I love the rapid and never-ending acceleration of a two-stroke, but because of that, they’re prone to overheating.
The top 3 reasons a two-stroke engine overheats includes:
- Incorrect oil mix
- Engine running lean
- Cooling system fault
In this post, you’ll learn why your two-stroke engine is overheating, how to diagnose it and how to fix it.
Air or Water cooled?
Your 2-stroke is either air-cooled or water-cooled and while some of the causes of overheating are the same for either type, most aren’t.
Here are the main differences between both types:
- Air-cooled – an air-cooled bike uses airflow across the motor to help cool it, the engines are designed with adapted cooling fins to increase surface area and promote cooling.
- Water-cooled engine – a water-cooled motor employs coolant which is pumped around internal engines passageways to help keep the engine cool.
Even though both systems are different there is an overlap of causes that can cause them to overheat. Let’s quickly run over the list of where they overlap before getting into a bit more detail on each coolant system type.
- Fuel type issue
- Faulty spark plug, incorrect gap or incorrect plug heat range
- Piston carbon buildup
- 2-stroke oil quality or quantity issue
- Carburetor issue
- Head gasket issue
- Vacuum leak
- Ignition timing issue
If your engine is water-cooled you can jump ahead here.
1 Incorrect Oil Quality / Quantity
Good oil quality and quantity are essential for all engines. Oil not only lubricates but also cools and cleans the inside of an engine, breaking down harmful carbon deposits. A four-stroke engine employs a reservoir of oil, a filter, pump, and oil galleries to move oil around and lube the inside of the engine.
A 2 stroke, as you know, does lubrication a little differently. All two strokes need oil mixed with the gas, and the mix is mission-critical. Without the correct quality and quantity of oil, the piston will become so hot it fuses with the cylinder, and your engine is seized.
Getting the mix right is sooooo important. Every machine is different, and so has a different mixing ratio. A common mix ratio is 32:1. That’s 32 parts gas to one part oil.
So why does it matter? It’s all to do with the AFR (Air Fuel Ratio). All gas internal combustion engines need air (oxygen) and gas. Just gas alone inside a cylinder won’t combust.
And the air and gas have got to be mixed to the correct ratio, it’s known as stoichiometric, and it’s 14.7:1, that’s 14.7 parts air to 1 part gas. This is the sweet spot.
An engine is either running at stoichiometric, running lean, or rich.
- Running at stoichiometric – 14.7 parts air to one part gas
- Running Rich (aka fat) – any value below 14.7 parts air to one part gas
- Running lean – any value above 14.7 parts air to one part gas
The two-stroke carburetor is tasked with metering the gas and air, but remember, the oil is already mixed with the gas, and the carburetor can’t differentiate between gas and oil only.
So it follows if you put too much oil in your mix, your engine is proportionally receiving less gas and is therefore running lean. And a lean running engine runs FAST & HOT, and that’s a killer recipe for a two-stroke.
The oil mix has got to be as per manufacturers specs as the carburetor is calibrated (jetted) to supply the precise ratio of air to fuel (gas & oil).
2 Engine Running Lean
A lean running engine is an engine that isn’t receiving enough gas proportionally to the volume of air it receives. It’s an engine that receives more than 14.7 parts of air for every one part of gas.
A lean condition causes an engine to run much faster, and combustion chamber temperatures are far above normal operating temperatures. This usually causes piston scorching and scoring, blown head-gaskets, and seized motors.
Establishing that an engine is running lean is often the easy part of fixing the problem, finding the root cause is sometimes a head-scratcher.
Symptoms of lean running:
- Popping in carburetor
- White plug
- Burnt plug
- Excessive engine heat
- Erratic idle
- Hanging idle
- Lack power
- Over revs
Common causes of a lean running two-stroke:
- AFR needs adjustment
- Temperatures, altitude and humidity changes
- Bad gas or wrong gas type
- Exhaust or air filter modification
- Engine modification
- Vacuum air leak
- Carburetor needs re-jetting
- Carburetor fault
- Failing head gasket
- Mechanical issue
So as you can see, there’s quite a list, and this isn’t a complete list.
Begin by checking the basics:
- Any changes in the environment and ambient temperatures will change ratios.
- Bad gas or especially ethanol blends can cause lean conditions.
- Mods like fast flow exhausts and bigger air filters will throw the ratio out.
- Your carburetor must be clean. Partial blockages in both the pilot and main will cause gas starvation.
With all the easy questions asked, go ahead and try adjusting the air and gas mix screws.
3 Cooling Fins Blocked
Your engine’s cooling system is either an air-cooled or water-cooled setup. Air-cooled motors are more likely to suffer from overheating conditions. An air-cooled motor only cools when you’re moving fast enough to get airflow across the cooling fins.
Fins increase the surface area of the cylinder and head, and that helps radiate engine heat more quickly. Air-cooled engines are pretty simple cooling systems. The checks are simple, look for broken or missing fins, obstructions like caked mud, debris, or accessories that block airflow to the engine.
Air-cooled means your engine needs airflow. Riding slowly on duty quad trails in hot weather may naturally cause the dike to run hot.
Water-cooled 2-stroke Cooling System
Not all two strokes are air-cooled. Their water-cooled cousins are a ton more sophisticated, but not complicated. The system contains a lot more components, and therefore the maintenance checklist is longer too.
A water-cooled 2-stroke cooling system includes the following components:
- Rad cap
- Hose pipes
- Water pump
- Water jackets and passageways
- Fan switch/sensor
Common problems with water-cooled systems include:
- Old coolant – coolant should be changed every 2 years, old coolant losses its ability to cool, prevent freezing, lubricate and prevent corrosion inside the motor.
- Faulty rad cap – a leaking rad cap won’t pressurize the system and that causes the coolant to boil
- Failed water pump – a water pump is employed to move coolant around the system if a pump fails the coolant won’t move and obviously it overheats.
- Coolant leaks – a leaking pump will cause coolant to run low and causes air locking which promotes overheating.
- Stuck thermostat – thermostats can stick open or closed, when stuck closed, they prevent engine coolant flow, trapping the super hot coolant around the engine.
- Air locked coolant system – running low on coolant, means the coolant that is in the system needs to work harder to cool, low coolant also increases the chance of air locking.
- Internal rad blockage – rads can block up internally, corrosion and debris from old coolant may congeal inside the passageways slowing coolant flow. Backflusing the rad often soleves the problem.
- External rad core blocked – caked muck and debris in the cooling fins of the radiator will prevent airflow, which is crucial to colling the coolant inside the rad. A pressure washer will make short work of this problem.
- Cooling fan faulty – a fan’s job is to help airflow across the radiator and engine, thereby helping to cool. The fan typically turns on at lower speeds where the ambient airflow isn’t sufficient. Fans are an electrical component and do wear out.
- Fan switches faulty – the switch is a thermo switch and is responsible for monitoring the coolant temperature and at the appropriate temperature either signals the ECU to turn on the fan or in less sophisticated bikes it completes the fan circuit turning it on. Faulty thermal switches are a common cause of overheating at lower speeds.
To fix overheating conditions, begin with some maintenance
- Remove guards – any rad protection covers, while protection covers make great sense, they do lift the operating temperature of your engine.
- Clean – Cleaning the rads, clear caked muck and dust is a great place to begin.
- Repair fins – Straighten out bent rad fins using a flat screwdriver, the bent over fins prevent airflow through the radiators.
- Fit a fan – Not all water-cooled kit comes with a cooling fan as standard, fitting a fan will make quite a difference especially on slower-paced rides. Fitting is pretty simple but will require draining the system and removing the rad.
- Remove stat – If operating in hot conditions, go ahead and remove the thermostat. Removing the restriction inside the pipework will speed up coolant cycles. Change the coolant every two to three years.
- Relace coolant – Old coolant will cause your engine to boil in summer, freeze in winter, promotes corrosion, metal electrolysis, seal damage and pump failure. Air to change the coolant every two to three years, and while doing so backflush the system and swap out your thermostat.
Pressure test the system and check for leaks
If you spot coolant leaks on the ground or on the engine, that will be your problem, investigate and fix leaks before retesting.
You may need a coolant system tester to check the system conclusively. The kit pressurizes the system using adaptors fitted to the radiator, it is also capable of testing the rad cap itself.
Common easy-to-solve leaks include: failed rad caps, old clamps, perished hoses, water pump gaskets, and seals. At the other end of the scale, we have a failed head gasket.
You’ll find the tools I recommend here on the Dirt bike tools page and you’ll find common spare parts here on the Dirt bike parts page.
You may find the following posts useful:
Riding motorcycle without coolant
Dirt bike kick start won’t move
- About the Author
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John Cunningham is a technical writer here at ATVfixed.com. He’s a Red Seal Qualified Service Technician with over twenty-five years experience. He’s worked on all types of mechanical equipment, from cars and trucks to ATVs and Dirt bikes.