Snow Blower Surging or Running Rough? Causes and Fixes

A surging snow blower rises and falls in pitch instead of holding a steady note — revving up, dropping away, revving again, sometimes every second or two. Some people describe it as hunting, others just say the machine runs rough. It is the same fault, and nine times out of ten it is not a broken engine. It is the engine being starved of fuel, or given too much air, and trying to correct itself.

That is good news, because it means the fix is usually a cleaning job rather than a repair bill. What follows works out which version of the fault you have from what the machine is doing, then takes each cause in turn.

What surging sounds like

A healthy small engine holds one note. A surging one cycles: rrrRRR…rrr…rrrRRR. The revs climb and fall on their own, with your hands nowhere near the throttle. On a snow blower you will usually notice it in one of three ways.

• It revs up and down continuously, even sitting still on the driveway.
• It idles roughly and hunts, but smooths out the moment you push it into snow.
• It runs fine for the first minute, then starts hunting once the engine warms.

Those three are different faults with different fixes, which is why “my snow blower surges” on its own is not enough to go on. The table below sorts them.

Work out which fault you have

Find the line that matches what your machine is doing. The cause on the right is where to start — not the only possibility, but the one that is right most often.

What it does	Most likely cause	Where to look
Revs up and down constantly, hot or cold	Blocked main jet in the carburettor	Carburettor jet, below
Surges at idle, smooths out under load	Blocked idle circuit, or a slight air leak	Idle-only surging, below
Runs rough and never settles	Stale fuel in the bowl	Stale fuel, below
Only runs with the choke on, dies when you push it off	Fuel starvation — a badly blocked jet	Choke section, below
Starts, runs a few seconds, dies	Fuel not reaching the bowl at all	Stale fuel and jet, below
Fine cold, surges once hot	Air leak that opens as the engine expands	Gasket and air leaks, below
Surges and backfires	Timing or a very lean mixture	When it is not the carburettor, below

The usual cause: a blocked carburettor jet

The main jet is a brass screw with a hole through it, and that hole is roughly the width of a human hair. Petrol left standing over the summer evaporates and leaves a varnish behind, and it only takes a trace of it to narrow that hole. The engine then runs lean, the governor opens the throttle to compensate, the extra air makes it leaner still, and the cycle you are hearing is the machine chasing its own tail.

This is far and away the most common cause of surging on a machine that sat unused since last winter. If your snow blower ran fine in March and hunts in December, start here. Our guide to cleaning a snowblower carburettor covers the full strip-down; the short version is below.

Cleaning the jet

1. Switch the machine off, close the fuel tap, and let the engine cool completely. Petrol vapour and a hot exhaust are a bad pair.
2. Take off the cover to reach the carburettor, and photograph the linkages before you disturb anything — the governor and choke rods go back one way only.
3. Drain the float bowl. If what comes out is orange, smells like varnish, or has grit in it, you have found your problem.
4. Remove the bowl nut — on most machines the main jet is inside it, or screwed into the body behind it.
5. Hold the jet up to the light. You should see a clean circle. If you cannot see through it at all, that is the fault.
6. Clean it with carburettor cleaner and compressed air. Never push wire through it — widening that hole by even a fraction changes the mixture permanently.
7. Reassemble, refit the linkages as photographed, and refill with fresh fuel.

If you would rather not take the carburettor off at all, it can often be done in place — see cleaning a Craftsman carburettor without removing it, which applies to most similar machines.

Stale fuel — why the jet blocked in the first place

Modern petrol starts degrading within about thirty days, and pump fuel containing ethanol also draws moisture out of the air. A snow blower that sits from March to November is holding fuel that is eight months past its best. That is what leaves the varnish, and cleaning the jet without changing the habit means doing it again next year.

Two things prevent it, and you only need one. Either add a stabiliser to the tank at the end of the season — the same approach used for fuel stabiliser in lawn mowers — or run the machine dry before storing it. There are arguments for each; running a snowblower out of gas before storage weighs them up.

Air leaks around the carburettor gasket

The gasket between the carburettor and the engine hardens with heat over the years. Once it cracks, air is drawn in past the carburettor rather than through it, the mixture goes lean, and you get the same hunting note. This is the cause behind the classic pattern of an engine that behaves cold and starts surging once it is hot — the metal expands, the crack opens, the leak grows.

There is a quick test. With the engine running and hunting, briefly spray a little starter fluid around the joint between the carburettor and the engine. If the revs change as you spray, air is getting in there and the gasket is the culprit. Where to spray starter fluid on a snowblower shows the spots worth testing.

1. Let the engine cool, then remove the carburettor as above.
2. Peel off the old gasket. Scrape the mating faces clean — any ridge of old material left behind will leak again.
3. Fit the new gasket dry unless the manual says otherwise. Sealant on a paper gasket usually causes more problems than it solves.
4. Tighten the bolts evenly and only to snug. Over-tightening warps the flange, which creates the leak you were trying to fix.

A stretched governor spring

The governor is what holds engine speed steady as the load changes — it is why the machine does not bog down when you hit a deep drift. It works through a small spring, and if that spring is stretched, wrongly hooked, or fitted in the wrong hole on the linkage, the governor will overcorrect: too much throttle, then too little, then too much. The result sounds exactly like a fuel fault but no amount of carburettor cleaning will cure it.

Check that the spring is on the hole the manual specifies, that it is not visibly longer than its neighbour, and that the linkage moves freely without binding. A spring that has lost its tension is a cheap part and a ten-minute job.

A clogged air filter

Less common on snow blowers than on summer machines — and some models do not have a filter at all, as whether snowblowers have air filters explains — but where one is fitted, a blocked filter restricts airflow and upsets the mixture. It is a two-minute check, so do it before you take anything apart.

Hold a paper element up to a light. If you cannot see light through it, replace it. Foam elements can be washed in warm soapy water, dried completely, and lightly oiled.

If it surges only at idle

An engine that hunts at idle but pulls cleanly through snow has a blocked idle circuit rather than a blocked main jet. They are separate passages in the same carburettor, and the idle one is narrower, so it blocks first. The cleaning job is the same — the difference is that you need to get cleaner through the pilot passage and the tiny air-bleed holes around it, not just the main jet.

Many people never notice this fault, because the machine works fine once it is in the snow. If that is you, it is not urgent — but it is an early warning that the fuel in the bowl is going off, and the main jet is next.

If it only runs with the choke on

This is the same fault, further along. Closing the choke restricts air, which richens a mixture that has gone too lean — so running on choke is the machine telling you it cannot get enough fuel through the jet. It will usually surge as well, and die the moment you push the choke off.

Do not leave it running on choke as a workaround. It washes oil off the bore and dumps unburnt fuel into the sump. The fix is the jet. Why a snowblower dies when the choke is off goes through it in detail.

If it runs fine cold and surges once hot

Two things behave this way. An air leak that opens as metal expands, covered above — and a fuel supply that cannot keep up once demand rises, usually a partially blocked tank vent or fuel filter. If the engine also loses power rather than just hunting, a snowblower losing power when hot covers that pattern specifically.

A quick way to separate them: loosen the fuel cap when the fault appears. If the engine picks up within a few seconds, the tank vent is blocked and the carburettor is innocent.

Craftsman, Toro and Ariens — what differs

The fault is the same across brands, because they mostly share the same engines. What differs is access.

• Craftsman — the bowl nut is usually reachable without pulling the carburettor, which makes this the easiest brand to clean in place.
• Toro — primer-bulb models have a separate primer circuit that blocks independently of the main jet. If priming does nothing and the bulb stays firm, that passage is blocked, not the jet. Starting procedure matters on these too: see how to start a Toro snowblower.
• Ariens — generally good access, but check for oil around the carburettor first. An engine that is leaking oil can show symptoms that look like a fuel fault.

When it is not the carburettor

If you have cleaned the jet, fitted a new gasket, checked the spring and it still hunts, stop taking the carburettor apart. The remaining causes are a worn throttle shaft letting air past, a failing ignition coil, or a valve clearance that has closed up. A machine that surges and backfires points that way — why a snowblower backfires explains the overlap.

At that stage the parts are cheap but the diagnosis is not, and a small-engine shop will find it faster than a second weekend in the garage will.

Stopping it coming back

• Use fresh fuel. Buy what you will use in a month, not a season. Ethanol-free petrol, if you can get it, keeps far better.
• Stabilise or drain before storage. One or the other, every spring, without exception. This single habit prevents most surging.
• Run it once a month out of season. Five minutes keeps fuel moving through the jet and the seals wet.
• Change the oil at the end of the season, not the start. Old oil left standing over summer holds acids. Which oil your snow blower takes covers the grades.
• Keep the filter clean where one is fitted, and replace rather than wash paper elements.

The short version

A surging or rough-running snow blower is almost always fuel. If the machine has been standing since last winter, clean the main jet and put fresh petrol in it — that fixes the large majority. If it hunts only when hot, suspect the gasket. If it hunts only at idle, the idle circuit is blocking and the

main jet will follow. And if cleaning changes nothing, the fault is electrical or mechanical rather

than fuel, and it is worth paying someone to find it.

Frequently Asked Questions