Internal combustion engines need three things to run. Fuel, Fire and Compression. If the correct amount of fuel (and air) is in the cylinder, and the mix is properly compressed (to somewhere near 1/9th of it's uncompressed volume), and it gets a spark, the mix will explode. Physics. It cannot do otherwise.
Granted, lots of things need to happen properly in an engine for all that to come together and happen at the right time in the piston cycle, but that remains the core principle of internal combustion engines. To troubleshoot an engine that is not running, or not running properly, those are the three main issues to consider.
Lots of things can cause a loss of compression, but they all come down to the cylinder having a leak. Mechanical wear, blown gaskets, defective seals, dirty / leaky valves or bad valve timing can all cause a loss of compression. Fortunately, serious compression loss is fairly rare...and that is good, because it is always the most expensive problem to solve. Engines are built, first and foremost, to continue to hold compression through their service life. The good news is that if you have verified fuel and fire, checking compression is among the easiest test to do on an engine. Still, it is rarely a sensible place to start your troubleshooting. Failures in the fuel supply and ignition systems are far more common.
Outside of flat running out of gas, ignition failure is by far the most common cause of an engine failing to run. And that is great, because it is the easiest problem to check directly and is often the cheapest problem to fix. The easiest ignition check is to unplug a spark plug wire, remove the spare plug, reconnect the plug wire, ground threads of that plug to the block, and crank the engine. If you can see a nice blue-white (or even reddish) spark you can PROBABLY say ignition is ok and move on. I say "probably" because if you do see a spark, you still do not know if the spark happening at the right time. If you don't see one, you do not yet know why. Maybe the plug is bad, or maybe other ignition components are bad and you still need to track that down (or maybe the kill switch is turned on!). But at least you know the next path to follow. But, there is actually something that is even easier to check, if only indirectly. And since we are trying to zero in on the main problem area quickly, easy counts for a lot. And the easy test involves the third leg of the stool...and is often a great place to start your troubleshooting.
So, we have already assumed you are not out of gas...but on a small engine, you better make sure the fuel valve (often called a "petcock") is open and fuel is actually flowing. Next up on smaller machines, including most motorcycles, is the carburetor, which regulates the amount of fuel that is supplied to the engine under various throttle and load conditions. Larger, more sophisticated, less polluting, engines get their fuel via a fuel injection system. Let's start with carbs.
Carbs have tiny passages in them, through which tiny streams of fuel flow, that ultimately get mixed with the air coming into the engine. Just a little schmutz in any of those passages can put a carb out of commission. A failed fuel pump or clogged fuel filter can also stop the music (in fuel injected motors too). If your machine has been sitting for a few months with fuel in it, there is a very good chance your carb is plugged up and needs to be cleaned. But let's not jump to conclusions.
While you can look at spark plug and get a pretty good idea of whether or not it is sparking, you cannot just look into a carb and see whether or not it is working properly. Fuel injection systems in gasoline engines are almost always computer controlled, and with proper diagnostic tools, can often tell you if and why they are sick. But for a carbureted engine that has no blinky lights, it is not so easy. What to do?
Well, obviously, you cheat. (And the cheat actually works on fuel injected engines too.)
One of the products you will find on the shelf at every auto parts store is a can of "starting fluid" or "starting spray". The can will contain a cocktail of flammable chemicals, usually including ether, which vaporizes easily and happens to burn like crazy. A spray can of starting fluid is a great diagnostic tool. Remove the air cleaner so you have a clear shot at the intake, give it a one second blast of ether, and immediately crank the engine. If it fires and runs for a few beats (perhaps for a few seconds), you now know that you have reasonably good ignition, firing at (at least approximately) the right time. You know you have at least marginal compression. And you know that you just supplied the missing element...fuel, more specifically, vaporized fuel. (Engines do not run on liquid fuel. They run on vaporized fuel.) Now you have a pretty good idea where to look...the fuel delivery system. Ignition may not be perfect, but it was good enough to fire the ether. Compression may not be perfect, but it was good enough to explode the ether. So right now the fuel delivery system is the prime suspect. Conversely, if the engine did not fire with ether, you PROBABLY have an ignition problem and should at least do the easy ignition test (pull a spark plug and check for good spark).
If you see good spark AND the motor will not fire on ether...well, now you are in for an education. It is time to dig deeper.
So...you aren't supposed to talk about religion and politics in gentele company, right? Well, I am going to put on my big steel-toed boots and stomp right into the middle of a hot religion topic in motorcycling...using car tires on motorcycles.
"What kind of lunatic puts a car tire on a motorcycle?", you might ask. I cannot say with absolute certainty, but the evidence suggests to me that they are, at their core, shortsighted cheapskates; the sort who try to seal up a cavity in their tooth with superglue to avoid paying a dentist, only to reap an abscess and need dental surgery later.
Why would someone even consider this insane idea? Well, motorcycle tires are not cheap, particularly not good motorcycle tires. And the labor charges to install them are often almost as much as the tires themselves. So it can cost almost as much to put two tires on a bike as it costs to put four tires on a car. That, understandably, rubs some folks the wrong way. Worse still, motorcycle tires may last 10k miles or even less, so they get changed much more often than car tires. In short, tires represent a significant cost of ownership to a motorcycle rider.
Nobody who has followed my work will be surprised I say that those who are not willing to pay what it costs to operate a bike safely, shouldn't own or ride motorcycles. But what are the safety issues? They all come down to the key mission of the tires...keeping the bike stuck on the roadway rather than sliding over/off it. Car tires present two obvious significant challenges to that mission.
So this is kind of a no-brainer. Bikes lean when they turn. They must lean to turn. The rounded cross section of a motorcycle tire produces a near-uniform contact patch (that part of the tire surface that is in contact with the roadway), no mater what the lean-angle of the bike. The car tire, in contrast, has a rectangular profile and typically, very stiff sidewalls, so as the bike leans, the contact patch of the of the tire shrinks as the main body of the tire lifts off the roadway. Not good.
The other big issue is the tire's grip on the road...traction. Traction and tread life are the core tradeoffs in tire design. Hard rubber wears off slower than soft rubber, but soft rubber grips better than hard rubber. You may be able to visualize soft rubber oozing into the tiny pits in a paved roadway, while a harder rubber only makes contact with the tops of the roadway texture. Some motorcycle tires, particularly those for touring bikes that do lots of highway miles, finesse this issue with a multi-compound design. The center of the tread area is made with a harder rubber for good wear when the bike is rolling straight ahead, but the outer portions of the tread surface, between the center area and the sidewalls, are built with a softer compound that provides better grip when the tire is leaned into a turn. Car tires, which are not only designed for a heavier vehicle (and so have a harder tread compound for suitable wear in their intended application) but are also designed for a vehicle suspension system that is designed to keep the tire upright in a turn, making a multi-compound approach less useful.
BUT WAIT, THERE'S MORE
Or perhaps, there is less. In this case, less contact with the bead seating surface. "Bead" may be an unfamiliar term in the context of tires but here it is in a nutshell. It is the part of the tire that seats against the wheel and provides both the air seal that keeps the tire inflated but also provides the structural connection between the tire and the bike. The bead on car tires are designed to seat / seal / hold to the rims of car wheels (duh). The bead of motorcycle tires and seating profile of motorcycle wheels are different. Perhaps they did not need to be, but bikes weigh much less than cars and the cornering forces placed on car tires are different from those put on motorcycle tires, so the tire seating region is smaller than is found on car wheels. Or to consider it from the other perspective, a car tire expects a different, larger bead seating region on the wheel, and therein lies the rub...or perhaps the lack of a rub. The seating surface on a motorcycle wheel is not right for a car tire.
You may hear someone say, "But it works fine!". Well, that depends on your definition of "fine". Yes, as a rule, a car tire can be induced to hold air when mounted on a motorcycle wheel. And depending on the bike and the car tire selected, the tire will clear the surrounding suspension structure of the bike so the bike can roll. Let's even say that we are willing to accept the traction and handling compromises introduced by a car tire (I cannot imagine why you would, but let's say so, for the sake of argument.) What else is there to consider? What about emergencies?
The car tire shown in this article came into the shop mounted on a motorcycle wheel. It did something very interesting when it was deflated. It spontaneously dismounted itself from the motorcycle wheel. That is to say, the tire bead became unseated from the wheel without any additional force being required. OMG!
If you have never mounted new tires on car or motorcycle rims, you may not be aware that, once seated on the wheel, removing that tire from the wheel requires considerable side force. Getting the tire unseated from the wheel is the main mechanical challenge in any tire change job, followed by the challenge of getting the tire off the wheel after it is deflated and then, finally, by the difficulty of getting the new one onto the wheel. That may not sound surprising...but the first item on the list is critical from a safety point of view.
If we do not know anything else about tires, we know that they sometimes go flat. But we also know that they rarely come off the wheel just because they have lost air. Yes, a massive blowout can shred a tire and leave you with a bare wheel, but that is rare and virtually never happens to tires until they are used long past the end of their service life. On a car, a tire dismounting from a wheel causes damage to the wheel and perhaps the bodywork. It could even cause a loss of control and an accident. However, on a motorcycle, it WILL cause a loss of control, and that, for motorcyclists, is often deadly.
Of all the hazards a car tire presents to a rider, the most insidious is the risk of a low pressure spontaneous dismount of the tire. Because car tire sidewalls are made for much greater loads than motorcycles offer, an inattentive rider may be on an under-inflated tire without realizing it. The tire may not "go flat". However, at some point, as inflation pressure continues to fall due to a small leak, the tire may no longer keep its grip on the motorcycle wheel. That dismount event is most likely to occur when the rider enters a curve...and is most likely to occur when entering a high speed curve. How's that for a nightmare scenario?
Granted, motorcycle riding is not an enterprise for seriously risk averse folks. But it is an enterprise that tends to weed out the reckless. Using a car tire on a motorcycle may not get you killed, but it absolutely lowers your chances of survival. It's not worth it to me. Make your own call. (But I will never mount one for anybody in my shop.)