Why Is My 2 Stroke Overheating (Top 3 reasons)


Dirt bike

I just love the rapid and never-ending acceleration of a 2 stroke, but because of that, they’re prone to overheating.

The top 3 reasons a two stroke engine over heats, includes:

  1. Incorrect oil mix
  2. Engine running lean
  3. 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.

1 Incorrect Oil Mix

Proper oil quality and quantity is important for all engines. Oil not only lubricates but it 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 little differently. All two strokes need oil mix with the gas, and the mix is mission critical. Without the 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 has 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 2 stroke carburettor is tasked with metering the gas and air, but remember the oil is already mixed with the gas and the carburettor 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 carburettor is calibrated (jetted) to supply the precise ratio 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 receiving. It’s an engine that is receiving more that 14.7 parts air for every one part 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 a 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 carburettor
  • Missing
  • White plug
  • Burnt plug
  • Excessive engine heat
  • Erratic idle
  • Hanging idle
  • Lack power
  • Over revs

Common causes of a lean running 2 stroke:

  • AFR needs adjustment
  • Temperatures, altitude and humidity changes
  • Bad gas
  • Exhaust or air filter modification
  • Engine modification
  • Vacuum air leak
  • Carburettor needs re-jetting
  • Carburettor fault
  • Mechanical issue

So as you can see, their’s quite a list and this isn’t a complete list.

Begin by checking the basic – any changes in environment and ambient temperatures will change ratios. Bad gas, or ethanol blends can cause lean conditions. Mod’s like fast flow exhausts and bigger air filters, will throw the ratio out.

Your carburettor must be clean, partial blockages in both the pilot and main will cause a gas starvation which is a lean condition.

With all the easy questions asked, go ahead and adjust the air and gas mix screws.

3 Cooling Fins Blocked

Your engines cooling system is either an air cooled or water cooled set up.

Air cooled motors are more likely to suffer from an over heating condition. An air cooled motor only cools when your moving fast enough to get air flow 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 quad trails may also cause overheating.

Not all 2 strokes are air cooled. Their water-cooled cousins are a ton more sophisticated though, but not complicated. The system contains a lot more components and therefore the maintenance checklist is longer too.

A basic water cooled system includes:

  • Rad
  • Rad cap
  • Hose pipes
  • Water pump
  • Thermostat
  • Water jackets and passageways
  • Fan
  • Fan switch/sensor

Common problems with water cooled systems, include:

  • Old coolant
  • Faulty rad cap
  • Leaking water pump
  • Stuck thermostat
  • Air locked coolant system
  • Internal rad blockage
  • External rad core blocked
  • Cooling fan faulty
  • Fan switch faulty

To fix overheating condition, begin with some maintenance. Remove any rad protection covers, makes great sense but it does lift the operating temperature of your engine.

Clean the rad, clear caked muck and dust, compressed air works best.

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.

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 3 years, old coolant will cause your engine to boil in summer, freeze in winter, promotes corrosion, metal electrolysis, seal damage and pump failure.

Pressure test the system and check for leaks, common areas are old clamps, perished hoses, water pump gaskets and seals.

John Cunningham

John Cunningham is an certified mechanic and writer on ATVFixed.com. I’ve been a mechanic for over twenty years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of ATV ownership, from maintenance, repair to troubleshooting.

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