Motorcycle Carburetor Troubleshooting


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Some Basics on Troubleshooting Motorcycle Carbs

If your bike is not running properly or not running at all, then you may need to troubleshoot your motorcycle's carburetor unless you happen to have a newer machine that uses fuel injection.  Carburetors were used extensively on older bikes and they are still used on different motorcycles today, so we'll look at how you can try to troubleshoot problems related to a carburetor.  In theory, the carburetor is a relatively simple device, but modern motorcycle carbs are precision made devices that need to be kept clean and in tune for your engine to operate properly. 

Motorcycle CarburetorFlat Slide Motorcycle Carb

Before going any further on the subject of troubleshooting motorcycle carbs, it is important to understand the basics of what is necessary for an internal combustion engine to run.

  1. Compression

  2. Ignition Spark

  3. Air/Fuel Mixture

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 carb 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. 

Motorcycle Carburetor Troubleshooting

Typical Carburetor Float Bowl Components

The next question that naturally comes up is: "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 can give off sparks when they are running.  Also, be careful around gas water heaters and furnaces when working with gasoline.  Some natural gas appliances can have an open flame pilot light.  In addition, 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 a part of gasoline combustion yourself!  Work on your motorcycle carburetor in a safe place where there is adequate ventilation.  

In addition, make sure that you have some old rags handy because when you move the carburetor around, because you will often have gas coming 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, or 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 when it gets in your eyes.  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. 

Carb JetsWhen 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 an air compressor.  This may be helpful on some motorcycle carburetors, but BE CAREFUL.  Some motorcycle carburetors have diaphragms which can be ruptured if too much air pressure is applied.  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 do it.   

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 pouring 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.

Carburetor Leaks Gasoline

Now let's look more specifically at a carburetor problem that can be quite common, and that is when a carburetor leaks gasoline.  If you have the problem of a carburetor that leaks fuel, then there are a few possible issues that might be causing this.  Some of this page is written from the perspective of a motorcycle carburetor, but the same principle apply to other engines with carbs.  First of all, has the carburetor been disassembled just prior to the problem appearing?  If yes, then it is possible that the float assembly was messed up when it was taken apart, and now the float inlet needle valve can not shut off the inflow of fuel.  As a result, the fuel level in the float bowl will overflow and the carb will leak gas.  If this is the case, then you will need to remove the carburetor from the bike (or other machine) and take off the float bowl and check the float assembly and float height setting.  This scenario is probably not the most common case, because most people will not have just taken apart their carburetor when they encounter this problem.  More likely, the bike just sat for a while and the problem was noticed when the bike was taken out to use again.  Another possibility is that a small piece of debris worked its way from your fuel tank into the carburetor.  This small foreign object can lodge between the inlet needle and the valve seat.  This can cause the float valve valve to remain stuck open allowing fuel to overfill the float bowl causing the carb to leak fuel.  This should not happen if you have a good fuel filter.  If for some reason you don't have a fuel filter installed, then you might want to pull the carburetor from the bike, remove the float bowl, and carefully remove the float assembly and inlet needle valve.  Look for any signs of junk that might have been causing the needle valve to not be able to close properly.  You can use compressed air to try to blow out any junk you might find, but always be careful when doing this so that you don't damage anything on the carb.  Another good way to clean out debris is to use a can of carburetor spray with the extension straw.  With this, you can reach into small areas and spray out any junk that might have accumulated.  The writer of this article once had a brand new motorcycle engine that had a carb that leaked fuel.  The fuel tank was filled for the first time and all of a sudden gasoline was leaking on the floor!  What?!  It was a brand new machine!  The carburetor was taken apart and checked for debris.  Nothing obvious was found, but after things were cleaned out and reassembled the gas leak stopped.  Somehow the float must have been stuck and unable to close the inlet needle valve.  Weird!  This also happened on a brand new lawn tractor engine.  So, sometimes debris from the manufacturing process can cause the inlet needle valve to stick open even on a brand new engine.  It shouldn't happen, but just be aware that it can happen.

Carb Leaks Gas             

The most likely culprit for why your carburetor leaks gas is probably going to be described below.  As gasoline has been reformulated for better emissions characteristics, it has resulted in fuel that has much worse storage characteristics.  In other words, 20 or 30 years ago, a person might have gotten away with leaving a motorcycle to sit with fuel in tank and carb for extended periods of time.  Many years ago, the writer of this article had an old motorcycle that sat for over 10 YEARS with old fuel in the tank.  Tires were flat from sitting so long, but amazingly the engine still started up and ran fine!  BUT, that was a long time ago.  Times have changed!  These days, gasoline can deteriorate in a matter of MONTHS.  When it does, it can break down and leave deposits that can mess up your carburetor.  Deposits left behind by the deteriorated fuel can make the inlet needle valve stick open, so the carb leaks gas all over the place.  As another example, the author of this article had an engine that was working fine.  Then it sat unused for a few months during the winter.  When the gas tank was filled in the Spring, fuel began to leak all over.  The float inlet needle valve was stuck open by the junk fuel deposits left by the "modern" gas.  These reformulated fuels might burn "cleaner", but what about all the emissions caused by improperly running engines that have carbs gummed up by deposits?  What about all the gas that overflows on the ground (sometimes unnoticed until the tank runs empty), because the newer junk gas gums up carburetors so easily?  Oh well, it's all in the name of "progress".  Give me that outdated 30 year old fuel formulation any day, and give me those low fuel prices from 30 years ago as well!

The solution to the gummed up carburetor is again: disassembly and cleaning.  Adding a fuel stabilizer like Sta-bil or PRI-G should help slow down the deterioration of stored fuel.  PRI-G fuel stabilizer is especially good stuff and really works well.  You add the stabilizer along with fresh gas.  Then you run the engine long enough to be sure that the treated fuel has filled up the carburetor.  It won't do much good to add stabilizer to the tank and run the engine for too short of a time, so that the treated fuel never makes it's way through the fuel lines and into the carburetor float bowl.  If you run the engine for around 5-10 minutes, then that should be fine.  Alternatively, some people prefer to skip the fuel stabilizer, drain the tank, and then just run the engine until the float bowl runs empty.  This can work as long as you are pretty sure that float bowl is actually empty.  You can also remove the drain screw on the bottom of the float bowl and drain the bowl that way.  Easiest and most convenient way is probably to use a fuel stabilizer.  So if your carburetor leaks fuel from the overflow tube (or elsewhere), then you might need to take it apart and hope that you can get it cleaned out without too much trouble.  In the future, be aware of the junk gasoline fuel that we have to live with these days.  Hopefully this information helps a little as you troubleshoot your motorcycle carburetor or any other carburetor that you might encounter.