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:
- Warm the engine
- Locate the air/fuel mixture screw
- Adjust the screw clockwise for less fuel/anti-clockwise for more fuel
- 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, 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 its happiest when the engine gets a mix of 14.7 parts air (oxygen) for every 1 part gas. And it’s the job of the carburetor to make sure this ratio is maintained. The carburetor is calibrated and tuned from the 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 as re-bores, but temperature, elevation, and humidity will affect it too.
An engine usually bogs because it’s running too rich, but a lean running motor is often described as bogging down too. So let’s find out which type of bog you’ve got, rich or lean.
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:
- Air/fuel mix off
- Dirty air filter
- Carburetor float issue
- Leaking carburetor float needle
You may find these posts helpful; they deal with rich running engines:
Stoichiometric
Stoichiometric is the optimum air-fuel ratio (AFR), and for gas engines, it’s 14.7 to 1. Your ATV engine runs at its 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, whichever 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 (under certain conditions, gas goes stale after one month)
- Carburetor jet blockage
- Faulty carburetor
- Blocked gas filter
- Blocked gas tank
- Faulty head-gasket
- Tight valve
- Intake manifold/gasket leak
You may find these posts helpful:
Rich or Lean Condition
We’ll need to understand which type of bog your ATV suffers from. To do that, we could start adjusting by 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 the choke. If the bog improves, your bike will suffer 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 means a lean running motor. Your plug should be a tan/brown color. That’s stoichiometric.
Check the spark plug condition and read the color – Check the ATV Plug color guide for what each color means.
How To Diagnose Throttle Bog
Your carburetor has a few different circuits that come into play at various stages of the throttle application. So, identifying where the bog is in the throttle range will help you find the problem fuel circuit.
- Pilot jet (circuit) – idle to 1/4 throttle
- Needle Jet – 1/4 to 3/4 throttle
- Main jet -3/4 to full throttle
In addition, your carburetor is likely fitted with an 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 supplies gas at almost every stage of the throttle. However, you should know the wider the throttle is open, the less the pilot circuit contributes to the mix.
How To Adjust ATV AFR
When we’re adjusting the air/fuel ratio, we’re adjusting gas flow through the pilot jet. Adjusting the AFR isn’t difficult, but you will need patience. It’s a case of changing 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 two-stroke is very similar, it differs as said in that it may also have an airscrew.
Four-stroke fuel screw:
- Turn clockwise – less gas
- Turn anti-clockwise – more gas
To adjust a four-stroke, begin by cleaning the air filter, cleaning the plug, or replacing it and idle the bike to warm the engine.
- Go ahead and locate the fuel mixture screw. It is 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 flathead screw.
- Using a suitable screwdriver, start the bike, allow it to idle, stab the throttle a couple of times, hear and feel it bog.
- Now, turn the fuel mix screw slowly clockwise until the bike stalls.
- Start the bike, now turn the screw anticlockwise until the engine stalls.
- 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 the mix screw makes no difference, you likely have a dirty carburetor. See below.
Dirty Carburetor
A partially blocked carburetor will cause bogging. Remember, bogging is 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 or 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 completely.
You’ll need some carb cleaner and some copper strands from the electrical cable to clean the tiny portholes of the jet and tube.
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 or a vacuum leak.
The carburetor 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. Does that 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.
Check out symptoms of poor valve adjustment – Valve adjustment symptoms
I wrote a post about checking and adjusting valves which you may find helpful – ATV valve adjustment in 6 steps
Head-gasket – Damaged gasket will allow air into the cylinder un-metered.
Manifold – Damaged or cracked manifold or gaskets will allow a vacuum leak.
I wrote a post that covers unmetered air, which you may find helpful – ATV idles up and down
You may find the following posts helpful:
I’ve written a ton of ATV troubleshooting posts. Hopefully, you won’t need them, but if you do, we have you covered.
- About the Author
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John Cunningham is a technical writer here at ATVfixed.com. He’s a Red Seal Qualified Service Technician with over twenty-five years experience. He’s worked on all types of mechanical equipment, from cars and trucks to ATVs and Dirt bikes.