Motorcycle misses at high rpm

Motorcycle misses at high rpm

Carburetor problems generally fall into three areas: too lean a mixture, too rich a mixture, or an incorrect adjustment. Luckily there are telltale symptoms that will tell you which problem you're dealing with. Carburetors are relatively simple devices. However, as with all mechanical devices, carburetors require periodic tuning and service, and will eventually wear out.

Before attempting to fix a carburetor problem on your motorcycle, you need to come up with the correct diagnosis. When a carburetor is running lean, the fuel-to-air ratio is off because the carburetor is delivering too much air. Typical symptoms of a lean mixture are:. When a vehicle is running rich, the fuel-to-air ratio is off because the carburetor is delivering too much gasoline. Typical symptoms of a rich mixture are:.

Incorrect adjustment can produce any of the previously noted symptoms. On multicylinder machines, with separate carburetors for each cylinder, the following symptoms are typical of an adjustment problem:. This condition is generally caused by the incorrect fitting of after-market accessories such as exhaust systems, air filter systems, or replacement carburetors of a different type or size.

In addition, if the fuel level in the float chamber is set too low, insufficient fuel will be drawn through the main jet. Others have an air adjusting screw. Turning this screw clockwise will reduce the amount of air entering the carburetor, and will, therefore, richen the mixture refer to a shop manual for correct settings. If no changes have been made to the bike, and it previously ran well, a lean mixture can be traced to a leaking inlet manifold or leaking exhaust often at the interface of header pipe and cylinder head.

This condition is primarily caused by dirty air filters, but it could also result from the incorrect fitting of after-market accessories such as exhaust systems, air filter systems, or replacement carburetors of a different type or size. In addition, if the fuel level is set too high in the float chamber, a rich mixture will result. To remedy, clean your air filter, have your mechanic check the exhaust and carburetor fittings, or both. This situation is mostly caused by poor maintenance.

With the inherent vibration of all engines, carburetor parts—primarily adjusting screws—tend to rotate, and therefore change their positions. Low-speed running jets and multicylinder balancing screws are the items most prone to self-adjust during normal operation and often require periodic corrections.

John Glimmerveen. John Glimmerveen is a former competitive motorcycle racer. He later worked as a race technician for several international race teams. Updated August 04, By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

It only takes a minute to sign up. I have a Yamaha XS I want to tune my carbs yes, I know that the B34s are notorious for being lean in the first place. I have attempted adjusting the pilot screw, which did alleviate the popping symptoms and gave me a much smoother ride at 25 mph.

In order to truly tune my carb, I want a better understanding of the symptoms of lean vs the symptoms of rich.

Terms often used are: Stuttering Hesitating Running Rough etc etc. In your post you indicate that some of the symptoms you have read are contradictory. I will try and clarify between the two conditions and attempt to give you guidelines in troubleshooting between the two. Pilot Circuit - small effect on idle. Higher effect at lower RPM's with decreasing effect to full throttle.

5 Reasons Why Your Motorcycle Is Sputtering

Main Jet - Wide open. Fuel is metered through the main by the jet needle at different throttle positions. Choke circuit - Initiated by a valve or butterfly that increases the vacuum and opens the circuit up. Cold starting and warm up. Wide open throttle yields no power. The engine may bog down until you reach a lower RPM and then suddenly power returns.

Runs better at higher altitudes - AFR becomes normalized due to reduced atmospheric pressure allowing fuel into the venturi easier. Backfires - Popping on deceleration for a lean idle circuit or backfires in general. Runs on choke - The vehicle may run on choke when it's warm but stalls if the choke is turned off. Engine runs hot - Due to more oxygen than fuel combustion temperatures are hotter reflecting on a temp gauge.

Hanging idle - The engine idles high and then drops and stalls. Typically a lean condition caused by an air leak between the butterfly and the head or a vacuum line that is not attached. Sharp Odor - The exhaust smell may be sharp and burn your nose. This is NOx or Nitrogen Oxide. It's created by high temperatures in the combustion process between nitrogen and oxygen. Although NOx is present in all exhaust it can be more pronounced in higher combustion temperatures associated with lean conditions with more oxygen.

Reduced Fuel Mileage - Your using more fuel per cycle so your mileage becomes worse. Rough Idle - Combustion takes a just a bit longer with more fuel and at lower RPM's the engine can idle rough.

Unlike a miss-fire on a spark plug this idle seems fine if the idle is turned up just a bit. Spark Plugs Black - Carbon buildup on insulator and electrodes of plug. Wipes off your finger as sooty and back. Carbon is a conductor of electricity and getting considerable carbon buildup from a rich condition with ground the inner electrode down the positive insulator.

motorcycle misses at high rpm

When this condition occurs there will be no spark. You can clean the sparkplug with carburetor cleaner and re-use. Exhaust Exit is Black and Sooty - Excessive carbon buildup in the exhaust is thick and overly grimy. A properly jetted carburetor will leave a nice grey scale color in the end of the exhaust pipe.A backfire is a loud pop or bang sound that comes from your exhaust pipes when your motorcycle is running.

Lean and Rich Symptoms in Motorcycle Carburetors

A motorcycle exhaust backfire is an annoying and potentially dangerous problem to have. Why is my motorcycle backfiring? A motorcycle backfire happens because of the presence of uncombusted fuel in the exhaust pipe. This issue is caused by:.

I have worked on dozens of motorcycles over the years and backfire issues seem to come around every once in a while. I have a minor in automotive engineering and spent time in some of my classes studying and fixing backfires.

Click here to see my list of recommended tools to use on your motorcycle. Most motorcycles from on have electronic timing which makes things a lot easier. Electronic timing has simplified how to troubleshoot a backfiring problem because there are less parts to check off as the culprit. The way electronic timing works is by sending a voltage signal to your ignition coil when one of the cylinders is in its compression stroke and about ready to fire.

When the ignition coil receives this voltage signal it dumps its built up voltage which is carried through your spark plug wire, through your spark plug, and finally ends up as a big spark right at the top of your cylinder.

The culprit could be things like the timing chain is one tooth forward or backwards of where it should be, or an exhaust valve is sticking open for a little bit too long which is a pretty common problem with older motorcycles. When you have a classic motorcycle that still has points and a condenser then things get a little more tricky because now not only do you have all those mechanical timing issues to worry about, but you also have to figure out if your ignition is manually timed correctly.

My Triumph originally had a points system and I got tired of it so I installed an electronic system. A point is basically two small metal discs that open and close as your engine spins around. Every time the discs separate it sends the signal to the ignition coil to send the spark to the spark plug.

motorcycle misses at high rpm

A point has to be set manually by following the written instructions from your motorcycle shop manual or repair manual. Once the timing marks are lined up you have to set the points a specific distance away from each other. Again, this all depends on what model of motorcycle you have as they are all different. If your timing is off and the spark gets to the end of the spark plug a little too late then the exhaust valve in your cylinder head will have started to open, and the spark plug finally sparks.

In order for the combustion process to take place successfully inside the cylinder there needs to be a certain amount of air and fuel.

Pilot Air/Fuel Screw Adjustment Explained - Single Carb - Part 1

A rich condition on an engine is not a a healthy way to run and should be fixed. Running rich not only drastically decreases your miles per gallon, but can cause severe backfires in the exhaust pipe. If there is too much fuel present in the cylinder when the spark plug ignites then not all of the fuel will be successfully burned during the combustion process, there will be excess fuel that is pushed out of the cylinder head through the exhaust valve.

This unburned fuel will come in contact with atmospheric air and be surrounded by the extremely hot exhaust header. In the presence of fresh air and high temperatures, the fuel combusts and creates a loud pop or bang sound. I was once troubleshooting a 2 cylinder motorcycle and each cylinder had a separate exhaust pipe.

motorcycle misses at high rpm

I would put my hand by the end of each exhaust pipe in turn to see how much air was coming out. If one side is weaker than the other then you know you have a problem on that one cylinder.

As I was reaching down to feel the air coming out, there was a loud bang and fire erupted out of the tailpipe, missing my hand by about an inch. I no longer put my fingers right by the exhaust pipe of a running motorcycle! The culprit of getting too much fuel in your cylinder is almost always the carburetor.It can be incredibly frustrating when a motorcycle is having issues that affect your ability to ride it.

Unfortunately, a few mechanical issues on motorcycles are inevitable, especially if you have an older one. 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.

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.

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. Vacuum leaks can cause several problems with the functionality of a motorcycle, especially sputtering. 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. 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.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

It only takes a minute to sign up. The first time I took the bike out for a ride I noticed that when you accelerate hard and the engine is at high revs it feels as if the bike has got a dead spot and as if it's losing power. I didn't think much of it and rode the bike everyday to work the bike did not give any trouble. I rode the bike home while it was misfiring. I thought that the bike probably had a faulty spark plug because i have experienced a faulty spark plug before.

I replaced all four spark plugs and then took the bike for a test drive and it was running smooth after a while of riding the engine started to misfire again when I accelerate hard then it misfires all the way whether you accelerate or decelerate till you reach a stopping point in the road then the engine cuts out.

I switched the engine off and smoked a cigarette then I started it up again and then the engine was fine and running smooth on my way home the engine started to misfire again.

motorcycle misses at high rpm

When i got home i went to a motorcycle dealer and explained him the problem he advised me that the problem sounds to him like a faulty coil. I then bought a coil and replaced it taking turns in the two coils to check which coil is faulty no matter which coil i replaced the new coil with the bike kept on misfiring.

The bike misfires so bad now that it misfires at start up point where it just used to misfire when the engine is hot. Another question, should the bike run on LRP fuel or unleaded? If the bike has been sitting in a garage for an extended time, the membranes in the carbs might gotten hard.

If that's is the case the mix is too poor, when the engine has to deliver power.

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The plugs is probably light grey in that case. If the RPM limiter sets in too early, the sparks would be wet and black, because of the unburned fuel. My first thought was that the coil was faulty It can be a CDI problem too Try cleaning the carbs with pressurized air take them out, dismantle them and clean them, or ask somebody to do it. Beware, you risk damaging them if you make any mistakethen you can try some carb cleaner spray. My 2 stroke does that too when the engine is cold, but it gets smooth again after meters When it goes up to rpm, when the power valve is supposed to kick in, it will just start lagging and the valve won t open.

I know 4 strokes are different, but maybe yours has a rpm limiter too How could you know? It s kinda time consuming but it s worth trying: Download your bike's user manual from the internet make sure you get the exact model and YEARand search for the electrical diagram.

Then go to the bike and compare what is in the diagram with what is on the bike About the fuel question: check the manual. It has to be written somewhere what kind of fuel you are supposed to use. Every owner's manual has this specification. I would suspect an electrical problem with the coils or CDI. Analogue CDI systems of this era used components that can degrade over the decades and act intermittently as you describe particularly over different temperatures.

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How to Diagnose Carburetor Problems in Your Motorcycle

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