Popping and banging noises from your ATV is annoying, but it’s also a warning sign that you probably shouldn’t ignore.
An ATV makes a popping noise because the engine is running lean, and the top 8 reasons an ATV engine runs lean include:
- Temperatures, altitude and humidity changes
- Carburetor out of adjustment
- Engine modification
- Carburetor fault
- Bad gas
- Vacuum leak
- Valve lash off
- Mechanical issue
In this post, you’ll learn what a lean condition is, why your ATV makes a popping noise and what you can do to fix it. The following post assumes your bike is fitted with a carburetor and not fuel injected.
Symptoms of Lean Engine
Lean running engine symptoms vary by engine and by modifications, but the usual telltale signs include:
- Popping
- White plug
- Hot running engine
- Misfiring
- Hanging idle
- Rough running
- Bogs on takeoff
What’s Lean Running?
A popping sound in the engine is a classic symptom of a lean running engine. Owning an ATV, you’ll be aware no doubt of the delicate balance of air to fuel (known as AFR – Air Fuel Ratio) that must be maintained by your carburetor.
Any change to how your motor breaths will have a direct effect on this balance. A ratio of 14.7 parts air (oxygen) to 1 part gas is the sweet spot for an ATV engine and is known as stoichiometric. A lean engine is a cylinder that isn’t getting enough fuel in relation to the volume of air it receives. Or another way to look at it is “engine is receiving too much air.”
A lean condition is an engine with an AFR above 14.7:1. (stoichiometric) A rich (fat) running engine is an engine that is receiving too much gas for the volume of air it’s receiving. Or another way to look at it – “engine isn’t receiving enough air.”
A rich condition is an engine with an AFR below 14.7:1. (stoichiometric)
Your carburetor is designed to measure and mix gas in the ballpark of the stoichiometric ratio. It does so using fuel jets with very precise orifices through which gas flows. Adjusting screw(s) on the body of the carburetor helps a technician fine-tune the mix. A single air-fuel mix screw is the norm, but some carburetors may have a separate screw for both (one for air and one for gas). See adjusting below.
Dangers of a Lean Condition
A lean engine runs hotter than normal, and that can lead to all sorts of expensive repairs. The lean condition, as you know, could be caused by something simple, but it could also be a symptom of a more serious underlying fault.
1 Temperature
Air temperature, altitude, and humidity all affect the volume of oxygen in the air. Colder air, for example, causes an engine to run lean because the air is denser.
At colder temperatures, the engine is sucking in a larger volume of oxygen-heavy air than at higher temperatures.
Oxygen and gas (AFR) must be mixed to stoichiometric, and unlike a car, most ATVs don’t have a closed-loop computer making real-time adjustments for oxygen volume.
Instead, we make small adjustments to the air-fuel mix screw manually and or move the carburetor clip position and in some cases, a carburetor may need a re-jet.
2 Carburetor Adjustment
Carburetors are set from the factory, but component wear, vibrations, rough terrain can all cause carburetors to fall out of adjustment. Most carburetors are fitted with one fuel mix screw. Some may have two adjustments screws – an air screw and a fuel screw.
If fitted, the air mix screw will be on the air filter side of the carburetor, and the fuel mix screw will be on the engine side of the carburetor. Adjusting the fuel mix screw anti-clockwise adds more fuel to the mix, and turning it clockwise removes gas from the mix. A normal setting for the fuel mix screw is about 1.5 – 2 turns.
An airscrew works in the opposite way. Turning anti-clockwise removes air from the mix, and turning clockwise adds air to the mix.
Below we’ll adjust a carburetor with a fuel mix screw only, often referred to as the pilot mix screw. Begin with a warm engine and find your base setting. Having a good screwdriver as pilot mix screws are made from brass, which is a soft metal and will break if abused.
Step 1
- To find your base setting, engine off, and using a screwdriver, count accurately the number of turns clockwise to seat the mix screw. No need to tighten the screw. Just seat it. A ballpark is usually about two turns
- Now, go ahead and set your mix screw back where it was. Knowing your base setting means you can return to it at any time if everything turns to crap.
Step 2
- As we believe your engine is running lean, you’ll need to move the mix screw anti-clockwise until the engine bogs and stalls. Move about 1/8 of a turn at once and test. Small changes are better as it’s easy to overdo it.
Step 3
- Move the screw clockwise now until the engine bogs and stalls.
Step 4
- Now set the screw where the revs are highest within that window. The bike should rev cleanly without bogging.
Step 5
- Now set the idle screw so that the engine idles smoothly at about 1000 rpm.
Black smoke or a black fouled spark plug indicates a rich running engine and means you’ve gone too far. If it takes more than 2 and 1/2 turns to dial in your engine, it suggests you may need to re-jet. Just under two turns is about right for a fuel mix screw. The bike should rev cleanly without bogging.
3 Modification
If you’ve had some light modification like fitting a bigger air filter or free-flow exhaust, you’ll need to adjust the carburetor. If you’ve done a re-bore or sometimes just resetting valve lash throws the cab out a hair.
Mods that help your engine suck in more air or breath more freely are great for power, but you’ll need to add more fuel to keep the ratio at that sweet spot.
Try adjusting the fuel mix, but if that doesn’t work, you’ll need to go balls deep, move a clip, or re-jet.
Re-jetting
OK, but what’s a jet? It’s a brass screw with an orifice that is very precisely measured. Each jet is marked with a number indicating the orifice size. The bigger the orifice, the more fuel the jet can supply the engine. If you’ve tried adjusting your fuel mix screw and your more than 2 and 1/2 turn out, it’s likely you need to re-jet. Re jetting your carb isn’t technical. It’s more trial and error and plug reading.
To successfully re-jet, you’ll first need to carefully assess how your carburetor is fueling at the different stages of the throttle. You’ll need to pull your carb apart and see what size pilot, main and clip position you’re running.
Most ATV carburetors are either mechanical slide or vacuum-actuated CV (constant velocity) type. These carbs usually have three main fueling circuits. Idle, Midrange and Main.
1 Idle circuit (Pilot jet) – affects idling to 1/4 throttle
2 Midrange circuit (Clip position) – controls 1/4 to 3/4 throttle
3 Main circuit (Main jet) – controls 3/4 to full throttle
In addition to these circuits, your carburetor may have an accelerator pump. It squirts a shot of gas into the venturi of the carb, but only when you nail the throttle from idle.
The pump squirt lasts about one second and is timed to miss the slide face (adjustable) as it opens.
It’s only needed when you nail the throttle as the WOT (Wide Open Throttle) causes an instant lean condition.
When not needed (gentle throttle application), the gas in this circuit is leaked back to the gas bowl using a leak jet. The leak jet is sized to allow more or less fuel leak back, which directly affects squirt volume and duration. The leak jet on your ATV may also need to be re-sized.
4 Carburetor Fault
Carburetors are precision bits of kit, not unlike a musical instrument. If it’s even slightly off-tune, you’ll know about it. A dirty carburetor is a very common cause of a lean running engine. The jets inside the carburetor may become dirty and prevent a full flow of gas to the engine.
Carb gumming has become a real problem with small engines that lay idle for a few months. Blended gas like ethanol under certain conditions starts to go stale after just one month, and left in the carb can turn into a congealed mess.
The best way to avoid this problem – use a gas stabilizer. It’s an additive for the gas that will keep it fresh for at least 12 months.
I advise using it in all your small engines, including two-stroke chainsaws, etc. It protects the whole fuel system from gumming, moisture, and corrosion. You’ll find the stabilizer I recommend here on the ATV parts page.
It never hurts to clean your carburetor and should be done at least once per year. You’ll need to remove it from the engine and strip it completely. You can use carb cleaner, or I prefer an ultrasonic tank. It won’t damage any plastic or rubber components. When rebuilding, treat her to a new gas filter too.
You could find after cleaning that your problem persists, and that’s possible. A carburetor fault, unlike a flat wheel, isn’t very obvious to the eye. So if you’ve cleaned it, adjusted it, your gas is fresh, and you can’t dial it in, go ahead and change it out.
5 Bad Gas
Old gas is losing its zing and can cause your engine to misfire and run lean.
And as you know ethanol-blended gas older than one month may already be in the process of turning stale.
If you suspect your gas is a little off, go ahead and drain it, or if less than a half tank, top it up with fresh and ride on. I use my ATV all through the summer and only use a gas stabilizer at the end of the season. I use it in all my small engine kit, riding mower, saws, generator, dirt bike, old cars, and my ATVs.
You could use it all year round, and you should if you’re only occasionally using your ATV. Old gas turns to a sticky, gummy mess inside a standing fuel system and is a carburetor killer.
Faulty Petcock
A petcock or fuel valve is fitted to all bikes, most are a simple mechanical tap, but some are vacuum operated. As the engine is cranked, a vacuum is applied to the valve, which, when all is going well opens wide and allows gas to flow freely to the carb.
If as you’ve guessed the vacuum hose to the valve leaks, the effects on the engine are twofold, firstly the hose leak leans out the mix, and secondly because of the leak the valve doesn’t open fully, causing fuel starvation which serves to further the lean condition.
You can eliminate this as a possible cause by simply bypassing the valve and testing the bike. If you find it’s failed, you can replace it with an OEM or fit a basic mechanical tap, but you will need to cap the vacuum pipe and remember to turn off your gas.
6 Vacuum Leak
What’s a vacuum leak? It’s an air leak, on the engine side of the carburetor. Loose carb bolts or a weak interface gasket or manifold is enough to cause a vacuum leak. A leak on the engine side of the carburetor will cause a lean condition and most likely an erratic idle.
Finding a vacuum leak can be a right pain in the ass, but a can of WD 40 helps. With the engine idling, try systematically spraying around the gaskets, manifold, carb adjustment screw. The WD helps seal the leak temporally, and your ear will pick up the change in how the engine sounds.
7 Valve Lash
What’s valve lash? Valves, as you know, open and close sequentially to allow air/gas mix in and spent gases out. The timing is mission-critical and is driven by a camshaft which is, in turn, driven by the crankshaft.
So what’s the problem? Rockers are employed to push on the valve tip to open it at the appropriate time and hold it open for the correct duration.
The gap between the valve tip and rocker is important, and changes as an engine naturally wears. They’re adjustable and should be as part of scheduled maintenance but are often skipped as some are a pain in the ass to access.
If the lash isn’t adjusted to spec, you can get all kinds of symptoms, depending on if the valves are too tight or too loose.
8 Mechanical Issue
A lean condition caused by a mechanical issue is usually more serious. It can mean a damaged valve or valve seat, blown head gasket, excessive blow-by.
Running a few simple tests like a leakdown test will rule this out.
You may find the following posts helpful:
Other Possible Causes
Float Tang needs adjustment
Faulty carburetor valve
You’ll find the tools I recommend here on the ATV tools page and you’ll find common spare parts here on the ATV parts page.
You may find the following posts helpful:
How to tell if ATV jumped time?
Why is two-stroke overheating?
- 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.