Why Your 2 Stroke Is Bogging & How To Fix It

Is your 2 stroke dirt bike bogging? Does it stop accelerating when you open the throttle quickly?

If your bike has bogged going up a hill and caused you to stop, then you know how frustrating it is.

Fortunately, this article will cover all the most likely and unlikely problems that make a 2 stroke or 4 stroke dirt bike bog or sputter, why it’s happening, and then how to fix it.

Not Getting Enough Fuel

If the engine is starving for fuel, it doesn’t have anything to ignite and produce power. This is when the dreaded “bog” happens.

These are the top reasons why your dirt bike might not be getting enough fuel:

• Vapor-locked gas tank
• Fuel line pinched/kinked
• Carb float height is set too low
• Lean fuel mixture

Bike Bogs When Throttle Is Opened Quickly

This is similar to the above reason. Some dirt bikes, for a few different reasons, can’t handle going from zero throttle to wide open too quickly.
The most likely causes for bogging when the throttle is opened quickly are:

• Poor jetting
• No accelerator pump on the carb (4 strokes only)
• Accelerator pump needs adjustment
• Engine isn’t fully warmed up

We’ll look at poor jetting in a minute, but let’s look at the other reasons why your bike bogs from quick throttle openings.
An accelerator pump is simply a device (usually mechanical) on a 4 stroke carb that shoots an extra amount of fuel at low RPM throttle openings.

This is because the air/fuel mixture tends to be lean when you first open the throttle slide. The extra gas helps prevent that “lean bog” and also provide a little more low end torque.

Carburetors on many simple trail bikes are not built for high performance, so they don’t always have an accelerator pump.

If you’re riding your trail bike like a motocross bike and it bogs because you’re full throttle most of the time, you need to consider buying a more suitable dirt bike for your riding style.

2 stroke bogging at low RPM/before powerband

The “powerband” on a 2 stroke dirt bike is the “hit” of power that you feel when you get up in the high RPM range – this is one of the reasons why they’re so fun, but what if it bogs before you even get there?!

If the engine is bogging before it gets to the powerband, then the air-fuel mixture is most likely off. When the engine isn’t burning the proper amount of fuel and air, it won’t produce power and accelerate, giving you the annoying and dangerous “bog”.

Incorrect air fuel mixture can be caused by:

• Lean jetting
• Rich jetting
• An air leak
• Dirty air filter
• Powervalve not working
• Float height not properly set
• And more

How to fix a bog before the powerband

The first step is determining whether it’s a lean bog or rich bog. A lean bog will sound like the engine is dying when you open the throttle, and a rich bog will be a sputtering and inconsistent acceleration.

Next you need to figure out what throttle position you’re at when it’s bogging. At 0-1/4 throttle openings, it’s most likely the pilot jet circuit. This could be a simple air screw adjustment, or you may need to change the pilot jet size.

For 1/4-1/2 throttle openings when bogging before the powerband, the needle is the most likely culprit. Adjusting the needle height with the clip position is the simplest step to see if it corrects your issue.

If it’s bogging while using 1/2 to full throttle before the powerband, then the main jet is most likely the problem. A simple main jet change will tell you if it’s too lean or rich, and that can be done without removing your carb from the bike.
Pro Tip: You can mark throttle positions on your throttle grip for accuracy

Did You Leave The Choke On Too Long?

There is a simple solution for this one, but it’s an easy one to forget. Turning the choke on is important for starting a cold engine, but it should be turned off once the engine is warm enough to stay running.

The choke creates a richer air/fuel mixture to be able to start and idle on first start up. Leaving it on will essentially give you the same result as rich jetting, making it run poorly and sputter or bog.

The other scenario would be a choke assembly that is broken and stuck “On or Open”. Test this when the engine is fully warm by turning the choke on and off and see if your bike runs any differently.

If it runs the same in both positions, then the choke is either stuck on or off.

Engine Isn’t Fully Warmed Up

An engine that isn’t up to full operating temperature (180 degrees or higher) can’t ignite and burn the air/fuel mixture as efficiently.

Don’t do any testing, such as jet testing, until the engine is nice and hot. If it’s warm but you can still touch it with your bare hand then it’s not fully warmed up.

Rich Jetting

Jetting is a complex art to perfect, but it’s not difficult to be good at with some practice.

A properly jetted dirt bike will start easily hot or cold, idle, and run throughout the RPM range with good throttle response and without hesitation or bogging.

Jetting that is too rich will hesitate and stumble when under load. It will also not idle well, depending on which jet circuits are too rich.

Lean Jetting

Lean jetting is still a jetting problem. Not only will a lean air/fuel ratio cause bogging, it will also burn down your engine faster because there’s not enough fuel to “cool” the piston down.

A “lean bog” is most commonly noticed when you try to accelerate and it doesn’t. It sounds kind of like the engine is drowning out, or if you could spell it out, it would read: “Bwwwwahhhh”.

To correct a lean bog, you must recognize which jet circuit needs attention. To learn more about jetting and how to do it yourself read this article. 

Lean bog vs Rich Bog

There is a difference between a lean bog and a rich bog. You can tell by how your bike sounds when you turn the throttle.

A lean bog will stop accelerating and sound like the engine is dying; the same as if you were to hold the kill switch down. A rich bog may allow your bike to continue accelerating, but it will sound like it’s sputtering and hesitating.

The air screw is the first step to getting proper jetting. For a foolproof way to get started in a few minutes click here.

Fouled Spark Plug

A fouled spark plug will cause the engine to “misfire”. This is simply just a hesitation or a big and usually noticed when accelerating under load.

It can show up as an occasional problem as well as a constant problem that occurs every time you try to accelerate.

A fouled spark plug happens when there is more fuel going into the combustion chamber (engine) than it can ignite and burn.

The top causes of a fouled spark plug are:

• Jetting too rich
• Piston/rings are worn
• Ignition/spark is weak

We already discussed why rich jetting causes bogging. If you don’t fix that problem, it will eventually cause the spark plug to foul and make it run even worse. 

Worn or scored piston and rings can also cause a plug to foul easily. A worn top-end has less compression and can’t fully burn the air/fuel mixture.

It also allows more oil to sleep past the rings, which increases the chance of a fouled plug. 

Bad CDI Box

Lastly, a faulty or weak ignition box is a possible culprit to fouling your spark plug. It is not as common, but it still happens and is worse looking into after you’ve tried the easier options.

The quickest way to test the CDI ignition is to swap it with a known working unit if available.

Loose/Broken Spark Plug Cap

A poor connection to the spark plug will definitely give you a bad running bike. It will sputter and bog and not be able to accelerate very well.

Check the plug cap and wiring, if it’s loose, push it firmly back on the spark plug. If it doesn’t feel like it’s “clicking” onto the plug, look at the internal cap to see if it is damaged or missing.

Replace if necessary and be on your merry way having fun riding again.

Loose Spark Plug

A loose spark plug causes bogging because it doesn’t reach far enough into the combustion chamber to ignite all the fuel. A spark plug can become loose from vibration over time, or if it wasn’t properly tightened to begin with. 

Of course, you should never over tighten a spark plug because then it can break off and you’ll be stuck with trying to remove the other half from the cylinder head.

Just refer to your owners service manual for the manufacturer’s recommended torque spec when installing the plug.

Dirty Air Filter

Getting the correct air/fuel mixture is key to being a jetting master. However, if the air coming in is inconsistent, it’s hard to get a consistently good running bike.

A dirty air filter not only flows less air to the carburetor and engine, but it’s more likely to allow dirt pass by and reduce the lifespan of the engine.

Less clean air means the jetting will become more rich, which can cause your dirt bike to bog. Keep the air filter clean, whether you have a 2 stroke or a 4 stroke.

Cleaning/replacing the air filter is regular maintenance that you need to be doing to any dirt bike you own.

Faulty Stator

The engine is the life of your bike, but the stator is what uses the engine to power everything else electrical. A bad stator can cause some major issues and potentially not allow your bike to run well or at all.

If the stator doesn’t work, the battery (if your bike has one) won’t charge, the lights will stop working (immediately if there’s no battery), and there won’t be power to the ignition that keeps the engine running.

Stator failures are few and far between on most quality brand dirt bikes, but if you’ve checked everything above, then it’s worth testing the stator.

Electrical Short/Broken Wire

Electrical gremlins can seriously test a man’s wits. An electrical wire or connection that is corroded or broken can cause intermittent bogging problems.

Places to check for bad grounds/wires:

• Spark plug wiring/cap
• CDI ignition wiring
• Kill switch wiring/connection
• Battery connectors/wiring (if applicable)
• Mastery ignition switch/key (if applicable)

Look for rust/corrosion, any broken or loose wires. Dirt bikes don’t have that much wiring compared to a car or a street bike.

Wires in locations that might get rubbed against something, such as the forks, are one spot where a wire might have rubbed through and is causing a “short”.

Timing Is Off

The ignition timing can’t easily be changed on most dirt bikes, but the cam timing can on 4 strokes. The cam or camshafts may be incorrectly timed due to installation error or because the cam gear jumped a tooth.

If the timing is off by one tooth then your bike will be hard to start, run poor (bog), and may cause serious engine damage if internal parts get too close.

Cam timing can “jump” out of place. This will make your good running 4 stroke into a very poor running 4 stroke.

It will bog, misfire, backfire, pop, and just not sound right. If the cam timing jumps a tooth, it needs to be fixed right away to prevent any more possible damage to your engine.

When you correct the cam timing, always replace the timing chain with a new one. The cam jumped a tooth because the timing chain was worn out and got too loose.

Top 15 Reasons Your Dirt Bike Is Bogging:

• Not getting enough fuel
• Choke was left on
• Choke is stuck
• Engine isn’t fully warmed up
• Jetting is too rich
• Jetting is too lean
• Fouled spark plug
• Spark plug is loose
• Spark plug cap is loose/broken
• Air filter is dirty
• Throttle opened too quickly
• CDI box is bad
• Stator is bad
• Electrical wiring short/broken wire
• Timing is off

Your next step?

Want to learn how to learn how to fix bogging, hard starting and plug fouling by taking a few minutes to dial in a jet circuit? Click here to learn how, even if you’ve never done it before.