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Some Basics on Motorcycle Carburetor TroubleshootingIf your bike is not running properly or not running at all, then you may need to do some motorcycle carburetor troubleshooting. In theory, the carburetor is a relatively simple device, but modern motorcycle carburetors are precision made devices that need to be kept clean and in tune for your engine to operate properly.
Before going any further on the subject of motorcycle carburetor troubleshooting, it is important to understand the basics of what is necessary for an internal combustion engine to run.
All three things listed above are needed in order for a standard reciprocating piston engine to run. Let's take quick look at 1 and 2 before going into carburetors in #3, because you really need to eliminate possible problems with #1 and 2 before trying to troubleshoot problems with your motorcycle carburetor. 1) Compression: An engine needs to develop adequate pressure (compression) in order for the engine to run properly. If you try to start your motorcycle and you feel normal resistance on the kick start, or when you crank the engine with the electric start and it sounds normal, then it's likely that your engine compression is just fine. On the other hand, if you try to spin the engine (with kick start or electric starter) and it spins very easily and very quickly, then this can be an indication of low/no engine compression. If that's the case, then it's possible that your engine has some serious problems. A busted connecting rod, bent valve, hole in piston, blown head gasket, etc... can all cause low/no compression. On the other hand, if you try to start your engine and it will not spin over at all, then that is an indication that your engine may be seized. A piston could be seized in the cylinder, or a crankshaft can sometimes be locked up because of bearing failure. In any case, most of these conditions are serious problems that can require major engine work. Hopefully none of these is true about your engine, and you can move on to the next step. 2) Ignition Spark: The next component needed for proper motorcycle engine operation is ignition spark. Probably the easiest way to check this out would be to unplug the spark plug wire boot off the existing spark plug and plug it onto an extra spark plug. Then hold the threads of that spark plug against the metal part of the engine making a good ground. You may want to wear leather gloves to minimize your chance of being shocked. The ignition system produces high voltage (but low amperage), so the shock hurts momentarily, but it more just scares the unsuspecting victim. With the plug threads touching a good metal engine surface, crank the engine (or have someone else do it for you) and see if you can see a spark jumping the plug gap. It might be best to do all this in lower ambient light conditions (indoors or when it's starting to get dark), so that you can more easily see the spark. If you see a spark, then you know that you've got this necessary component of engine operation - at least to some degree. It's possible to have spark, but for the ignition timing to be so far off that the engine won't run right. It's also possible that the spark is too weak to properly ignite the air/fuel mixture in the cylinder under compression. These conditions can be harder for the amateur mechanic to troubleshoot, but for simplicity sake and in the interest of brevity of this article, we'll just say that you are looking to make sure that you have spark (a "Yes" or "No" question), and we'll assume timing and spark strength are adequate. In most cases, that's probably true. For this discussion, if you've got spark, then we'll assume it's otherwise adequate. 3) Air/Fuel Mixture: Now more on to the subject of motorcycle carburetor troubleshooting. The 3rd component needed for proper engine operation is an adequate air/fuel mixture. This is the job of the carburetor, and on some newer motorcycles it is being handled by fuel injection. Most bikes are carbureted, so this article will look at some basic motorcycle carburetor troubleshooting principles. One of the first questions you need to ask yourself (or ask another person who knows the history of the bike in question), "Did the motorcycle run normally the last time it was parked?" That is an important piece of information to know, because modern formulations of gasoline are known to break down and decompose relatively easily and quickly. As a result, it is a very common (and unfortunate) situation to have a motorcycle that ran great when it was parked for the winter, but then some time later it won't start or run properly anymore. This can be caused by partially or fully clogged carburetor circuits. If you've never seen it happen, then you may be surprised at how easily ordinary gasoline can decompose and plug up carburetor jets and passages. Since this problem is fairly common, it will be the primary attention in this article of motorcycle carburetor troubleshooting. Also keep in mind that once you get into your carburetor, then that might be a good time to consider buying a carburetor rebuild kit that will replace the most commonly worn parts.
If the bike was parked in good running condition, and then later (usually an extended time later) the bike would not start or would not run properly, then it's very likely that your carburetor is gummed up - thanks to "new and improved" gasoline formulas. Years past, with earlier formulations of gasoline, this was not as serious of a problem. In fact, the author of this article knows of one of his own motorcycles that sat without being started for over 10 years. Surprisingly, the bike started right up and could be driven around almost immediately! That was a long time ago. These days, this is NOT normal, but that was the situation with older gas formulations that did not break down as quickly. In the past, you could sometimes get away with just letting a bike sit with old gas in the bike for a long time. Not anymore! The next motorcycle carburetor troubleshooting question that naturally comes up... "What can I do if I have a plugged up carburetor?" The simple answer is: "Clean it." First, you will need to remove the carburetor from the motorcycle. Make sure you shut off the fuel petcock from the gas tank so you don't spill fuel all over when you disconnect the fuel line to the carburetor. As always, be careful when working around gasoline. Don't even think about doing it while you are smoking or around any other open flame or sparks. Even electric motors or natural gas water heaters and furnaces can give off sparks. In the case of some natural gas appliances, they can have an open flame pilot light. Some auto-ignite gas appliances produce a spark to ignite the natural gas. Sparks or open flames can ignite gasoline fumes and cause an explosion or fire. That may be good when the explosion takes place in the combustion chamber of your engine (that's what makes an internal combustion engine run), but you don't want to be apart of gasoline combustion yourself! Work on your motorcycle carburetor in a safe place where there is adequate ventilation. Also, make sure that you have some old rags handy because when you move the carburetor around, because you will often have fuel leaking out of it. On most carburetors, you will want to flip the carburetor upside down and remove the 4 small screws that hold the float bowl onto the main carb body. Some carburetor float bowls are held on with a formed wire spring clip. In any case, remove the float bowl. Once you remove the bowl and look inside, then you will know more whether or not your carburetor was plugged. If it is plugged from old gas, then one of the first things you may notice is that it stinks. It's not the normal smell of gasoline, but more like a lacquer or wood finish smell. In addition, the inside surfaces of the float bowl may be coated with a dirty film. Sometimes it looks green, brown, white in color. These are deposits left behind by the decomposed gasoline as well as corrosion of carburetor metals. It's deposits like these that may have also plugged up the tiny openings in the carburetor jets and passages. You can try to clean out the float bowl with some aerosol can carburetor cleaner. You may need to use a stick or brush to try to remove the residue in the bowl. Just be careful not to get the carburetor cleaner spray on your skin or in your eyes - the stuff burns and hurts like crazy. As you begin disassembling your carburetor, play special attention to how things are assembled, so that you can put things back together properly when you are done. If you unscrew adjustment screws so that you can spray out those passages, then make sure you first screw them in until lightly seated (do NOT tighten too much - just until lightly seated). Count the number of turns it took until the screw seated, and then you can unscrew and remove the adjustment screw. When you are done cleaning the passages and put that screw back in, you want to return it to the same position where it was before. You tighten the screw until it seats lightly, and then you back off and unscrew it the number of turns that you counted earlier.
Another area that you want to make sure is clean is the float inlet needle valve. It is the valve that controls the flow of fuel into the carburetor float bowl. It is similar to how a toilet tank float valve works. When the level reaches a set limit, then the float shuts off the inlet valve. You can usually slide out the float pivot pin and then carefully remove the float and needle. Pay attention to how it is assembled, so that you can put it back together exactly has you found it. Make sure the inlet needle and seat are clean and free of deposits. If the inlet needle can not close properly, then you carburetor (and engine) will be flooded and gas will be leaking out. After you have made sure that all the jets are clean, and once you have sprayed out the passages in the carburetor body, then you can put the float bowl back on. Make sure that the float bowl o-ring or gasket is not torn and that it is positioned correctly before putting the float bowl back on. If everything else is OK with the carburetor (and engine), then that may be all that is needed to get your motorcycle running again. Don't forget that you will want to remove the old gasoline from the gas tank and fill up with some fresh fuel. No sense in going to all the trouble of cleaning up your carburetor and then trying to run the old gas (that plugged it up in the first place) into your freshly cleaned carburetor! Decomposed gasoline will not burn properly in your engine anyway. Also, if there is a fuel filter, then now is a good time to replace it. For many bikes, this cleaning procedure may be all that is needed to get a motorcycle running properly again. While doing this motorcycle carburetor troubleshooting, you might also want to consider buying a carburetor rebuild kit to replace the old, worn out parts in your carb. If you would like to see a visual example of what's being described here, check out this helpful video made by a mechanic giving a quick overview of some of the carburetor cleaning information discussed above.
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Motorcycle Carburetor Troubleshooting
When you open your float bowl, you will usually see a main and a pilot jet. There may be other jets as well depending on your particular carburetor. You should unscrew the carburetor jets. The jets often have slotted heads (remove with blade screwdrivers), and sometimes they have hex heads (remove with socket wrenches). Once you unscrew the jets, you can spray carburetor cleaner into the carb passages where the jets were screwed into. Take the jets and try to hold them up to the light and see if you can see light passing through the tiny orifices. If the jets themselves are plugged, then you can try to spray them out with carburetor cleaner. If that doesn't work, then you can try to take a toothpick or small strand of stiff plastic (like a plastic bristle from a stiff brush) and try to poke that through the jet openings to clear out the deposits. Some people use a wire to do this, but you have to be very careful not to deform the hole in the jet. Most carburetor jets are made out of brass and a steel wire can gouge the softer brass. If you gouge the jet orifice, then you will be changing the fuel flow characteristics of that jet. If your jets are plugged real bad, then you can soak them in a small glass or metal container with some carburetor cleaner. You can also buy new jets for your carburetor but make sure that you buy the exact same jets. Generally speaking, you want to make sure that you remove the visible deposits in the carburetor. If you see gunk anywhere in the carburetor, then be sure to clean it out. Carburetor cleaner is relatively cheap (especially when compared to the cost of a new carburetor), so make sure that you get things good and clean. Make sure that all the tiny holes are unplugged. Those tiny orifices and passages are there for a reason, and if they are blocked, then you carburetor will not work properly. Some people recommend blowing out passages with compressed air. This may be helpful on some motorcycle carburetors, but BE CAREFUL. Some motorcycle carburetors have diaphragms (like CV type carbs) that can be ruptured under the high
pressure of compressed air. You do not want to tear one of these diaphragms, so be very careful if you do decide to use compressed air. If in doubt, then don't use compressed air.