Snowblower Making A Poping Noise

A popping sound from your snow blower isn’t a big worry. In fact, when the cold starts until it warms up, it's quite natural. But hey you know your machine, and if it's out of the ordinary 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 snowblower makes a popping sound, how to diagnose them and what you’ll need to do to fix them.

Mower gas cap

Vented Gas Cap

Where’s The Pop From?

While one person may describe a 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

Mower gas tank

Winged & Threaded caps

What Is The Pop Sound?

The pop sound is backfiring and it’s 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 diagnoses. 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 others reasons too, but for now, here’s a three of the less complex and 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 filter (if fitted)

If that doesn’t work, roll the sleeves, time to dig deep.

Vented mower gas caps

Vented gas caps

Carburetor fault

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

Diagnosis 

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.

My best advice, if your carburetor is more than ten years old or it’s 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”.

How To Clean Carburetor

  • Turn gas off
  • Remove housing
  • Remove gas line
  • Take picture of control lever locations and gasket orientations
  • Remove carburetor fasteners
  • Turn carburetor sideways to aid choke and governor lever release

How To Tear Down Carburetor

  • Remove the gas bowl
  • Remove the main jet
  • Remove float & valve
  • Remove emulsion tube
  • Remove idle jet (if applicable)
  • Remove adjuster needles (count turns to remove)

How To Clean Carburetor

  • Use carb cleaner or ultrasonic tank
  • Use wire brush strand to clean emulsion tube and jets
  • Clean idle jet
  • Soak in carb cleaner or ultrasonic tank

Rebuild in reverse order, taking care to fit the adjuster screws to the correct number of turns. Orientate the gaskets and control levers correctly.

Some larger engine blowers may have an electrical solenoid fitted to the carburetor fuel bowl. It’s job ironically is to prevent run-on, (a form of backfiring). It controls backfiring by closing the valve and blocking gas to the engine.

If the valve is faulty it may cause a lean condition, check it’s function with 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
  • Armature
  • Flywheel
  • Spark plug

    In addition, if your snowblower is fitted with safety switches, they will need to be checked also. A faulty safety switch has been known to cut spark intermittently which creates a rich condition and of course the backfiring.

    Diagnosis

    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 diagnoses.

    To find an intermittent spark issue a spark test tool is required and when installed inline as per pic. Using the tool in this way allows the snowblower to be tested 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 inconsistent spark is a sign the armature is at fault.

    Valve Adjustment Out

    Valves in your snowblower’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.

    Diagnosis

    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”

    Process 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, 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 damaging other components like transmission and engine.

    However, should the auger shear pins not break, the flywheel inertia is 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.

    Flywheel and crankshaft relationship is mission critical. Together, they are responsible for piston movement, opening of valves and the correct 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, back firing, pull cord snapback, oil leaks.

    The flywheel nut will need to be removed in order to check shear key condition. The process is as follows:

    • Remove blower housing
    • Remove flywheel nut (use impact wrench). No impact? Use a piston stop tool and ratchet and socket
    • Examine keyways – Both flywheel keyway and crankshaft keyway should align

    Replacing The 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). The flywheel should be torqued to spec and so a torque wrench is advised.

    You can find all these tools here on the “Small engine tools page”

    The process as follows:

    • Remove two armature fasteners and set aside
    • Place the puller tool on the crankshaft and tighten
    • When in place and tensioned, strike the tool (use minimal force)
    • Repeat the process, tighten and strike (just once)
    • When removed clean flywheel and crankshaft keyway
    • Refit the flywheel and fit the new shear key
    • Tighten the flywheel nut using the torque wrench and piston locking tool

    Removing the armature:

    • Remove the pull starter assembly
    • Remove the blower housing (cover)
    • Remove plug wire
    • Remove armature control wire
    • Remove the armature fasteners (2)
    • Remove the armature

    Fitting the armature requires a special procedure, as follows:

    • Refit the armature and thread in fasteners but leave loose
    • Place a feeler gauge (check spec) between the flywheel and armature legs (mechanics hack – use business card)
    • Push the armature towards the flywheel and tighten both fasteners
    • Remove feeler gauge and refit blower housing

    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 (need 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.

    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 allows excessive oxygen into the combustion chamber. That’s a lean condition and that means backfiring.

    Rings – Worn out rings means 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 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 a 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 getting in and to 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 diagnoses 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.

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    Auto Technician and Writer at Lawnmowerfixed | Website

    John Cunningham is an Automotive Technician and writer on Lawnmowerfixed.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write "How to" articles that help fellow gear-heads with all aspects of mechanical repairs, from lawn mowers to classic cars.