6 Reasons Your Motorcycle Is Losing Power When Accelerating

Acceleration on a motorcycle is one of the exhilarating parts of the riding experience. Not only is it fun, but it’s also an important part of riding; without it, we couldn’t go anywhere.

So it can be incredibly frustrating when there are accelerating problems with a motorcycle. You give it some gas and sometimes it does the opposite of what you want: loses power and actually slows down or maybe even stalls. Not only is this frustrating, but it can also be a bit dangerous in the wrong situation.

So, why is your motorcycle losing power when accelerating? There are six main reasons why a motorcycle loses power when accelerating which are:

Plugged jet in the carburetor
Broken carburetor spring
Vacuum leak on the carburetor or intake boot
Carburetor spring is on backwards
Extra slack in the throttle cable
Poor timing advance

I have experienced this very thing on a lot of the motorcycles I have owned. Through trial and error, I have been able to figure out the reasons why a motorcycle loses power when accelerating and how to fix it which will be explained.

Plugged Jet In The Carburetor

A usual culprit to running problems on a motorcycle is the carburetor. 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. The jets are tiny passageways that the fuel flows through to combine with air so the engine gets the proper air-fuel mixture. These include both the main jets and the pilot jets.

These jets often get clogged with either debris or components in gas, such as ethanol. When these get clogged, fuel cannot be delivered so your engine will be getting too much air and not enough fuel, thus causing loss of power when accelerating.

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, take the jet and spray some cleaner through it. Make sure you can see it coming out the other end. You can also blow some compressed air through it to make sure you got any debris out. Do this for all jets on the carburetor to make sure all of them are clean. Having the right tools makes a job like this a lot easier. Click here to see my list of recommended tools to use on your motorcycle.

I have created a video series about restoring motorcycles and within this series includes a detailed 25 minute video on how to clean and rebuild carburetors. 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!

Broken Carburetor Spring

As small of a part as it is, the carburetor spring is a vital part to the function of the carburetor as well as the rest of your motorcycle. If it malfunctions in any way, you’ll quickly be able to notice.

A broken carburetor spring is often a culprit to poor acceleration on a motorcycle. These springs are located where the throttle cable comes down and connects to the carburetor. This spring is what opens and closes the butterfly valve (also known as a throttle valve slide) when you’re accelerating.

If this spring is broken, it will not be able to open the butterfly valve and let any air in. As you accelerate, your motorcycle will continue to spray fuel but won’t have any air to mix it with. This causes your motorcycle to run rich on fuel. The improper air-fuel mixture will cause your motorcycle to lose power.

How to fix it: If you suspect that you have a broken carburetor spring, you can easily diagnose it by turning your motorcycle off, remove the pod filters or airbox, and peer inside the carburetor and see the butterfly valve reaction when you twist the throttle. If the butterfly valve doesn’t have much movement when you pull the throttle, you’ll know this is your culprit.

It will be easiest to completely take off your carburetor to replace the spring. 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 can be caused by either faulty gaskets on the carburetor or cracked or loose intake boots.

While a carburetor is trying to make the right air and fuel mixture to deliver to the engine, it happens inside the carburetor which should be sealed without disturbance for it to work right. 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 referred to as a vacuum leak.

A vacuum leak leads to too much air being added to the air-fuel mixture the carburetor makes. When your motorcycle engine is getting too much air and not enough fuel, it won’t run right. In fact, the more gas you try to give the motorcycle, the more air will be sucked into the unwanted holes which will ultimately make you lose power.

How to fix it: Examine the intake boots on your carburetor. It’s pretty common for them to crack or become brittle over time. Ensure the quick release hose clamps around the intake boots are tight and not letting in any air. If needed, replace the intake boots.

You’ll also need to examine the gaskets on the carburetor. These are located between the float bowl and the rest of the carburetor. These can often also become brittle and crack. Replace them if needed after examining them to ensure a tight seal between the parts.

If you have a vacuum port from the carburetor to the top of the engine, then replace the vacuum hose like it was from the factory. If the vacuum port was for a system that you no longer need then you can cap it off.

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. But it is absolutely possible for a carburetor spring to be put on backwards.

A few years back, I bought a Honda CX500 from a friend who attempted to start a motorcycle rebuild but ended up getting overwhelmed and sold it to me to finish it. When I went to start it up, I noticed it was having issues with accelerating. I had checked through everything I could possibly think was the reason. Luckily, I checked on the carburetor spring even though I knew it was new. The previous owner had placed it on backwards.

This is a problem that is usually caused by someone rebuilding the carburetor themselves. 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.

When this spring is backwards, it will do exactly the opposite of what you want it to do. Originally when you go to accelerate, you twist the throttle which should enable the cable to pull on that spring to open the butterfly valve to let air in the carburetor. A backwards spring will close the butterfly valve.

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 a simple 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 skill level, you may be able to do this with the carburetor still on the motorcycle. But it’ll be a lot easier if you remove the carburetor and fix the spring. Test the spring by twisting the throttle while the motorcycle is off and see the reaction of the butterfly valve. They should be opening every time you twist the throttle and close completely when you let go.

Extra Slack In The Throttle Cable

You may have noticed that your throttle is connected to a cable. This cable is known as the throttle cable and runs down to connect to the carburetor. This cable tells the carburetor how wide the butterfly valve needs to open and when it needs to close, depending on how fast or slow you want to go.

You may have excess slack on the throttle cable where it is connected 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. You will have an uneven air and fuel mix which will cause your motorcycle to lose power when accelerating.

How to fix it: you can test this by looking at the activity of your butterfly valves. Twist the throttle while the motorcycle is off; if the valves don’t completely open with a full throttle twist, extra slack on your throttle cable may be your problem.

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

Sometimes if you have an older motorcycle, the throttle cable stretches a tiny bit. This is something tightening cannot fix. You will simply need to get a new throttle cable. Throttle cables are pretty simple to replace and can be replaced without having to remove the carburetor at all, depending on the anatomy of your motorcycle.

Poor Timing Advance

The previous stated solutions to your accelerating problems only apply to motorcycles with carburetors. If you have a fuel injected bike with acceleration problems, your likely culprit is poor timing advance.

The faster you go on a motorcycle, the sooner your ignition coil will tell the spark plugs to fire. More firing is performed at higher speeds whereas slower speeds have less firing. The engine timing is the mechanism that controls this.

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. This will probably be accompanied with some backfiring. If your motorcycle is also backfiring, see my article here to help with troubleshooting.

Most motorcycles up to the late 1960’s had mechanical timing with a points system and a condenser. By 1970, most motorcycles started using an electrical timing system. Though the advancements of the electrical timing system are an improvement to mechanical timing, they both are still susceptible to poor timing advancement.

Poor timing is usually caused by a fault in the electrical system or the cam on the mechanical timing system could be faulty. Also, if you or someone else has messed with the timing before, that could also cause timing issues.

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. They have special tools that can get the job done quickly and thoroughly and leave you knowing the job was done right.