Two-stroke engines are pretty simple machines that rely on a cycle of intake, compression, combustion, and exhaust. Despite their simplicity, these machines sometimes break down, refusing to start even after immediately switching them off. So, how do you fix a 2 stroke engine not starting when hot?
Here are the steps to fix a two-stroke engine not starting when hot:
- Check the fuel.
- Clean or replace the air filter.
- Check the spark plug.
- Ensure the compression is 110-130 psi (758.4-896.3 kPa).
- Ensure the oil to fuel ratio Is 50:1.
- Clean and adjust the carburetor.
- Replace worn crankshaft bearings and seals.
- Replace broken reed pedals.
- Let the engine cool down for 10 minutes.
- Remove, clean, and replace the magneto points.
- Check the ignition timing.
So your engine’s hot and won’t start? Don’t sweat it! Instead, read on for more information on how to solve the problem and learn about the precautions to take to avoid further engine damage.
1. Check the Fuel
The most likely cause of your 2-stroke engine not starting when hot is that you’re using insufficient or the wrong type of fuel. It’s essential to check the fuel level and quality before starting your engine.
An excellent way to tell if your two-stroke will start when hot is that the fuel tank should be at least half-full, and the string trimmer line should not contain too much oil.
Also, if you’re using either ethanol-blended gasoline or gasoline containing more than 10% ethanol, such as E15 gas, get rid of it immediately (source)!
Ethanol will not appropriately combust compared to gasoline containing less than 10%. So, when using too much ethanol, the machine won’t produce much power. Besides, once you expose an engine to ethanol, you’ll have to completely drain out old gasoline and refill the tank with new fuel to run again. That’s because ethanol corrodes the engine components.
2. Clean or Replace the Air Filter
A clogged air filter is another common cause of a 2-stroke engine not starting. The air filter protects the machine’s metal components from dust and other contaminants. So, if it’s dirty or clogged, the machine will struggle to start since there won’t be enough airflow into the cylinder.
So how do you fix a 2 stroke engine not starting when hot when the air filter is clogged? Here’s how to clean your air filter:
- Remove the air filter from the carburetor.
- Rinse it in a solution of mild liquid soap and warm water ( a drop of soap for every two cups of water).
- Let the filter dry for 30-60 minutes.
- Once the filter is dry, reinstall it in the same orientation to ensure proper airflow.
However, replace the air filter with a new one if the machine starts sputtering and dies after a few seconds or minutes.
Here’s a comprehensive YouTube tutorial on how to clean, replace, or lubricate air filters on two-stroke engines.
3. Check the Spark Plug
Another common cause of a 2-stroke engine not starting when hot is a faulty or damaged spark plug.
Here are some signs that your two-stroke engine needs a new spark plug:
- The engine takes longer to crank. A faulty spark plug can reduce the machine’s ability to produce enough power to turn, resulting in a slower ignition process.
- The engine begins sputtering or dies after a few seconds. This problem happens because the spark plug becomes clogged with carbon deposits from the fuel, preventing it from creating a high-voltage current.
- The engine backfires when starting. If the plug is dirty, it could cause an explosion of unburned fuel in the cylinder when you start the engine, making it backfire.
Also, if the plug gap is incorrect, it will affect the machine’s performance, just like with an air filter. The correct size for a gap in a spark plug for most string trimmers and other small 2-stroke engines is 0.7 to 0.8 mm (0.027-0.031 in) (source).
Still, some machines like outdoor power equipment might need a different value, so check the manual before adjusting or changing it yourself (if you’re uncomfortable or not experienced).
4. Ensure the Compression Is 110-130 PSI (758.4-896.3 kPa)
Some small 2-stroke engines may be harder to start when hot if their compression is low, so always check the compression levels before restarting the machine.
Compression refers to how much power an engine can produce, and low compression means that it’s having trouble pushing out enough power.
You can tell if your machine has low compression if it struggles to start when hot or only runs for a few seconds/minutes after starting (or doesn’t start at all).
Here are some guidelines on checking the compression:
- If the compression is less than 100 psi (689.5 kPa), get new piston rings (and possibly a new piston) to resolve the issue.
- If the compression is between 110 and 130 psi (758.4 and 896.3 kPa), you probably need to clean or adjust your carburetor.
- If the compression is higher than 130 psi (896.3 kPa), you might have to replace the crankshaft bearings and seals.
5. Ensure the Oil to Fuel Ratio Is 50:1
Another common cause of a 2-stroke engine not starting when hot is that the oil to fuel ratio is incorrect or faulty. This ratio refers to how much two-stroke oil you should put into your gas tank for ideal performance and protect it against wear and tear on your machine.
In general, the signs and symptoms of a wrong oil to fuel ratio include:
- The engine takes longer to start. This stalling happens because the oil adds more lubrication for optimal performance. Without any, your machine may not be able to turn over or start up at all.
- The engine sputters or dies after a few seconds. Your engine may turn off prematurely when the oil doesn’t provide enough lubrication for optimal performance.
- The engine overheats. Overheating happens when the oil doesn’t lubricate parts of your machine correctly, leading to unreasonably high friction and high temps (and possibly even damages to your engine).
So what does the correct ratio look like?
Well, ideally, you want one part two-stroke oil per fifty parts gasoline (as an example). Some machines might need different ratios due to their capacities, so check your manual or ask a professional to determine the ideal balance.
Here’s an excellent video with tips on fueling your two-stroke engine.
Pro Tip: Besides using the right oil-fuel mix, I recommend adding this STA-BIL Storage Fuel Stabilizer from Amazon.com to your gasoline. Using a fuel stabilizer will help prevent the gas from going bad before its time (which can cause backfiring and corrosion of your fuel tank).
6. Clean and Adjust the Carburetor
A dirty carburetor can also affect a small 2-stroke engine’s starting performance because it restricts airflow, so cleaning it out is an essential step if you have other problems as well.
Here are some guidelines on how to clean your carburetor:
- Remove the carburetor from the machine.
- Use a screwdriver to remove any screws securing the top.
- Remove any jets, gaskets, and springs with tweezers or pliers and soak them in a carb cleaner overnight.
- If necessary, use an air compressor and a brush attached to its nozzle to blow out the passages.
- After soaking everything overnight, reassemble the carburetor and reinstall it onto your machine.
Caveat: Do not use a wire brush to clean the carburetor because it will damage the metal.
If you need more information on how to clean a 2-stroke engine’s carburetor, watch this tutorial:
7. Replace Worn Crankshaft Bearings and Seals
While ensuring your two-stroke engine’s oil to fuel ratio is right, you should also check if the crankshaft bearings and seals are worn out or damaged because wear can affect how hot the engine runs.
A faulty bearing or seal could be due to dirt or moisture accumulating in the engine, so cleaning out all passages will help resolve this issue.
You can also buy a bearing and seal replacement kit to resolve this problem yourself without professional help.
To replace the crankshaft bearings and seals, follow these steps:
- Remove any screws holding the top on.
- Disassemble the carburetor with pliers, tweezers, and a screwdriver.
- Take out the piston, piston rings, and crankshaft assembly.
- Remove all of the old bearings and seals from the crankshaft, the connecting rod, the piston, and the piston rings.
- Use a small flathead screwdriver to pry out old bearings and seals.
- Use a small flathead screwdriver to push the new bearings and seals into place.
- Put everything back together, clean the engine, and start it up to ensure everything is working correctly.
Caution: When removing the crankshaft, be careful not to bend or scratch it. Also, note how you disassemble the parts so you can reassemble them easily.
8. Replace Broken Reed Pedals
Small 2-stroke engines might not start if the reed valves are swollen or damaged.
A swollen reed valve can be due to air leakages in your system caused by broken or cracked parts, so replacing it will help resolve this issue.
To replace broken or swollen reed valves, follow these five steps:
- Remove the reed assembly from your engine.
- Use a flathead screwdriver to pry out damaged valves and springs.
- Use tweezers to remove the rest of the broken parts.
- Install the new reed valves.
- Replace and tighten all screws and bolts and return your reed assembly to its original position.
9. Let the Engine Cool Down for 10 Minutes
While not as severe as other issues on this list, an overheated 2-stroke engine can also affect its starting performance because it needs to cool down first before getting started again.
Because small engines don’t have efficient cooling systems like 4-stroke and larger engines do, they overheat more quickly than their bigger counterparts unless you take the proper precautions.
Therefore, it would be best to let your engine rest for about ten minutes after shutting off its ignition to get rid of any excess heat. Checking out other parts on your machine that need attention while you’re at it will also help correct this issue.
10. Remove, Clean, and Replace the Magneto Points
The magneto points in your engine’s cylinder provide a spark to ignite the fuel.
As with other engine parts, these points may lose their efficiency over time, so it’s best to replace them every once in a while for optimal performance.
To clean and replace the magneto points, follow these steps:
- Remove all of the screws holding the top on.
- Disassemble the carburetor with pliers, tweezers, and a screwdriver.
- Clean out the carburetor with a cleaning brush.
- Remove broken magneto points using pliers and tweezers.
- Install new, dry magneto points in your engine’s cylinder.
- Put everything back together and start your machine to ensure it works properly.
11. Check the Ignition Timing
If you have a 2-stroke engine, it’s best to check the ignition’s timing periodically. This way, if there are any issues with your timing, you can fix them before they become worse and cause other problems in your machine.
To check the ignition timing, follow these steps:
- Make sure that the magneto points are not touching each other.
- Slide the flywheel towards you until it’s off its magnetic hold.
- Mark the flywheel to keep track of your timing.
- Remove the top so you can access the magneto assembly inside.
- Remove the magneto plate by removing all screws holding it in place.
- Pull out both magnets with pliers or tweezers.
- Mark both magnets with a marker so you can determine how far they are from the flywheel when putting them back together.
- Install new, dry magneto points.
- Replace the magneto plate and screws in their original locations to keep everything in place.
- Put your engine’s top back on and reconnect the flywheel to its magnetic hold.
Note: While it’s hard to determine precisely when this needs to be done, checking the strength of your magnets and replacing them if they’re weak is a good rule of thumb. If you use your 2-stroke engine for hours on end, especially in harsh conditions like dusty or wet areas, then you should check and replace them every six months.
You should periodically clean, repair, or replace a 2-stroke engine’s ignition and exhaust valves. Otherwise, you can expect your equipment not to start when it’s still hot after use. Checking the air filter, cleaning out the carburetor, replacing worn parts such as crankshaft bearings and seals, and more can bring your machine back to its prime condition.
To ensure that you’re doing the right thing for your 2-stroke engine, check your owner’s manual before performing any repairs or maintenance on it. That way, you’ll know what you need to do at all times. Happy fixing!