Motorcycle Won't Go Over 40-50 MPH? (Here's Why)

When I bought my first, used Harley-Davidson, the greasy dealership salesman told me that the Dyna Wide Glide I was purchasing was bogging a little at higher speeds, but that I just needed to blow some of the old fuel out of the carb and it would be in tip-top shape in no time.

Although it was a notoriously bad sales technique, I just grinned and nodded.

At that price, I knew I could get the bike home and do some basic diagnostics on the bike and get her running just fine.

If your bike won’t go over 40-50 mph, read this!

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It Just Wouldn’t Go Over 40-50 Mph, No Matter What

When a motorcycle is having a hard time getting up to higher speeds or starts fully bogging down around 40-50 mph, there are some pretty simple ways you can adjust this.

Before you throw your hands up and let an expensive mechanic manhandle your bike, I suggest looking into some of these simple solutions.

If your bike is running fine (up until you reach 40-50 mph) you know, in a sense, that you have all three things you need for a running motorcycle. Those three things are fuel, air, and spark. But you might not be getting enough of one of those three things.

There are several things that can be causing your bike to bog down around 40-50 mph, including issues with spark plugs, fuel injectors, carburetors, and the fuel- and air lines that connect to the carburetor.

The word carburetor can send a shock of fear through anyone who hasn’t had the distinct pleasure of working on one, but they are relatively simple mechanisms that control the air/fuel mixture, atomize it, and send it into your engine for combustion.

It pays to familiarize yourself with this mechanism because chances are if you’ve got a problem with air or fuel delivery, it concerns your carburetor.

Let’s take a deep dive into some simple (and one not-so-simple) issues that are affecting your bike’s performance upwards of 40 mph, and how to fix them!

Is Your Motorcycle Rated to Go Over 40-50 Mph?

If you are new to riding motorcycles, one common thing to look at is the specifics of your bike.

If you are putting around on a smaller 50cc to 80cc trail bike, chances are you aren’t going to go much faster than 40 mph.

Do a quick internet search for your make and model of motorcycle. Although some smaller trail bikes are comfortable for smaller adults to ride, higher weight might contribute to your bike not going as fast as the top speed suggests.

Carburetor

The first thing that I would check on your bike if it won’t go over 40-50 miles per hour is the carburetor. One of the first things I would recommend that you do is check your air/fuel screw.

Because carburetors are designed to properly mix air and gasoline (creating fuel), it has to be adjusted to your bike, and your bike alone.

Using your service manual or an online source, locate the air/fuel screw on your bike’s carburetor. Sometimes an aftermarket carburetor has been added to a bike to improve performance, so be sure to search for the carburetor diagram, as opposed to the motorcycle manual specs.

I found very quickly that a few hundred bucks had been knocked off the price of my Dyna Wide Glide because it was having issues accelerating over 40-50 mph, but it was a piece of cake to adjust and I’ll tell you how to do it.

Once you have located the air/fuel screw, turn your bike on and let it sit at idle until it warms up. Usually, 5 minutes is more than enough to do the trick.

Now, using a screwdriver that will fit in the screw hole, slowly turn the air/fuel screw all the way in. When it is turned in all the way, your motorcycle will sound…very bad and this is known as rough idle.

Now you are going to back out the screw 2-3 full turns. Your engine will perk up immediately as you do this, and you will be able to hear it idling correctly. Now take your bike out for a quick run. Does it accelerate naturally beyond 40-50 mph? Done and done.

Your Jets Are Clogged

Inside the carburetor are fuel jets which are little nozzles that allow the correct amount of gas from the fuel line into the carburetor. These can become clogged and negatively impact the performance of your bike.

Sometimes opening the carburetor at the fuel bowl, or even at the air intake, and spraying a good amount of carb cleaner in there can be enough to clear out blocked or clogged fuel jets.

Put the air filter or fuel bowl back on your machine and test it to see if it will now accelerate above 40-50 mph.

If this didn’t solve it, you might want to remove the carburetor, open it up, remove the jets, and unclog all the tiny holes individually (I prefer to use a small piece of wire or guitar string to do this).

Fuel Leaks Will Affect Your Acceleration and It Is Easy to Find and Fix Them

A fuel leak is any place in the fuel line between the gas tank and your carburetor where gas is leaking out. This can affect your ability to accelerate higher than 40-50 mph because your bike isn’t getting the amount of fuel it needs, even before it gets to the carburetor.

This one is a pretty simple fix because if you have a fuel leak, you’ll be able to smell it.

Look over your fuel line for any dripping gas that is coming from your gas tank, petcock (or fuel valve), broken, cracked, or degraded fuel hose, or throttle body.

If you find any places where the gas is leaking out, that might be the culprit for your acceleration problems above 40-50 mph. Go ahead and replace any damaged section of your fuel line to prevent losing fuel when you need it most.

Vacuum Leaks Are a Common Cause of Bogging

Just like how the fuel line delivers the correct amount of fuel, your vacuum lines deliver the correct amount of air into your carburetor and on into your engine.

If you have any leaks in your air intake system, it could cause your bike to bog down when you open your throttle to accelerate above 40-50 mph.

Because your air intake lines are often made of rubber or other materials that can degrade over time, it is important to visually inspect your air line for any sign of damaged, cracking, or degraded areas. If you don’t see anything too obvious, use this trick to find any air leaks from your vacuum line.

Turn your bike on and let it idle without turning the throttle at all. Take the can of carb cleaner that you used to get all that gunk out of your carburetor and spray it anywhere there is a junction on your vacuum line.

These will be places where the rubber is connected to metal or plastic, such as the air tube connection at the airbox or carburetor.

If your idle changes and sounds like it is revving, then you know you have a vacuum leak. This could be solved by simply checking all of your hose clamps or connectors to make sure they are good and tight.

If this doesn’t solve the issue, search around the area where you heard the engine revving because that’s where the vacuum leak is. Oftentimes, a tiny crack in any hosing or connection can cause vacuum leak problems.

Once you find any part or piece of your vacuum line that has degraded beyond repair, you must replace it to keep your engine from bogging down.

If you do these simple checks and repairs, your motorcycle should easily whoosh above 40-50 mph and you’ll have a hard time getting the grin off your face from flying fast!

Spark Plugs

Spark plugs are seated in the engine head to provide the spark that is necessary to ignite the air/fuel mist in a combustion engine.

Over time, they can become dirty or fouled by excess gas or tarnished from excess air.

When this happens, they won’t fire evenly, which can have a drastic effect on your acceleration.

You will need to remove your spark plugs to inspect them.

Removal of Spark Plugs and Testing

Here is how you remove and test your spark plugs:

Remove the spark plug wires by gently pulling the boot off the head of the spark plug.
Using the correct socket, slowly loosen the spark plug.
By hand, unscrew the whole thing out of the engine head.
Visually inspect your spark plug for tarnish.
Carefully replace old spark plugs with new ones.

It is really important to be careful when removing or replacing spark plugs because any improper technique can strip the threads on the spark plug seat, causing expensive repairs to your top end.

Running On One Cylinder

If you have a twin-engine, you have more than one cylinder. If one of your spark plugs is completely bad or cracked, you might be running on just one of those cylinders.

This will limit your ability to accelerate above 40-50 mph because, essentially, only half of your engine is powering your whole bike.

There are two simple reasons why your motorcycle is only running on one cylinder. Bad spark plugs and bad spark plug wires.

Start your bike and place your hands on the head by the spark plugs. If they aren’t heating up evenly, there’s a good chance you are only firing on one cylinder.

Visually inspect your spark plugs and replace them if need be. If you still feel you are only working with one cylinder, you’ll want to check your spark plug wires.

You can do this by using a voltmeter, or by connecting a good spark plug to the wire boot, placing it up against some metal, and starting the bike. If there’s no spark, replace your spark plug wire to get your engine running on all cylinders. Be smart and don’t shock yourself, dummy!

Throttle Cable Needs Adjusting

The throttle cable is what turns the throttle body and lets fuel into the engine upon acceleration. The cable can slack up over time and needs periodic adjustment.

If you have too much slack in your throttle cable, you can’t get to the full range of your acceleration power and start topping off around 40-50 mph.

Using the designation from your service manual, tighten your throttle cable to the correct specifications and it will allow you to accelerate higher than the speed you currently could not range above.

Sources

Why is my Motorcycle Bogging on Acceleration? | WestEndMotorsports.com

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