Troubleshooting 101:General engine troubleshooting

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When a bike suddenly stops working, there are three "systems" to check. Note that this does not apply to a bike which gradually stops working (then, think valves or carb cleaning). This is a pretty basic guide, but hopefully it will be helpful to anyone who's having trouble with their engine. If you can run through this and narrow down your problem board members will be much more likely and able to help you diagnose the exact trouble.

1. Fuel

Fuel is what gets consumed to make the fire that makes the engine turn. Fuel is stored in the tank, travels through the petcock (which has two potential blockers, in the petcock lever and the vacuum diaphragm, not to mention crud getting caught inside it somehow) and fuel lines into the carburetors, where it's atomized and sprayed into the cylinders.

Things to check on the fuel system:

  • Pull the fuel line off the carb and direct it into a gas can of some description. Fuel should only flow with the petcock lever in the ON or RES positions AND the engine running (or vacuum applied to the vacuum line on the petcock). Under those conditions, it should flow fairly copiously, not in a trickle.
  • Turn the petcock to ON (or RES if you may be low on fuel - check visually) and apply vacuum to the vacuum hose on the petcock. That's the line that runs to the top of the carb intake tube, not the bottom of the carb. With the fuel line attached to the carbs, pull some vacuum for a few seconds. Pull off each float bowl and ensure that it's full of fuel (you're looking for about 2/3 full of fuel -- totally full is wrong, and empty or 1/8 full is wrong). If you've got too much fuel in there, that float valve is messed up in some way. If not enough, then there's a blockage in the fuel supply line, or the float valve is stuck closed.

2. Air

Air arrives at the engine by way of the airbox, air filter, and carburetors. Air has to be mixed with the fuel in the right ratio to burn well. Too much or too little air and the fuel won't burn right. There's not a lot that can go wrong here, but pull off your seat and check to make sure there's nothing covering those two intake holes just behind the fuel tank. If that looks clear, check the air filter to see if it's disgustingly dirty (if it's just a bit dirty, that's not the problem). You could check for further problems like leaves or mouse nests stuck in the intake, but that's quite unlikely if the filter's properly in place.

3. Spark

Spark makes the fuel burn in the presence of air. Obviously, without spark there will be no fire. The spark system (in very basic terms) is composed of the battery, the ignition module, the coils, the wires connecting them all, and the sparkplugs. The battery supplies power to the ignition module and the coils. The ignition module tells the coils when to fire. When the coils fire, they send a jolt of high-voltage electricity down the sparkplug wires to the sparkplugs. The sparkplugs are nothing more than carefully adjusted wires which have a precise amount of space between them. The sparkplugs only work when they're touching the engine case, since that's how they complete the circuit.

Things to check on the spark system:

  • Fully charged battery. Make sure your battery has at least 12.4V (see here) by using a voltmeter. Less than that and things start to get dicey on the electrical side of things.
  • Connections all solid, particularly on the sparkplugs. Make sure the sparkplug connectors are seated all the way down on the plugs.
  • Try wiggling the clutch and sidestand around a bit. The switches on these two things (which can cut off the engine) have been known to get wonky. Usually these fail gradually and are pretty obvious. It could also be the neutral sensor switch, although on this bike it's less of a problem than the clutch and sidestand switches.
  • If that's all OK, unscrew each sparkplug in turn (leaving the other one installed), plug it into the sparkplug cap, and hold the threads of the plug against a piece of bare-metal engine case. Hit the starter button on the bike. You should see some kind of spark jumping across the gap, even if it's bright out. If you can't see a spark at all in direct sunlight, try making some shade and see if you just have a dim spark. If you just can't see a spark, proceed. If you can see a spark, do the same thing with the other plug and see if it's sparking.
  • If you have no spark on one or both plugs, you need to figure out what's disconnected. It could be a really dirty spark plug (heavy black buildup on the plug nose) or a broken spark plug (rare, but possible) or a wire disconnected somewhere. It's unusual for components like the ignition module or coils to fail, but it does happen from time to time. Much more likely is that you have a disconnected wire (although it may look connected) or a corroded connection. Go through each of the wires and unplug and replug each connection. Try starting again. If that doesn't help, you're beyond the scope of this guide.
  • These two sites are recommended for electrical troubleshooting:
HondaVFR.org
ElectroSport Fault Finding Chart

If nothing in this guide helps, then you're either doing something wrong or your bike has been gradually getting worse and you didn't notice it. Gradual problems are likely to be the valves going out of adjustment, or varnish slowly building up in your carbs if you don't ride very often. There's no easy way to check if it's the valves, but symptoms include running poorly at low speed or idle, ragged or variable idle, and hard starting.

Side note: reading spark plugs. Since spark plugs are the only part of the combustion chamber you can readily remove from the engine, you can use them to see how the engine's doing. Spark plugs should be a lightish tan color if all is working correctly. If a spark plug is really heavy black, the engine's running rich. If it's white, the engine's running lean. If it's lightly black, that still suggests a rich mixture, but maybe not enough to freak out about. Compare yours to the ones in the sparkplug guides.