Snowblower Loses Power When Hot

I've been a mechanic for more than twenty five years and I’ve seen a ton of hot running problems. The fix is usually straightforward, but first we'll need to do a little detective work. Stay with me here, this is not difficult..

A snowblower that loses power when hot, likely suffers from one of four faults:

  1. Bad spark plug
  2. Bad armature
  3. Valve lash outside spec
  4. Faulty gas cap

In this post you’ll learn how to diagnose the root cause of your hot start issue, and you’ll also learn how you can fix it. In addition I’ll list the less common causes of hot running issues and how you can fix them.

Mower gas cap

Vented Gas Cap

Bad Spark Plug

The spark plug is the number one culprit. A fouled, incorrectly gapped, damaged or simply incorrect plug type could be the problem. Let’s take a quick look at each possible plug issue.

A fouled – A fouled is basically a dirty plug. Why it’s dirty is a better question. There are many possible root causes from bad gas, wrong plug, faulty armature, mechanical issue, too much oil.

However, sometimes simply running the engine for short periods will foul the plug electrodes. Remove, inspect, clean and gapping the plug usually solves the problem. Replacing the plug is the best option, especially if the engine is overdue for a tune-up.

Incorrectly gapped – The spark plug electrodes need to be gapped to specification. A gapping tool is used but not necessary. You will need the correct spec from your owners manual. Typical plug gap is .030”.

The size of the plug gap is important as it directly affects the quality and reliability of the spark. Voltage is as you know encouraged to jump the electrodes, thereby creating a spark.

If the gap is too small, the spark is weak as the voltage doesn’t need to try too hard. Too big causes excessive resistance which may damage the armature.

Damaged – The spark plug lives in a hostile environment, heat, compression, fire, raw gas and fumes. A failed plug is not hugely common as they are pretty durable, but it does happen.

Plugs are inexpensive and easy to swap out.

Incorrect plug type – Many good mechanics have fallen into this trap. Replacing a plug with the incorrect type. Spark plugs look interchangeable, and many are.

However the codes on spark plugs also serve to indicate the heat range of a plug.

Heat range is important. A plug needs to get hot enough to burn off contaminates that naturally form inside the combustion chamber, but not so hot that they prematurely ignite the fuel mixture.

Manufacturers recommend a plug type for that reason, they have tested the optimum plug for their engine.

However, many plugs will fit and start a snowblower and that’s where incorrect plugs are often fitted incorrectly.

I never assume the plug type fitted to an engine is correct, I like to check the spec in the owner’s manual.

Mower gas tank

Winged & Threaded caps

Faulty Armature

The armature lives under the blower housing and next to the flywheel. The armature (coil) generates voltage as the flywheel spins. The voltage is as you know sent down the plug wire to the plug.

The armature is a hard working component and I replace a ton of them. They commonly fail out right, meaning the snowblower just won’t start, however I’ve had plenty that failed only when hot or worse were temperamental when hot.

Diagnosis: A spark test tool is required to nail this test conclusively. The tool is easy to use and you’ll find a link to it here on the “Snowblower maintenance tools page”.

Remove the plug wire and fit the tool, ground the tool and start the engine. Observe the tools test window. Spark should be a consistent cycle, gaps in the cycle indicate armature failure.

Repair: Note a feeler gauge is traditionally used to set the air gap between the armature and flywheel. I’ll show you a mechanics hack in case you don’t have a feeler gauge. However, you’ll find all tools needed on the “Snowblower maintenance tools page”.

To replace the armature, follow these steps:

  • Remove blower housing
  • Remove armature fasteners (2)Remove the armature control wire and discard armature
  • Refit control wire to new armature
  • Refit armature fasteners, don’t tighten just yet
  • Place appropriate feeler gauge (check manufacturers spec) between the armature and flywheel. (Mechanics hack – use business card)
  • Pushing the armature towards the flywheel, tighten the fasteners
  • Replace blower housing
Vented mower gas caps

Vented gas caps

Valve Lash Out Of Spec

Valves open and close sequentially inside your engine. They allow fuel mixture in and spent gases out. How they seal and the amount they open is mission critical to how the motor runs.

The opening or “lash” is adjustable and should be checked and adjusted every 3rd or 4th season. Lash is rarely adjusted unless there’s an issue.

We already know your engine components expand as they heat. That’s true for valves also. Problem is, if the valve lash is too tight, it may cause the valves to open slightly when they should be closed tightly.

A slightly open valve will cause low compression and a lean condition, neither conductive for peak power.

Diagnosis & Repair: Remove the valve cover and check valve lash as follows.

  • Remove spark plug
  • Remove the valve cover
  • Using a pencil, place it inside the cylinder against the piston crown
  • Rotate the engine by hand until the pencil is pushed farthest from the cylinder. Engine is now at TDC (Top Dead Center) and valves are closed
  • Place suitable feeler gauge between the valve tip and rocker (check manufacturers spec) If the gauge too tight or lacks resistance, adjust
  • Open the lock nut
  • Adjust the adjuster screw while checking resistance on the gauge
  • Tighten lock nut
  • Repeat on second valve (note – spec may be different)Refit the valve cover and replace gasket if necessary

Faulty Gas Cap

A faulty gas cap is pretty common. How, you might ask, can a gas cap be faulty? Great question. A snowblower gas tank is designed to be open to atmosphere. If the tank can’t breath, (i.e. air replaces gas consumed) then gas flow stops.

Problems are usually noticed after the engine has been running and working a while, some engines may stop altogether, others may just not perform at their best.

Problems usually arise when the original gas cap is misplaced, the MacGyver type replaces it with a regular plastic cap. Of course that cap doesn’t have a vent and you can see where this is going

Genuine gas caps employ a small vent to facilitate tank venting, but original caps can fail too.

Testing is simple, remove the cap and test performance, if it improves, go ahead and treat her to a new shiny vented gas cap.

She’s worth it!

Other Possible Causes Of Hot Running Issues

Hot running and starting problems are very often easily solved, but in some cases the problem is a little more challenging to diagnose conclusively.

In this section we’ll look at some other causes of hot starting issues, many of which can still be easily repaired by the owner.

Bad gas

Bad fuel shouldn’t be overlooked. Ethanol gas goes stale after just one month. It loses its Oomph and that can cause some varying symptoms. Obviously you’ll have an idea when you last filled the tank and have an idea how old or fresh that gas is.

In the workshop I deal with stale gas issues all the time, I can usually tell the state of the gas by the smell. It loses that sweet gas smell and turns acidic, I don’t recommend you sniff the tank.

Instead take a sample of gas in a jar, fresh gas is close to clear and stale gas is the color of pee (yellow).

The fix is simple, drain out the tank.

Vacuum Leak

As an engine heats up, the metal components expand, which often gives rise to vacuum leaks. Flexible gaskets sandwiched between metal components are designed to prevent these hot running vacuum leaks.

But as an engine wears so too do the gaskets, they become hard and brittle and no longer seal the components.

Carburetor gaskets and cylinder head gaskets are especially susceptible to vacuum leaks. The leak, although small, makes a big difference to performance. Although a snowblower engine is small and simple, it has very exacting fueling requirements.

A ratio  of air (oxygen) to fuel (gas) must be maintained. It’s known as the AFR (Air Fuel Ratio) and it is 14.1 parts air to one part gas.

Air that enters the engine without passing through the metering device known as the carburetor is unmetered and that results in too much air in the mix.

Too much air causes the engine to run lean. Running lean in itself causes the engine to run hot and may also exhibit other symptoms like lack of power, erratic rpm, stalling.

Engine overheating

An overheating engine will run poorly, as the engine overheats it expands and allows a vacuum leak. In this case, the gaskets are good, they have just reached their max tolerance.

The issue is not the gaskets, it’s the overheating engine itself.

Your snow blower is what’s known as an air cooled engine. Instead of a radiator, coolant and pump, it employs cooling fins on the cylinder head.

The fins increase surface area and together with an engine fan attached to the crankshaft, they cool the engine.

The engine fins tend to trap debris, although it’s more of an issue with lawn mowers than snow blowers.

The fan blades attached to the flywheel spin with the engine and coupled with the blower housing direct air over the engine cooling it.

Remove the blower housing and check the engine fins for debris, check also the cooling fan on the flywheel.

Choke

Choke is super important, especially to a snow blower. And as you know, low temperature engine starts are the toughest

Some snow blowers may be fitted with an auto choke system and for the most part they work well. Auto choke systems employ a mechanical thermostat that reacts to hot exhaust temperatures. As the motor heats, the thermo (usually wax filled) expands and pushes on a lever opening the choke proportional to engine heat, quite clever.

However, if the thermo malfunctions, the choke may not be turned off fully or at all. This causes the engine to run rich. A rich running engine will run poorly, sputtering and coughing and generally unresponsive.

Check the choke plate location when the engine is hot. Remove the air intake housing and view the choke plate. If the plate is closed, the choke is faulty.

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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.