By: Author John Cunningham. Published: 2021/06/15 at 3:40 pm
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:
- Bad spark plug
- Bad armature
- Valve lash outside spec
- 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.
1 Bad Spark Plug
The spark plug is the number one culprit for many intermittent faults. 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.
Fouled Plug – A fouled plug is a dirty plug. Why it’s dirty is a better question.
Inspecting the plug offers a clue as to what’s going on.
There are many possible root causes, from bad gas, wrong plug, faulty coil/armature, mechanical issue, and too much oil.
However, sometimes simply running the engine for short periods will foul the plug electrodes. Removing, inspecting, cleaning, 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.
Check out the “Spark plug cleaning video” here.
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 owner’s manual. The 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.
Check out the “Spark plug gapping video” here.
Damaged – The spark plug lives in a hostile environment, with 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 contaminants that naturally form inside the combustion chamber but are 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.
2 Faulty Coil/Armature
The armature (aka the coil) lives under the blower housing and next to the flywheel. The armature generates a 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 outright, meaning the snowblower just won’t start. However, I’ve had plenty that failed only when hot or worse 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.” To test the coil/armature, we’ll fit the test tool inline and run the engine.
We must pay close attention to the test window, especially when the snowblower is losing power.
Remove the plug wire and fit it to the tool, and ground the tool on the plug as per the picture. Go ahead and run the engine and observe the tool window.
Spark should be near a consistent cycle; large gaps in the spark cycle indicate armature intermittent failure.
Check out the “Check mower spark video.”
You may find the following post helpful – How ignition systems work?
To replace the armature, follow these steps:
Note a feeler gauge is traditionally used to set the air gap between the armature and flywheel. I’ll show you a mechanic’s hack in case you don’t have a feeler gauge. However, you’ll find all tools needed on the “Snowblower maintenance tools page” and check out the Amazon link below for armature prices.Amazon Snowblower Armature
Check out the “Check mower spark video,” which covers mower armature replacement. It’s a mower, but the snowblower armature replacement process is nearly identical.
Rebuild now in reverse order.
3 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. The 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 conducive for peak power.
Diagnosis & Repair: Remove the valve cover and check the valve lash as follows. Check out “Valve adjusting video” here.
4 Faulty Gas Cap
A faulty gas cap is common. How you might ask, can a gas cap be faulty? Great question. A snowblower gas cap is designed to allow air to come into the tank but keep gas vapors from escaping. If a gas tank is sealed, then the gas flow stops as it requires incoming air to displace the gas.
Problems are usually noticed after the engine has been running and working for a while, some engines may stop altogether, and others may just not perform at their best.
These types of symptoms usually arise when the original gas cap is misplaced; the MacGyver-type owner 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 air intake, 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 shiny new 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 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 of 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 the gas tank. Check out “Cleaning snowblower gas tank.”
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 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 14.7 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, and stalling.
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 snowblower 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 lawnmowers 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; also, check the cooling fan on the flywheel is in good shape.
Choke is super important, especially to a snowblower. And as you know, low-temperature engine starts are the toughest. Some small engines 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, which is quite clever. However, if the thermo malfunctions, the choke may not be turned off fully or at all.
A choke that’s sticking on or partially on will cause the engine to run rich. A rich running engine will run poorly, sputtering and coughing and generally unresponsive.
Check the choke plate position when the engine is hot.
Remove the air intake housing and view the choke plate. If the plate is closed, the choke may need adjustment.
Check out the “Choke test video” here.
The manual choke is shown here in the open position (choke off).
Need more info on the fuel system, carburetor components, and how they work, you can check them out here.
- 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.