your way through the bike.
Eventually you'll encounter a stubborn bike, it's probably an easy fix but like race day, be prepared for anything.