If there's a single shared experience that's common to all motorcyclists, it's the simple act of filling a tank with gas. Whether it's a shapely sportbike tank, a bulbous dual-purpose tank or an underseat tourer tank, the concerns are the same--get as much in as you can, don't spill any on the paint, and don't forget to turn the petcock from reserve back to on. But as long as gas isn't pouring out onto the ground, that's about as much thought as most motorcyclists give to the various components that make up their bikes' fuel systems, so that when the gas that goes into the tank doesn't make it to the engine, they're left high and dry.
The first place a fresh tank of gas goes is, of course, the gas tank. Perched high above the engine as it is on most bikes, it might seem an awkward place to put a large supply of a heavy liquid, and it is. But it has the advantage of simplicity when it comes to getting the gas from the tank to the engine, since gravity does the work a fuel pump would otherwise have to do. The trouble with gravity, though, is that it's never off duty Without some way to stop the flow of fuel from the tank when the bike is parked, a split hose or a clogged float needle in the carburetor is all it would take to let four or five gallons of volatile gasoline run out onto the garage floor. That's why most bikes have a valve in the fuel system called a petcock.
There are two basic types of fuel petcocks. The first is simply an on-off valve, with three positions: on, off--and reserve. Two tubes, one taller than the other, project upward from the pet- cock into the fuel tank. When the petcock is in the on position, fuel flows from the tank to the carbs through the taller of the two tubes. When the fuel level drops below the top of the tall tube, switching to reserve lets the remaining fuel flow out through the shorter tube, as well as warning you it's time to gas up soon.
The second type of fuel petcock is vacuum-operated. A diaphragm inside the petcock body controls the flow of fuel to the carbs. The diaphragm is connected to one of the intake ports by a rubber hose. In the closed position, fuel cannot flow past the diaphragm. A spring holds the diaphragm closed until the engine is started, when the negative pressure--or vacuum--inside the intake port overpowers the spring and opens the diaphragm, allowing fuel to flow past. When the engine is shut off, pressure inside the intake port returns to normal, and the diaphragm closes off the fuel supply.
In addition to a reserve position, there is an additional setting on most vacuum-operated petcocks, called "prime." Prime bypasses the diaphragm and lets fuel flow directly from the tank to the carbs. It's usually used only after an engine rebuild or carb overhaul, to quickly fill the float bowls without cranking the engine over for a long period.
Pump gas isn't always the cleanest stuff around, so petcocks often incorporate a filter to screen out impurities. The pickup tubes are sometimes enclosed in a mesh of filtering material to keep crud and dirt in the bottom of the tank from clogging up the works. Many petcocks also have a removable filter bowl on the bottom of the petcock body that collects whatever gets past the filter mesh. The bowl is easily accessible, and should be checked and cleaned periodically.
Gasoline's next stop after the petcock is the fuel line. Typically made of flexible rubber hose, it runs from the petcock to the carb or carbs. In the case of a single carb, the fuel line attaches directly to the carb body. In multi-carb applications, the fuel line often connects to a metal tube that then distributes fuel to the carbs. There isn't much that goes wrong with rubber fuel lines unless they crack or split due to age, are exposed to the wrong chemicals, become clogged, or get pinched or cut while the tank is being removed or replaced. Most fuel lines are secured at either end by a small spring clip that acts like a tiny hose clamp. If your bike's fuel lines are secured by real hose clamps, make sure the clamps aren't too tight, or they could split the hose and cause a leak.
The fuel line is where most riders who want extra filtration for their bikes' fuel supplies install additional fuel filters. There are many different kinds on the market, and most take no more than a few minutes to install. The clear plastic ones are easiest to monitor for dirt build-up, and although you can back-flush them to clean them, they're so cheap you might not want to bother.
Not all bikes use gravity to move fuel from the tank to the carbs. Due to the relative position of the fuel supply and the carbs, some carbureted bikes have a fuel pump. Typically the pump has a built-in pressure-relief valve to keep it from oversupplying the float bowls. When the float bowls fill up, and the float needles rise into their seats to cut off the flow of gas, the pump senses this rise in pressure in the fuel line and stops momentarily starting up again when the pressure in the line drops.
The pump is often connected to the engine's ignition system so that it doesn't pump unless the engine is actually turning over. This prevents the pump from working in case the bike falls over and the engine dies but the ignition is still on. In many Hondas, for example, the pump is connected to a fuel cut-off relay that is in turn connected to the ignition control module. The relay prevents battery current from getting to the pump unless the ignition module is transmitting ignition pulses to the relay indicating the engine is running. No ignition pulses, no current to the pump.
The last stop in the fuel system is the carburetors. Despite many different carb designs, virtually all use the same kind of float system to ensure a constant supply of fuel is available to the engine at all times. Below the carb hangs a reservoir called the float bowl. As fuel from the tank flows into the bowl, and the fuel level rises, it lifts a float inside the bowl. The higher the fuel level, the higher the float pushes a metering needle into a matching seat. when the fuel level reaches a predetermined level, the needle shuts off the flow of fuel. when the fuel level drops, the float descends, lowering the needle and letting more fuel in.
Most float bowls have two outside openings, a drain and an overflow tube. To drain the contents of the float bowl, turn off the pet-cock and remove the bowl drain screw. whatever dirt makes it past the petcock filter, the petcock filter bowl, and whatever add-on filter might be in the line, will collect in the carburetor float bowl. It should flush out when you open the drain. If for some reason you suspect it didn't, flush it again by spraying WD-40 into the bowl drain hole.
The other opening, the overflow tube, prevents the float bowl from overflowing should the float needle malfunction. The top of the tube, which projects upward from the bottom of the float bowl, is higher than the highest level the fuel should reach under normal circumstances. If the fuel level in the bowl exceeds this predetermined level, the excess runs out the overflow tube. The overflow tube is usually connected to a long rubber hose, similar to the fuel line hose but smaller in diameter, that is routed to empty onto the ground under the bike.
It's a good idea to drain the float bowls of any bike you intend to store for more than a month or so, in order to prevent the gas in the bowls from varnishing. This is especially critical with bikes run on so-called reformulated gas (RFG), which is gasoline mixed with an oxygenate such as ethanol or methyl tertiary butyl ether (MTBE) to increase the gasoline's oxygen content and reduce emissions. RFG varnishes far more quickly than regular gas when stored, and is much harder to clean once it does. Alcohol-based oxygenates absorb moisture out of the air, too, and if the concentration of water gets high enough, it can cause rust inside your gas tank. If you're running RFG in your bike, and you plan to park it for as little as three weeks, play it safe and drain both the fuel tank and the float bowls, and get as much of it as you can out of the rest of the fuel system.
Water can accumulate in the float bowls of bikes ridden frequently in the rain, or washed with pressure hoses, or improperly stored with a gasoline formulated with alcohol-based oxygenates. Water and gasoline don't mix, and water is heavier than gas, so it sinks to the bottom of the float bowl. The carb's fuel pickup is in the bottom of the float bowl, so when you try to start the engine, it's sucking water, not gas. Draining the float bowls will usually remove the water.
If in the course of normal operation you see fuel running out of the overflow tubes, it means the float system has malfunctioned somehow the float has a hole in it, for instance, or something is blocking the needle from rising into its seat. Sometimes you can dislodge the crud from the needle seat by whacking the float bowl with a plastic hammer or screwdriver handle. If that doesn't work, the next step is to remove the float bowl and visually inspect the float, and the needle and its seat, and clean or replace them as needed.
Sometimes the problem isn't too much fuel getting to the carbs, but too little, or none at all. Such problems will often manifest themselves the same way--the bike either won't start, or will but runs like it's starved for fuel, or slows while running and then stops dead. In all three cases, the first thing to do is make sure you have gas in the tank, and that the petcock is in the on or reserve position. Don't laugh--professional motorcycle mechanics can go on for hours about customers who trucked their dead bikes into the shop only to discover they were out of gas, or the petcock had been turned off.
If you hear a whoosh of air when you open the gas cap, check the cap vent before you do anything else. Most bikes have a tiny vent hole in the tank cap to let air in as the gas runs out. If this vent hole becomes blocked, the flow of fuel to the carbs is cut off gradually, and the engine will sputter and die as if it were running out of gas--which it is, even though there might be gallons left in the tank.
Next make sure gas is getting to the float bowls. We'll assume for the moment that your bike is a multi, with one carb per cylinder. First, make sure the vent tubes are clear and unkinked. If you're feeling heroic, you could blow into each tube, but the spray tube on a can of WD-40 works just as well. Now open each drain screw one at a time and note how much gas comes out. If only one or two bowls are dry and the others contain gas, odds are the blockage is in the dry carb bodies themselves. If all the bowls are dry however; start moving upstream in the fuel system.
Check the rubber fuel lines attached to the carbs for kinks, crimps or leaks. If a fuel line is leaking badly enough to affect engine performance, you'll know it right away by the puddle of gas under the bike. Crimped or kinked hoses can cut off the fuel supply gradually however; and go undetected until you remove the tank. Make sure any add-on fuel filters in the system are clean and unblocked.
In a well-ventilated area pull the fuel line off the petcock, put a container under the outlet pipe and turn the pet-cock on. If no fuel comes out, try the reserve position. If there's still no fuel, take off the filter bowl and inspect it. On vacuum petcocks, try the prime position first. If gas flows on prime, but the engine still won't start, check the hose that connects the vacuum diaphragm to the intake port. If the hose is blocked or kinked, the intake vacuum can't open the diaphragm and fuel won't flow If the hose is clear, there may be some malfunction of the diaphragm itself.
The procedure for testing electric fuel pumps varies from model to model. In general, it consists of disconnecting the pump's output hose and -putting it into a container; then running the pump for a specified amount of time and measuring the amount of fuel that comes out. You'll need your bike's shop manual to tell you how much fuel the pump should deliver, how to bypass the fuel cut-off relay so the pump will work without the engine turning over; and, since the fuel pump is an electrical device connected to several other electrical devices, how to track down rogue electrons that might be putting the pump to sleep.
By now you should have discovered whatever it is that's keeping fuel from getting to the engine. If you still haven't, however, the last resort is to drain the gas tank and remove the pet cock. It's extremely unlikely that enough crud will have collected in the bottom of the gas tank to completely block the flow of fuel to the petcock but it's not impossible. Years ago a buddy of mine chased a fuel supply problem for both days of a weekend road race and finally discovered a fist sized clump of dog hair inside the tank clogging the petcock pickup tubes.
As it turned out, the tank was one of several he had for his racebike, had been sitting with the cap open in the garage on a shelf above where the family collies slept. Your next ride doesn’t have to go to the dogs, however; if you check the fuel system filters regularly and drain the tank and float bowls If you plan on storing your bike for a month or more. Then all you'll have to worry about next time you fill up is not pulling up to the diesel pump by accident.
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