“It just happened on its own.” “I didn’t do anything.” – These are phrases any IT person hates, but do they describe your breakdown exactly? You don’t really know how this devilish machine is put together, but you’re on the highway / in the forest / in the urban jungle, and all your familiar mechanics (a grand total of one) are unavailable? Then let’s try to diagnose the breakdown ourselves!
Let’s start with the main thing: a suitable situation is one where you didn’t change anything. You didn’t touch the carb or injector, didn’t take the motorcycle to a service shop, and didn’t poke around inside it with your mischievous little hands trying to make it faster or more economical. Though if it won’t start, the fuel economy experiment clearly didn’t work out.
Why does an engine run?
Reciprocating motion, crank-and-rod mechanism, operating phases… nah, simpler. You pour in fuel, it sucks in air, you ignite it and off you go. That is: fuel, oil, spark. And if your engine suddenly stopped starting, it’s very likely that the problem is in one of these (assuming, of course, it’s not a heat seizure or a few other less popular cases). Shall we check them all? Off course!
Fuel
Fuel passes through a whole bunch of different parts on its way. A petcock, a filter, a carb/injector, etc.
When does the problem occur?
- Suddenly, on the road or almost immediately after setting off.
Symptoms?
- The starter spins as usual, but the engine doesn’t catch, or it catches but doesn’t start.
What to do?
First and foremost (besides stopping the abuse of the starter), check whether you actually have fuel. Surprisingly, many people are ready to blame anything at all except running out of fuel, especially if they “calculated” that it should run out later. It’ll be all the more annoying when that mistake is discovered.
If there is fuel in the tank, check the petcock as well, and for now that’s enough. Because beyond that you can spend quite a long time sequentially checking that:
- Fuel flows from the petcock/pump (close the petcock → disconnect the fuel hose → briefly open it / try to start; with a pump: turn off the engine → disconnect the fuel hose after the pump (that’s if you even find where the pump is) → try to start).
- Fuel reaches the carburetor (all carbs have a drain bolt; if you unscrew it, fuel should flow out. True, for that you need to know where it is. I don’t recommend disassembling a carburetor on the roadside).
- Fuel reaches the cylinder (the simplest way to check this is to unscrew the spark plug and plug the hole with your finger. Your finger will get blown off (since it’s about 10 atmospheres), but you’ll see fuel on it or you won’t).
Spark
And in this joke there’s only a bit of a joke, like any device that runs on electricity, the whole system is afraid of just two things:
- Contact where there shouldn’t be any (a short circuit).
- No contact where there should be (no spark).
When does the problem occur?
- Suddenly.
- During or after rain.
In the first case, a terminal may have come loose, for example from the coil (due to vibration or shaking), a wire may have chafed, or the spark plug or cap may have failed.
In the second case, most likely a short circuit caused by water is to blame.
Symptoms?
- The engine stalls as if you turned off the key.
- The engine won’t start and doesn’t even try to catch, and if you crank it longer it starts to smell like fuel.
What to do?
High voltage bites hard, don’t touch the metal parts of this system while the engine is running.
Unscrew the spark plug, connect it back to the wire, holding it by the insulation press the tail of the plug against ground (the metal frame, an unpainted part, or a bolt on the frame), briefly spin the starter and try to see the spark. It should be faintly visible even in daylight and should be blue. Yellow is bad – you need to look into it, though most often it’s not the cause. If there’s nothing, or if the plug is wet, there are problems with spark generation.
Check the contacts on the coil (that little cylinder where the spark plug wire goes), the wire itself for damage, and the cap for the presence of metal inside.
Example: I once had an interesting case: the retaining clip inside the cap crumbled. The metal core was there, but the cap kept slipping off and there was no spark. It was on a Honda Dio 34, and to get to the garage I had to open the floor cover to unscrew the spark plug, press the cap to the plug with a shard of ceramic tile from the roadside, press the shard down with my foot, and ride at 20 km/h, fixing everything after every bump. Ingenuity 😀
Air
This happens least often, which is why we check it last. Well, except for enduro bikes, but I think people can figure out without me that if the motorcycle is up to its ears in mud, it needs to be cleaned off.
When does the problem occur?
- On long rides in high humidity or rain.
- On long rides through mud.
- During poplar fluff season or when riding through dandelions 😀
In short, when there’s a lot of non-air in the air.
Symptoms?
- The engine starts running worse and worse until it stops completely.
- A removed spark plug will be black (if you rode more than 200 km with a clogging filter).
What to do?
First of all, check the air filter, paper filters get soaked with water and stop letting air through properly; dirt could clog the air intake opening; fluff also doesn’t help the engine “breathe” well.
Then it’s situation-dependent: you need to clean the filter and restore its ability to pass air.
A real-life example, or how I screwed up 7 years ago
Once I had a situation where, out of ignorance, I pushed my bike 10 kilometers home at night. It was in the city, which was good. But there were lots of hills, which was bad. And here’s how it happened.
I agreed on a scooter forum that someone would tune my carburetor, my machine pulled poorly uphill (no more than 30, though 50+ on the flat), and the “master” convinced me that the issue was the carb (in reality it was the poor condition of the engine plus a step on the variator weights).
I came to the meet of scooter riders. I met this guy, he twisted something in the carb, and half an hour later he left. I immediately checked in front of him, the moped rode back and forth on the sidewalk, acceleration seemed maybe more lively, maybe not. While I was thinking where to ride properly, he left. Not ran off, just had to go.
When I finally decided, I started up and rode off. When I tried to give it gas, the engine suddenly started choking and then stalled. It was 9 p.m. Then, together with the local “masters” from the meet, we tried to diagnose the breakdown. Several times they “dried” the spark plug, even though it didn’t look wet. They twisted the only bolt on the carb back and forth. We checked both the spark and the presence of fuel in the tank, there was enough of fuel. Nothing helped.
At 11 p.m. I made a brave decision: to push it. And off I went. After 3.5 hours and a short rain on the way home I pushed it to the garage, and collapsed into bed.
The next day, upon re-examining the unfortunate motorbike, I discovered a mysterious hose hanging nowhere. Reasonably assuming that, sure, there can be drain and vent hoses, but what if this wasn’t one of them, I started looking for where it could be connected. And I found a clean nipple on the housing. Connected it, fiddled a bit, started it.
Now, what actually happened
The carb on my bike was a Mikuni, and there is no mixture adjustment there (it’s plugged), so the “master” was just adjusting idle speed.
My motorbike didn’t have a fuel pump, but an automatic vacuum petcock that opened every time there was vacuum in the crankcase. And that “master” managed to disconnect the vacuum control hose while poking around.
None of the local “masters” thought beyond the presence of fuel in the tank, the fact that the spark plug didn’t look wet was noticed, it seems, only by me, though at the time I didn’t know what that meant.
That’s how ignorance of the “rule of three” and the inability to perform diagnostics at the most basic level made me push a bike for 10 km.
There was also the opposite situation
A friend and I went to a lake. I filled up in my neighborhood, he in his. Half a tank each. We arrived, bouncing over bumps, unpacked everything. He decided to ride to the store, and the motorcycle wouldn’t start. More precisely, it seemed to try to catch, but just wouldn’t. We started checking: fuel in the tank – yes; fuel reaches the cylinder too (we put a finger instead of the spark plug and crank the starter). There is spark, but the plugs are wet – meaning it’s flooding, the mixture isn’t igniting. And the spark is fine. Air is fine too. Something wrong with the mixture?
We drained some fuel from the tank, and it looked paler than usual and smelled weakly. Seems the guy was sold some kind of crap instead of gasoline. We drained the carb, drained the tank into a bottle, poured half a liter from me into his. Cranked — started.
That’s how a bit of knowledge about how your steed is built helps bring it back to life. Don’t be shy about knowing this, so you don’t end up looking like a fool.