Popping and banging noises from your ATV is so 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.
Symptoms of Lean Engine
Lean running engine symptoms vary by engine and by modifications, but the usual telltale signs include:
- White plug
- Hot running engine
- 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 “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 screws on the body of the carburetor help 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. 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.
Air temperature, altitude and humidity all effect 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 rich air than at higher temperatures.
Oxygen and gas (AFR), must be mixed to stoichiometric, and unlike a car, most ATV’s don’t have a closed loop computer making real time adjustments for oxygen volume.
Instead we make small adjustment to the air fuel mix screw manually.
2 Carburetor Adjustment
Carburetors are set from 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 2 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 air screw works in the opposite way, turning anti-clockwise removes air from the mix and turning clockwise adds air to the mix.
I like to begin with a warm engine, and know my base setting. To find your base setting, engine off and using a screwdriver, count accurately the number of turns clockwise to seat the mix screw.
The screw is brass, which is a soft metal and will break if abused. No need to tighten the screw, just seat it. A ball park 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.
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 over do it.
Move the screw clockwise now, until the engine bogs and stalls.
Now set the screw where the revs are highest within that window. The bike should rev cleanly without bogging.
Now set the idle screw so that the engine idles smoothly about 1000 rpm.
Black smoke or a black 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 your Pilot. Just under 2 turns is about right for a fuel mix screw.
The bike should rev cleanly without bogging, black smoke or a black spark plug indicates a rich running engine and means you’ve gone too far.
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 and 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 and re-jet.
OK, but what’s a jet? It’s a brass screw with an orifice that is very precisely measured. Each jet is marked 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 that 2 and 1/2 turns 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 throttle.
You’ll need to pull your carb apart and see what size pilot, main and clip position your running.
Most ATV carburetors are either mechanical slide or vacuum actuated CV (constant velocity) type. These carbs have 3 main fueling circuits.
- Pilot circuit – effects idling to 1/4 throttle
- Clip position – controls 1/4 to 3/4 throttle
- Main jet – controls 3/4 to full
In addition to these circuits, your carburetor may have a 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 effects 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 starts to go stale after just one month and left in the carb can turn to 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 2 stroke chainsaws etc. It protects the whole fuel system from gumming, moisture and corrosion.
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 very 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 losses it’s zing and can cause your engine to misfire and run lean. As you know, Ethanol blended gas older than one month is already stale, regular gas goes stale after 3.
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 ATV’s.
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.
A petcock or fuel valve is fitted to all bikes, most are a simply 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 it opens wide and allows gas to flow freely to the carb.
A rubber diaphragm inside the valve gets hard as it ages, it’s a pretty common failure. It as you’ve guessed causes a lean condition which can of course cause our engine to pop.
You can eliminate this as the possible problem by simply bypassing the valve and test the bike. If you find it’s failed you can replace it with a 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, 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.
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 a 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.
Anyway if the lash isn’t adjusted to spec, you can get all kinds of symptoms, depending if the valves are too tight or too loose.
8 Mechanical Issue
A lean conditioned caused by a mechanical issue is usually more serious. It can mean a damage valve or valve seat, blown head gasket, excessive blow by.
Running a few simple test will rule this out.