Why is My ATV Bogging Down? (This will fix it)

It’s so annoying, just when you need the extra power, you hit the gas and Waaaa….

An incorrect air to fuel ratio mix is the number one cause of ATV bogging. There are two types of bog, a lean and a rich bog. The fix is usually pretty simple, adjust the AFR (air fuel ratio) as follows:

  1. Warm the engine
  2. Locate the air/fuel mixture screw
  3. Adjust the screw clockwise less fuel/anti-clockwise more fuel
  4. Test engine and repeat if needed

In this post you’ll learn how to tell if your engine is running rich or lean, and how to adjust your AFR (Air Fuel Ratio). But there may be other reasons your engine is bogging, and so we’ll cover them here too.

Air/Fuel Ratio (AFR)

Your engine runs at its best when the air to fuel ratio is 14.7:1. That means your engine is at it happiest when the engine gets a mix of 14.7 parts air (oxygen) for every 1 part gas.

And it’s the carburetors job to make sure this ratio is maintained. The carburetor is calibrated and tuned from factory but is fitted with an air/fuel mix adjustment screw.

The adjuster screw is needed, as engines move outside the optimum AFR as they age or after major engine repairs, such re-bores, but temperature and elevation, humidity will effect it too.

An engine is said to be running Rich, stoichiometric or Lean.

An engine usually bogs because it’s running too rich, but a lean running motor is often described as bogging down too. So lets, find out which bog you’ve got a rich or lean bog.

Rich Condition

A rich running engine is an engine that is receiving too much gas proportional to the amount of air it’s receiving. Or, “the engine isn’t receiving enough air”, is another way to look at the problem.

Symptoms of a rich condition include:

  • Bogging
  • Spluttering
  • Black smoke
  • Hard starting
  • Plug fouling
  • Stalling

What Causes a rich Condition?

A rich condition may be caused by:


Stoichiometric is the optimum air fuel ratio (AFR), and for a gas engines it’s 14.7 to 1. Your ATV engine runs at it very best here, and it’s this ratio a mechanic is chasing when tuning your bike.

Lean Condition

A lean condition is an engine that isn’t receiving enough gas in relation to the amount of air it’s receiving. Or, too much air, which ever way you want to look at it.

It means the ratio of air to fuel is above the optimum 14.7:1.

Common symptoms of a lean condition include:

  • Engine only runs on choke
  • Engine won’t start
  • Engine starts then stops
  • Popping and banging
  • Hanging idle

A lean running bike is common and mechanics will usually identify the condition and make finer adjustments to the AFR at service intervals.

What Causes a Lean Condition?

A lean condition is commonly caused by normal wear and tear on engine components, vibration can also cause maladjustment of the air fuel mix screw.

But there are many other possible causes, some of which include:

  • Bad gas (gas goes stale after 1 month)
  • Carburetor emulsion tube/main jet blockage
  • Faulty carburetor
  • Blocked gas filter
  • Blocked gas tank
  • Faulty head-gasket
  • Tight Valve
  • Intake manifold/gasket leak

Rich or Lean Condition

We’ll need to understand which type bog your ATV suffers from. To do that, we could simply start adjusting adding gas or taking it away and see which improves our condition.

However, I prefer to use a few mechanics tricks to give us some idea before adjusting the fuel ratio.

1 – apply choke, if the bog improves, your bike suffers from a lean condition and we’ll need to add more gas.

2 – remove the air filter, if the bog improves, your bike suffers from a rich condition and we’ll need to reduce gas.

3 – pull the spark plug and read it. A black sooty plug indicates a rich running motor. A grey/white plug indicates a lean running motor, your plug should be a tan/brown color, that’s stoichiometric.

How To Diagnose Throttle Bog

Your carburetor has a few different circuits that come into play at different stages of the throttle application. So, identifying where the bog is, will help you fault find the correct circuit.

  • Pilot circuit – idle to 1/4 throttle
  • Needle – 1/4 to 3/4 throttle
  • Main jet -3/4 to full

In addition your carburetor is likely fitted with a accelerator pump that only comes into play when you nail the throttle quickly.

So, if your bog is the instant you touch the throttle, likely pilot adjustment, cleaning or re jetting.

If bogs when you nail the throttle from idle – check out the accelerator pump timing, diaphragm and leak jet.

If it bogs at full throttle – check the main jet for issues, dirt, resizing etc.

If it bogs between 1/4 and 3/4 only – check out the needle clip position or needle wear.

In any event, begin by adjusting the pilot air/fuel mix as the pilot supply’s gas at every stage of throttle.

How To Adjust ATV AFR

When we’re adjusting the air/fuel ratio, we’re adjusting gas flow to the pilot jet.

Adjusting the AFR isn’t difficult, but you will need patience. It’s a case of adjusting and testing several times to find the sweet spot. The sweet spot is different for every individual bike.

You should note, this process covers a four stroke bike and while a 2 stroke is very similar, it differs in that a four stroke engine carb screw adjusts gas flow and a two stroke engine adjusts air flow.

4 stroke fuel screw:

  • Turn clockwise – less gas
  • Turn anti-clockwise – more gas

2 stroke air screw:

  • Turn clockwise – less air
  • Turn anti-clockwise – more air

To begin, clean air filter, clean the plug, or replace it and idle the bike to warm the engine.

1 Go ahead and locate the fuel mixture screw. It is may right under the bowl, some can be a pain to access. The air filter and trunking must remain in place while adjusting.

The adjuster located on the carburetor, will likely be a brass flat head screw.

2 Using a suitable screwdriver, start the bike, allow it idle, stab the throttle a couple of times, hear and feel it bog.

3 Now turn the fuel mix screw slowly clockwise until the bike stalls.

4 Start the bike, now turn the screw anticlockwise until the engine stalls.

5 Start the engine, and turn the screw clockwise 1/8 turn at a time until you find a crisp response from the throttle, usually around the highest idling rpm.

Now adjust the idle speed until it’s smooth. Ride the bike, it should perform well under load and not just when revving.

If adjusting mix screw makes no difference, then you likely have dirty carburetor, see below.

Dirty Carburetor

A partially blocked carburetor will cause bogging, remember bogging is basically fuel starvation. If applying the manual choke or priming helps the engine run better, and you’ve tried the adjuster screw, then it’s likely you have a dirty carburetor emulsion tube and main jet.

In many cases you’ll only need to remove the carburetor bowl to access the main jet and emulsion tube. But some engines are compact and you may need to remove the carburetor complete.

You’ll need some carb cleaner and some copper strands from electrical cable to clean the tiny port holes of the jet and tube. I wrote a whole post about it here.

Un-metered Air

As you know a lean condition is usually caused by a lack of gas. But too much air in the mix can cause a lean condition too.

Air that enters the combustion chamber without passing through the carburetor is known as un-metered air.

The carburetor remember measures incoming air and is calibrated to add a set amount of gas. But if air sneaks in some other way, the ratio is off, make sense?

So where could air sneak in? The usual places are:

Tight valves – Valves that are tight/damaged may be slightly open and that will allow un-metered air in.

Head-gasket – Damaged gasket will allow air into the cylinder un-metered.

Manifold – Damaged or cracked manifold or gaskets will allow a vacuum leak.

John Cunningham

John Cunningham is an certified mechanic and writer on ATVFixed.com. I’ve been a mechanic for over twenty years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of ATV ownership, from maintenance, repair to troubleshooting.

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