Six Reasons Why Your Motorcycle Loses Power when Accelerating

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One of the most thrilling parts of riding a motorcycle is the acceleration. Not only is it fun, but it’s also an important part of riding; without it, we couldn’t go anywhere.

It can be very frustrating to have a motorcycle that is having problems. Sometimes, when you give the bike gas, it loses power or slows down. This can be frustrating and even dangerous if it is not the right situation.

Hence, why does your motorcycle lose power when it accelerates? There are six reasons why a bike loses power when it is accelerating.

  • The carburetor should have a plugged jet
  • Broken carburetor spring
  • Vacuum leakage on intake boot or carburetor
  • Carburetor spring is on the backwards
  • Additional slack in throttle cable
  • Poor timing

This is something I’ve experienced on many motorcycles that I own. I learned by trial and error how to identify the cause of a motorcycle losing power when it accelerates.

Plugged Jet in The Carburetor

The carburetor is a common cause of problems with motorcycles. Unfortunately there are a lot of things that can go wrong with a motorcycle carburetor, but luckily they’re pretty simple to work on and fix.

If you’re having acceleration problems, the first thing you’ll probably want to look at is the jets inside the carburetor. These are tiny passageways through which fuel flows to mix with air. These include both main jets, as well the pilot jets.

These jets are often clogged with debris or other components of gas like ethanol. If these become clogged, fuel can’t be delivered to your engine. Your engine will get too much fuel and not enough air, which causes loss of acceleration power.

How to fix it if you suspect this is the case with your motorcycle acceleration issues, you’ll need to take your carburetor off and partly take it apart. This may be a bit annoying to have to do, but it’s good practice to examine and clean your carburetor occasionally anyway. As a disclaimer, spraying your carburetor with carb cleaner and calling it good doesn’t count as cleaning it; you will need to do a much more thorough cleaning than that.

You’ll need to remove the float bowl (remove all of them if there are multiple) and remove the jets. If you’re not sure where the jets are in your specific carburetor, you should easily be able to find a diagram online to locate it. It’s usually easily removable with a flat head screw driver.

Next, grab the jet and spray some cleaner down it. It is important to be able see the other end. To remove any debris, you can blow compressed air through the carburetor. This can be done for all the carburetors jets to ensure they are clean. This job is much easier if you have the right tools. Click here for my recommendations of tools you can use on your bike.

A video series has been created on how to restore motorcycles. It includes a detailed 25-minute video on how you clean and rebuild carburetors. You will also find other components that are difficult to handle such as the body 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!

Broken Carburetor Spring

Although it’s a small part, the carburetor Spring is crucial to the operation of the carburetor. If it malfunctions in any way, you’ll quickly be able to notice.

An ailing carburetor spring can often cause poor acceleration. These springs are located at the junction of the throttle cable and the carburetor. This spring is what opens and closes the butterfly valve (also known as a throttle valve slide) when you’re accelerating.

If the spring is broken, it won’t be able open the butterfly valve to let air in. As you accelerate, your motorcycle will continue to spray fuel but won’t have any air to mix it with. Your motorcycle will run out of fuel. A wrong mixture of fuel and air can cause your motorcycle’s power to drop.

How to fix it You can diagnose a damaged carburetor spring by turning off your motorcycle, removing the airbox or pod filters, and looking inside the carburetor to see the reaction of the butterfly valve when you turn the throttle. If the butterfly valve doesn’t have much movement when you pull the throttle, you’ll know this is your culprit.

To replace the spring, you can take out your entire carburetor. It’s really not worth it to try to fix the spring itself because they’re inexpensive, even if you have to buy a whole carburetor rebuilding kit to get it. While you’re at it, you should clean the rest of the carburetor to prevent future problems.

Vacuum Leak

Air getting in to places it shouldn’t could be another culprit of losing power while accelerating on your motorcycle. This could be due to faulty gaskets or loose or cracked intake boots.

The carburetor’s job is to produce the right mixture of air and fuel to be delivered to the engine. It happens inside the carburetor. For it to function properly, it must be well sealed. If air sneaks in from the faulty gaskets or the intake boots, it can disrupt the proper mixture it’s trying to make. This is known as a vacuum leak.

Vacuum leaks can cause too much air to be added to the mixture of air and fuel that the carburetor produces. When your motorcycle engine is getting too much air and not enough fuel, it won’t run right. The fact is that the more you give your motorcycle gas, the more air you will get into the unwelcome holes, which can eventually cause you to lose power.

How to fix it Take a look at the intake boots on your carburetor. It’s pretty common for them to crack or become brittle over time. Make sure the quick release hose clamps around your intake boots are secure and do not allow air to enter. If necessary, replace the intake boot.

You’ll also need to examine the gaskets on the carburetor. These are located between your float bowl, and the rest the carburetor. They can become cracked and brittle. If necessary, replace them after inspecting them to ensure a tight sealing between them.

You can replace the vacuum line from the carburetor to your engine as if it came from the factory. You can cap the vacuum port if it was used to power a system no longer in use.

Carburetor Spring Is On Backwards

A few of you may be a bit skeptical on this next possible reason as to why you’re losing power when accelerating on a motorcycle. It is possible to put a carburetor shock spring backwards.

A few years ago, I purchased a Honda CX500 from my friend. He tried to rebuild a motorcycle but was overwhelmed. So he sold it to me. It was having problems with acceleration when I tried to start it. I tried everything. Even though the carburetor was new, I found it and checked it. The previous owner had put it backwards.

This problem is most often caused when someone rebuilds the carburetor. If you or someone else has recently rebuilt the carburetor and notice that you’re acceleration problems, your likely culprit could be a backwards carburetor spring.

If this spring is in reverse, it will do the exact opposite of what you want. When you want to accelerate, twist the throttle. This should allow the cable to pull on the spring to open that butterfly valve and let air into the carburetor. The butterfly valve will be closed if the spring is reversed.

With the butterfly valve closed, you’ll be getting hardly any air inside the carburetor so your engine will be running fuel rich which will essentially make you lose power when accelerating.

How to fix it This is an easy fix. You’ll simply need to remove the carburetor spring that connects to the throttle cable and place it the correct way.

Depending on your level of skill, you might be able do this while the carburetor is still on the bike. But it’ll be a lot easier if you remove the carburetor and fix the spring. You can test the spring by turning the throttle with the motorbike off. This will show you the reaction of your butterfly valve. They should open when you twist the throttle but close when you release it.

Extra Slack in The Throttle Cable

The throttle cable is probably something you’ve noticed. This cable is called the throttle cable. It connects to the carburetor at the bottom. This cable tells a carburetor the width of the butterfly valve and the time it should close. It is dependent on how fast you want to go.

There may be excess tension in the throttle cable connecting to the carburetor. This in turn will lessen it’s ability to open the butterfly valve because it’s not giving it a full pull. Your motorcycle will lose power when it accelerates due to an inequal fuel and air mixture.

How to fix it This can be verified by looking at the activity in your butterfly valves. Turn the throttle when the motorcycle is not in use. if the valves don’t completely open with a full throttle twist, extra slack on your throttle cable may be your problem.

A few inches above the knob connecting to the carburetor at end of throttle cable is a tightening nuts. You can attempt to tighten that so there isn’t any slack.

Sometimes, if your motorcycle is older, the throttle cable may be a bit stretched. This is something that tightening can’t fix. It is enough to simply get a new throttle cables. It is easy to replace throttle cables.

Poor Timing Advance

These solutions are only applicable to carburetors-equipped motorcycles. Bad timing advance could be the root cause of your fuel-injected bike’s acceleration problems.

The spark plugs will start firing faster the faster you ride a motorcycle. Higher speeds result in more firing, while slower speeds produce less. This is controlled by the engine timing.

If you have poor timing advance, spark plug firing becomes delayed and isn’t timed right which will cause your motorcycle to not accelerate properly. It is likely that your motorcycle will start to backfire. You can also check my article for help with diagnosing problems with your motorcycle.

Most motorcycles up to the late 1960’s had mechanical timing with a points system and a condenser. In 1970, the majority of motorcycles used an electrical timing system. The electrical timing system’s advancements are a step forward from mechanical timing. Both are susceptible to poor timing advancement.

Poor timing could be due to a fault with the electrical system, or the cam on the mechanical system. You or someone else may have caused timing problems in the past.

How to fix it fixing the timing on a motorcycle yourself is extremely tricky if you don’t know what you’re doing. I don’t recommend you attempt to fix it yourself because if it is done wrong, you could possible cause detrimental damage to your motorcycle.

If you suspect this is the reason you’re having acceleration issues on your motorcycle, it’s best to take it in to a mechanic to have it fixed. These mechanics have the right tools to quickly and efficiently fix your problem.

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