By: Author John Cunningham. Published: 2021/06/14 at 10:03 am
A popping sound from your snowblower isn’t a big worry. Popping noise on cold starts is normal until it warms up. But hey, you know your machine, and if the popping is out of the ordinary or more pronounced, then yes, we’ll need to check it out.
Common causes of a popping sound from a snowblower engine include:
- Fueling issue
- Ignition system fault
- Valves out of adjustment
- Broken flywheel key
- Internal engine fault
In this post, you’ll learn all the reasons a snow blower makes a popping sound, how to diagnose them, and what you’ll need to do to fix them.
It is important to use a choke until an engine warms up; if the choke is shut off too soon or the choke is not working correctly, popping from the intake will result.
Where’s The Pop Coming From?
While one person may describe the noise as a pop, another may describe it as a backfire. Whatever we call the noise, it’s a cause of concern. Popping usually occurs through the intake system and can be heard loudest around the air filter. A louder noise, sometimes described as a backfire, is generally heard from the muffler.
Both noises are backfires; they just sound a little different
What Is The Pop Sound?
The pop sound is backfiring, and its name gives it away a little. Backfiring happens when a fuel mixture ignites outside the combustion chamber. It commonly occurs inside the intake system or in the exhaust system.
The reasons the fuel mixture ignites in these areas of the engine are varied, but where they ignite (intake or exhaust) is often a clue as to what’s going on. Knowing where the noise is coming from does help with diagnosis. But more on this later.
A fueling-related problem is just one of the common causes of backfiring. An engine that gets too little gas (lean) or gets too much (rich) has a tendency to backfire.
A lean running engine runs hot, and that causes erratic fuel combustion; a rich running engine causes raw gas to enter the exhaust system where it combusts.
There are many other reasons too, but for now, here are three of the less complex, easy-to-check, and fix causes:
- Bad gas – drain out old gas and replace
- Bad spark plug – replace plug
- Dirty air filter – clean or replace the filter (if fitted)
If that doesn’t work, roll your sleeves; time to dig deep.
Carburetors work hard, and as a measuring device, it’s important that they’re accurate. The air (oxygen) to gas ratio is everything; even a little off, and the engine won’t stand for it. It will protest by stalling, backfiring, running rough, surging, and the list goes on.
Yep, the carburetor is high on the list of possible suspects. A carburetor that’s not performing correctly is likely, simply dirty. Removing, stripping, and cleaning will fix it. However, I replace a ton of faulty carburetors every season.
Unfortunately, you generally can’t tell a faulty carb from a good one by simply looking at it.
If your carburetor is corroded or gummed up, go ahead and treat your blower to a new one. They aren’t expensive; you’ll find many popular carbs here on the “Mower carburetors page.”
Check out the “Carburetor cleaning videos” here, the videos cover mower carburetors, but the process is identical.
How To Clean Carburetor
Turn Gas Off – Most will have a gas tap fitted.
Access – Remove covers to access the carburetor
Remove Gas Line – Use pliers to pull the gas line.
Remove Bolts – Remove carburetor fasteners
Remove Links – Turn the carburetor sideways to aid choke and governor lever release. Picture Take a picture of control lever locations and gasket orientations.
How To Tear Down Carburetor
Remove – Remove idle jet (if applicable)
Remove – Remove the gas bowl.
This is important; some engines may employ a bowl fastener jet combo. You’ll identify it by its portholes. If you have such as fastener, it will need a thorough cleaning. (use a strand of wire)
Remove – Remove float & valve.
Remove – Remove the main jet.
Remove – Remove the emulsion tube.
Remove – Remove the adjuster air/fuel metering screw (count turns to remove).
Clean – Use wire brush strand to clean emulsion tubes and jets.
Clean – Clean idle jet.
Spray – Spray in carb cleaner or ultrasonic tank.
Rebuild in reverse order, taking care to fit the fuel mix screw to the correct number of turns. Make sure to orientate gaskets and control levers correctly.
Some larger engine blowers may have an electrical solenoid fitted to the carburetor fuel bowl. Its job, ironically, is to prevent run-on (a form of backfiring).
It controls backfiring by closing the valve and blocking gas from the engine.
If the valve is faulty, it may cause a lean condition; check its function with a key on and off; the action should be smooth.
Ignition System Issue
Ignition systems on snowblowers aren’t complex; they usually consist of the following components:
- Control switch
- Spark plug
In addition, if your snowblower is fitted with a safety switch, it will need to be checked also. A faulty safety switch or On/Off switch has been known to cut spark intermittently, which creates a rich condition and, of course, the backfiring.
An intermittent or poor spark will cause a popping or backfiring. The easiest item to check is the spark plug. A fouled plug is a sign you may have found the issue. However, a fouled plug is usually a symptom rather than the root cause. Nevertheless, swapping out the plug is a great place to start the diagnosis.
To find an intermittent spark issue, a spark test tool is required. Install inline as per pic and check the test tool window while running.
This is a great way to find an intermittent spark which as you know will cause backfiring. Observing the test tool window should show a near-continuous healthy spark; observing an inconsistent spark is a sign the armature is at fault. For video help, see “Checking spark video.”
Valve Adjustment Out
Valves in your snow blower’s engine open and close in sequence. Your engine has two, one to allow fuel mix (air & gas) in, and the second, the exhaust valve, allows spent gases out.
The valves are driven by a cam and pushrods (OHV) engine. The valves are calibrated to open by a set amount, and this is adjustable. It’s known as valve lash and should be checked by the mechanic every three years. Valve lash, however, is often forgotten until one of the symptoms of excessive lash presents – backfiring.
Remove the cam cover and check lash. You can bet if they haven’t been checked in three years or more, they need adjusting.
How To Adjust Valves
To nail this like a pro, you’ll need a few tools. A feeler gauge and a few wrenches. You’ll find a link to the tools you need here “Small engine repair tools page.” Check out this post, “Mower engine valve adjustment,” or check out the video here.
The process is as follows:
- Remove spark plug
- Remove cam cover
- Place non metallic blunt object inside the cylinder (pencil)
- Turn over engine by hand until the pencil is pushed to the top of the cylinder (valves are now closed)
- Slip appropriate gauge between the valve and rocker (check engine lash spec), gauge should offer resistance. If not, adjust.
- Loosen adjuster lock nut
- Turn adjuster as necessary, while checking gauge resistance
- Repeat on second valve (lash spec may be different)
- Replace the cam cover gasket
Broken Flywheel Key
A flywheel key is a small alloy metal key. Its function is twofold. It helps locate the flywheel to the crankshaft. It does this by fitting precisely in a cut-out from both the flywheel and the crankshaft.
Its second and most important function is to break if the auger suddenly stops. (hits a solid object, the frozen newspaper, for example). The auger’s shear pins are designed to and should break in such an event. This, as you know, prevents inertia from damaging other components like the transmission and engine.
However, should the auger shear pins not break, the flywheel inertia would be sufficient to twist the crankshaft if it weren’t for the shear pin. Instead of twisting the crankshaft, the flywheel breaks the shear key and in doing so saves the engine. Hard to believe something so small, inexpensive, and insignificant has such an important job.
The flywheel and crankshaft relationship is mission-critical. Together, they are responsible for piston movement, the opening of valves, and the right moment to fire the spark plug. All these components must move and operate within a tight window.
Symptoms of broken shear key include rough running, hard starting, backfiring, pull cord snapback, oil leaks.
The flywheel nut will need to be removed in order to check the shear key condition. The process is as follows:
1 Remove – Remove blower housing
2 Remove – Remove the flywheel nut (use an impact wrench). No impact? Use a piston stop tool and ratchet and socket
3 Examine Key – Both the flywheel keyway and crankshaft keyway should align
How To Replace The Shear-key
To replace the shear key, the flywheel will need to be removed. This will require a puller tool. In addition, some flywheels may require tapping (thread cutting). It’s all covered here in this video, “Replacing the flywheel key.”
You can find all these tools here on the “Small engine tools page” or check out the Amazon torque wrench link below.Amazon Torque Wrench
The process is as follows:
Remove Armature – Remove armature fasteners.
Remove Flywheel – Place the puller tool on the crankshaft and tighten it. May need to use a piston stop tool; otherwise, use an impact wrench.
When in place and tensioned, strike the tool (use minimal force). Helps loosen the flywheel crank hold.
Clean Keyway – When removed, clean the flywheel and crankshaft keyway.
Refit Flywheel – Refit the flywheel and fit the new shear key.
Torque To Spec – Tighten the flywheel nut using the torque wrench and piston locking tool. Alternatively, improvise by locking the Ring gear.
Torque spec is important; the flywheel is designed to slip on the crank when needed, and overtightening could cause crank damage.
How To Fit The Armature
Fit Loosely – Refit the armature and thread in fasteners but leave loose.
Gauge – Place a feeler gauge (check spec) between the flywheel and armature legs (mechanics hack – use a business card).
Push – Push the armature towards the flywheel and tighten both fasteners. Remove the feeler gauge and refit the blower housing. Nice work!
Internal Engine Fault
An internal engine fault is not what you want to hear; that usually means it’s going to be expensive. But, at this stage, it is what it is, right? All these possible issues may be tested by using the leak-down tester. The test kit fills the engine with air (needs compressed air), and a gauge indicates a fault by how much pressure is lost over a given period. But usually, you hear the air escaping.
You’ll find a link to the leak-down tester I use here on the “Small engine tools page.”
Air that leaks from:
- Air filter – intake valve issue
- Muffler – exhaust valve seat issue
- Dipstick – rings worn
- Engine block – head gasket
- Valves – A badly seating valve will cause low compression and also allow excessive oxygen into the combustion chamber. That’s a lean condition, and that means backfiring.
- Rings – Worn-out rings mean the engine is low on compression, and that causes poor combustion, which means unburnt fuel in the exhaust system.
- Worn camshaft – A worn camshaft will reduce valve opening, which in turn reduces either fuel mixture or spent gas, depending on the valve.
- Bent pushrod – Pushrods transfer cam movement into the valve opening. To do this in OHV-type engines, they use a metal rod. If the rod bends, it becomes shorter and won’t open the valve correctly.
Failed Head Gasket
Head gaskets are graphite material sandwiched between the two halves of the engine. Its function is to create a perfect seal, prevent compression from leaking out and prevent oxygen from getting in, and keep oil and compression separated.
A gasket fails over time; it’s under a ton of stress. The symptoms of a failed gasket vary depending on where the gasket fails. Smoke, lack of power, and poor running are common symptoms.
In addition, the operator may hear a slight hissing sound – that’s the combustion escaping.
How To Check Head Gasket
To check for head gasket failure, a leak-down tester is commonly used. As before, it fills the engine with air, and a gauge indicates a fault by how much pressure is lost over a given period. However, most mechanics just hear the air escaping, and the diagnosis is over – replace the gasket.
A compression test tool may also be used to confirm compression issues but won’t be as effective at pinpointing a gasket fault.
Check out the compression test video to see how we go about testing.
Check out this post, How hard to change mower head gasket?
- About the Author
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John Cunningham is an Automotive Technician and writer at Lawnmowerfixed.com.
He’s been a mechanic for over twenty-five years and shares his know-how and hands-on experience in our DIY repair guides.
Johns’s fluff-free How-to guides help homeowners fix lawnmowers, tractor mowers, chainsaws, leaf blowers, power washers, generators, snow blowers, and more.