It can be incredibly frustrating when a motorcycle is having issues that affect your ability to ride it. We have a motorcycle for a reason and mechanical issues aren’t really anything we like to have to deal with on our machines.
Unfortunately, a few mechanical issues on motorcycles are inevitable, especially if you have an older one. I’ve owned a lot of motorcycles the past few years and have run into every mechanical issue possible. One common issue I often ran into was sputtering problems.
So, what causes a motorcycle to sputter? There are several reasons why a motorcycle sputters. The most common reasons are carburetor issues such as a vacuum leak, fuel leak, or tuning issues. Other culprits could include corroded or cracked spark plugs or spark plug wires, a faulty ignition coil, a clogged air filter, or engine timing issues.
The three things you need to create an explosion in the combustion chamber of a motorcycle are air, fuel, and a spark. Examining the mechanics that deal with these three components will ensure you’ll find your sputtering issue. I’ll discuss the most common reasons for sputtering.
Sputtering Due To Carburetor Issues
Carburetors have been a great mechanical product for older machines and usually does a pretty good job with providing the right air and fuel mixture the engine needs to power the motorcycle. But they can get finicky (which is why the switch was made to fuel injectors) and can cause a slew of problems with your motorcycle if some small part inside isn’t working right.
If you have a carbureted motorcycle that is sputtering, you’re likely culprit is the carburetor. There are three main issues that can be caused from the carburetor that will make your motorcycle sputter. The first is a vacuum leak, second is a gas leak, and third could be that the carburetor needs to be tuned. Having the right tools on hand can make these repairs easy. Click here to see my list of recommended tools.
First, we’ll discuss the possibility of a vacuum leak within your carburetor. Vacuum leaks can cause several problems with the functionality of a motorcycle, especially sputtering. If there’s a vacuum leak, that means the carburetor isn’t giving the proper amount of air to the air and fuel mixture which will ultimately result in sputtering and poor engine performance.
Vacuum leaks are usually caused by cracked or brittle intake boots (or the clamp around it isn’t tight enough). This is located between the carburetor and the engine. Intake boots are inexpensive and can easily be replaced if you suspect this is your issue.
Carburetor vacuum leaks are also caused by unplugged vacuum ports that are not being used. Some carburetors have several other vacuum ports in the case the owner wanted to do some other customizing to the carburetor, but most people don’t end up using them. Vacuum port plugs are cheap and are easy to install.
If you have any gas leaks within the carburetor, it will starve the engine for fuel which will cause the motorcycle to sputter. Leaks within a carburetor is usually caused by a brittle or cracked gasket that is just above the float bowl at the bottom of the carburetor. You can usually tell if this is your culprit because you will easily notice the smell of gas and will be able to visually see any leaks that are happening in the area. Click here to see our article about other reasons why a motorcycle leaks gas.
Float bowl gaskets are also generally inexpensive. You will easily be able to look up a tutorial online about how to replace the float bowl gasket specific to the carburetor you have. If you have more than one carburetor, it’s a good idea to change all the float bowl gaskets because they’re probably worn out too.
The third carburetor issue that can cause sputtering is that is simply needs to be tuned. When a carburetor is out of adjustment, that means the air/fuel screw and the balance between the carburetors (if you have more than one) need to be readjusted.
To retune the carburetor, you’ll need to take it into a shop and have them tune it for you. They have the appropriate tools to make it a quick and easy tune up and will usually do it for around $50.
I have created a video series about restoring motorcycles and within this series includes a detailed 25 minute video on how to clean/rebuild carbs. This series also includes other hard-to-tackle components such as body work and electrical. I give dozens of tips and tricks that you won’t find anywhere else online. Click here for more information if you’re interested in viewing multiple videos that will help fix up your bike or if you’re interested in completely building your dream motorcycle!
Sputtering Due To Ignition Issues
If you’ve ruled out that your carburetor is working just fine or if you have a fuel injector, the next likely culprit would be ignition issues. There are three possibilities that could happen with ignition issues. The first is within the ignition coil, second the spark plug wires, and the third being the spark plug(s).
Start by checking the easiest spot first which would be the spark plugs. Take out your spark plugs and make sure there is no nasty build up on them or any type of corrosion. Also make sure the point isn’t worn down and that the gap between the center and the ground electrode is the appropriate distance. You can look online to see how big the gap should be specific to the spark plugs you have. You can find the tool to measure that at any auto store.
A malfunctioning spark plug will cause the cylinder it’s connected with to misfire which will ultimately cause performance issues and the motorcycle to sputter.
Next, check the spark plug wires. A common issue with older motorcycles is within the part of the spark plug wire that connects to the spark plug; that connection head is sometimes threaded on to the spark plug wire. Over time, that threaded connection becomes corroded and rusted.
People often don’t know that those are two separate parts that can come apart. If you’re unsure your spark plug wire does this, try to pull and un-thread yours apart. If it’s pretty snug on there, it’s probably not threaded and likely not the problem. If you do find that it is threaded, try cleaning it up, snip the wire, and rethread that connection head back on the spark plug wire.
Also check for any cracks on the spark plug wires. If there are any cracks anywhere on the wires, the spark will arc to your motorcycle frame because that’s the path of least resistance for the electricity and it won’t arc through the spark plug. With no electricity delivered to the spark plug, that will in turn cause that cylinder to misfire. You’ll simply need to get a new spark plug wire.
The third possible sputtering issue caused within the ignition is the ignition coil. It is possible for the ignition coil to not send enough spark to the spark plugs which will again cause misfires within the engine.
This is probably the least likely case with the ignition. This is best diagnosed through a mechanic if you suspect this is what is causing the sputtering problem. Ignition coils have really high voltage and is dangerous to handle if you don’t know exactly what you’re doing with it.
Sputtering Due To Clogged Air Filter
A clogged air filter on a motorcycle is another big possible culprit to sputtering issues. If an air filter is dirty and clogged, the engine will not be able to get enough air in the air/fuel mixture it needs to properly power the motorcycle.
Air filters need to be changed out routinely and people often forget to pay attention to them until it starts giving them problems. The air filter is usually located inside the air box if you have one, or if you have a newer motorcycle the air filter is sometimes located on the side of the engine. Examine it to determine how dirty it is. Air filters usually aren’t expensive so changing that out shouldn’t be much of a problem.
If you have pod filters, examine them to make sure there isn’t some sort of residue on them that’s preventing air from flowing through them. Click here to see our article about pod filters vs. an air box.
Sputtering Due To Timing Issues
It is possible for a motorcycle to sputter due to timing issues, though this is a less likely reason. It’s pretty hard to have the timing randomly start getting off by itself and is usually caused by either someone rebuilding the engine and getting the timing off or someone tried to fix the timing themselves previously.
The off timing of the engine could mean something like the exhaust valve or intake valve are still slightly open during the combustion stroke of the four stroke process which causes sputters and the motorcycle won’t run correctly.
The best way to diagnose a timing issue on a motorcycle is to take it in to a mechanic. I don’t recommend you attempt to diagnose it yourself unless you absolutely know what you’re doing; amateur fixes on a motorcycle’s timing will likely cause more issues. A mechanic can quickly diagnose the issue and usually won’t charge much simply for the diagnosis
How do you fix a motorcycle running rich? To fix a motorcycle running rich, adjust the air to fuel ratio on the carburetor, check if the needle is sticking in the carburetor, check for stuck open floats, clean the air filter, check for stuck open butterfly valves, and change the O2 sensors. Click here to see my article for more info.
Can you convert from a carburetor to a fuel injector? It is possible to convert from a carburetor to a fuel injector. Doing such a conversion does require mechanical expertise and a knowledge of electric wiring, so it is not recommended to do it yourself.