50cc Scooter/Moped Won’t Start, Has Spark and Fuel: Get it Going Again Quickly

So, you’ve found yourself stuck with a stubborn scooter that refuses to roar into life, despite showing signs of spark and fuel? We’ve all been there, and we understand how frustrating it can be – especially when the sun’s shining, the road’s clear, and all you want to do is feel the wind in your hair as you ride down the street.

Before you start panicking or thinking of taking your moped to the nearest garage, take a deep breath. Even if you don’t consider yourself a mechanical whizz, there are still a number of things you can do yourself to potentially get your scooter back on the road.

In this article, we’re going to dive into all the possible reasons your moped might be giving you the cold shoulder and how to troubleshoot these issues. These methods will work for 50cc, 125cc, 150cc, or really any cc moped or scooter!

Like all our articles, like this one about a moped that won’t start with the electric starter, we promise to keep things straightforward and jargon-free, so you won’t need an engineering degree to follow along. By the end of this post, we aim to equip you with enough knowledge to identify and fix the common problems that may prevent your scooter from starting, even when it has spark and fuel.

Remember, every moped owner starts somewhere, and troubleshooting your own can be a rewarding learning experience. So, grab your toolbox, roll up your sleeves, and let’s get into this mechanical mystery…

Understanding the Basics: How Does Your Moped Work?

Before we dive into the nitty-gritty of troubleshooting, let’s take a quick detour into understanding the basics of how your moped works. This will make the process of identifying issues easier, and make sure we’re all on the same page.

At its core, your 50cc scooter operates using a simple principle known as internal combustion. While the term might sound complex, it’s just a name for the process your moped uses to generate power. This process takes place in the engine and relies on four key elements: air, fuel, spark, and compression.

Air and Fuel

Air and fuel are the primary ingredients your scooter needs to create energy. The engine draws in air, mixes it with fuel (usually gasoline), and this air-fuel mixture is what powers your moped. Without the right balance of air and fuel, your scooter might struggle to start or run properly.

Spark

The spark, provided by your scooter’s spark plug, is what ignites the air-fuel mixture. This tiny, timed explosion is what creates the energy needed to power your scooter. If your moped isn’t getting a strong, consistent spark, it could lead to starting issues.

Compression

Compression is the process of squeezing the air-fuel mixture in the engine’s cylinder before it’s ignited. The higher the compression, the more potent the explosion and the more power gets delivered to your wheels. Low compression could be the unseen culprit if your scooter refuses to start.

Now that we’ve got the basics covered, let’s dive into how to properly check for fuel and spark, the most common reasons why your scooter might not be starting.

Properly Checking for Fuel and Spark

We’ve all heard the phrase, “it’s not always what it seems,” right? Well, this applies to your moped too! Even though you might think that your scooter has sufficient fuel and spark, it’s crucial to verify this properly.

A lack of one of these two vital elements are the most common reasons that a scooter won’t start. We’ve seen countless examples of scooters and mopeds that won’t start, but “have a spark and are getting fuel”, only to quickly realize this isn’t the case.

There are a few little tricks you can try to make absolutely sure your issue is not fuel or spark related. We should prewarn you, these are tricks of the trade and may not be the safest activity in the world, so always proceed with caution. We are talking about handling flammable liquids and high voltage electricity here!

How to Test for Fuel Problems on a Moped/Scooter

Starting Fluid Test

Starting Fluid, also known as Aerostart or Easystart, is a volatile, flammable liquid that can help you determine if your moped is getting fuel to the cylinder. Here’s how you can perform the Starting Fluid test:

1. Remove the air filter to expose the carburetor intake.
2. Spray a small amount of Starting Fluid into the carburetor intake.
3. Try to start the moped.

If your moped starts up and runs for a few seconds before dying, it’s a good indication that fuel isn’t getting to the cylinder during a normal start process.

However, while effective, this method should not be used frequently. The reason is that Starting Fluid is very volatile and can cause the air-fuel mixture to ignite too early (a condition known as pre-ignition or knock), potentially damaging the engine. It also lacks the necessary lubrication for the components, which can lead to wear over time.

Direct Fuel Introduction

Another method to check fuel delivery is by introducing a small amount of fuel directly into the cylinder. Here’s how:

1. Remove the spark plug from the engine.
2. Carefully pour a small amount of fuel (about a teaspoon) directly into the spark plug hole. Use a funnel or syringe to prevent spillage.
3. Reinstall the spark plug and lead and try to start the moped.

Like the Starting Fluid test, if your moped starts and runs briefly before dying, it’s likely that your moped has a fuel delivery issue.

How to Test for Spark (Ignition) Problems on a Moped/Scooter

Visually Check For a Spark

A visual inspection can be a quick and straightforward way to check for a spark. Here’s how you do it:

1. Remove the spark plug from the engine.
2. Reconnect the spark plug to the spark plug wire.
3. Hold the metal side of the spark plug against a metal part of the engine to ground it (we highly recommend you use a pair of insulated pliers to hold it, you can get a nasty shock!).
4. While keeping a safe distance, try to start the moped.

You should see a bright blue spark jump across the spark plug gap. If the spark is weak (yellow or orange) or non-existent, you may have a problem with your ignition system.

Be cautious when performing this test. The spark generated can give you a significant shock if you touch it, so always hold the spark plug on the insulated cap with insulated pliers.

Spark Testing

A safer and more reliable method to test for a spark is by using a spark tester. This tool is designed to simulate the conditions inside the cylinder and can show you if your ignition system is producing a spark as it should. Here’s how to use it:

1. Remove the spark plug cap from the spark plug.
2. Connect one end of the tester to the spark plug and the other end to the spark plug wire.
3. Try to start the moped.

The tester has a small window through which you can see if a spark is generated when you try to start the moped. A bright blue spark is a good sign. If the spark is weak or non-existent, you may have a problem with your ignition system.

It is important to know, that whilst the tester will prove the ignition system up to the spark plug, it will not prove that the spark plug is working correctly. To check if the actual spark plug is working correctly, these really handy, cheap spark plug testers are great. You just put the spark plug in the hole and turn it on, watching closely for a spark.

We recommend that any moped or vehicle owner should have a spark tester and a spark plug tester in their toolbox. They’re cheap, easy to use and safer than a visual inspection, and they provide a more accurate representation of what’s actually happening inside the engine.

If you discover your moped has no spark, this post gives you a step-by-step guide on how to fix it.

50cc Scooter/Moped Won’t Start, Has Spark and Fuel

OK, now that you’ve double-checked your 50cc moped and found that it has both spark and fuel, you might be scratching your head about why it still won’t start. Don’t worry, we’ve got you covered.

The thing is, engines are like puzzles; all the pieces need to fit together and be in the right place for it to work properly. If it’s not starting despite having fuel and a spark, there might be something else throwing a wrench into the works. Let’s dive into some of the reasons this could be happening.

Air Supply Issues

Every engine, including the one in your 50cc scooter, runs on a mixture of fuel and air. This mix is important, as too much or too little air can cause your scooter not to start or to run poorly. So what air supply issues can affect your moped?

Air Filter

Your air filter’s job is to clean the air that goes into your engine. Over time, this filter can get clogged with dust, dirt, and other debris from the environment. When this happens, the amount of air reaching your engine gets restricted. Without enough air, the fuel in your engine won’t be able to burn properly. This can prevent the engine from starting or cause it to run unevenly.

Checking the air filter is usually pretty straightforward. It’s typically located inside the airbox near the engine. You’ll want to take it out and give it a good look. If it’s visibly dirty or clogged, it needs to be replaced. Make sure to replace it with one that is specified for your particular moped model.

Air Intake System

The air intake system is essentially the pathway the air takes to get from the outside environment to your engine. This system should be clear and unobstructed for the engine to receive the air it needs. Things like bent or crushed intake tubes, or blockages within these tubes, can affect the airflow.

Inspect all the tubes and parts of the air intake system to ensure they are in good condition and correctly connected. If you notice anything unusual like dents or severe bends, you might need to replace these parts.

https://www.youtube.com/watch?v=vbdSLxI9G0c

Compression Issues (2-Stroke & 4-Stroke Engines)

Compression is a key ingredient in the recipe for a healthy running engine. When the engine’s piston moves up, it compresses the air-fuel mixture inside the cylinder, making it ready to explode when ignited by the spark plug.

However, several issues can impact this compression, whether you have a 2-stroke or a 4-stroke engine. Let’s break down some of the most common compression problems and how they affect different types of engines.

Piston Rings

Piston rings create a seal between the piston and the cylinder wall, helping maintain proper compression. If the rings are worn out or damaged, they might let the air-fuel mix leak past them during the compression stroke. This is known as “blow-by” and it can seriously degrade engine performance.

Damaged Cylinder

Cylinder damage, like scoring or cracks, can be a major problem for both 2-stroke and 4-stroke engines. Scoring happens when debris or broken parts get between the piston and the cylinder, creating scratches or gouges in the cylinder wall. These scratches can cause the piston rings to lose their seal, leading to compression loss.

Faulty Valves

For 4-stroke engines, the intake and exhaust valves play a critical role in the compression process. If the valves are not sealing properly, they can cause a loss of compression. This can be due to a variety of issues, such as worn valve seats, broken valve springs, or bent valves.

Blown Head Gasket

The head gasket forms a seal between the engine block and the cylinder head, keeping the combustion gases in and coolant and oil out. If this gasket fails, you can lose compression, and engine coolant may leak into the cylinder, causing poor engine performance or even failure.

Damaged Reed Valves

For 2-stroke engines, the reed valve (also known as the intake valve) plays a vital role in controlling the fuel-air mixture entering the engine. If the reed valve is damaged or worn, it may allow the mixture to flow back out of the cylinder during the compression stroke, leading to a loss of compression.

Testing for Compression on a Moped or Scooter

The best way to diagnose compression issues is to do a compression test. You’ll need a tool for this: a compression tester. This device measures the amount of pressure generated in the cylinder during the compression cycle.

Here’s a step-by-step guide to conducting a basic compression test on a cold engine:

1. Purchase a Compression Tester: You can get a good one on Amazon. We’d recommend the BETOOLL HW0130 compression tester. It’s cheap, comes with all the attachments you’ll need and is nice and easy to use.
2. Prepare Your Scooter: Even though your scooter isn’t starting, ensure that it’s in a safe and level position. Remove the spark plug from the engine.
3. Install the Tester: Thread the correct attachment from the compression tester kit into the spark plug hole.
4. Conduct the Test: Turn the engine over a few times using the kick-starter or electric starter. Watch the gauge on the compression tester—it should rise to the maximum compression reading after a few cranks.
5. Check the Results: A healthy 50cc 2-stroke or 4-stroke engine should typically have a compression reading of over 100 psi, even when cold. If the reading is significantly lower, there’s likely a compression issue that needs further investigation.

Keep in mind that cold compression readings are typically lower than warm readings. So, if the reading is slightly below 100 psi (say 5 or 6 psi) it may still be within an acceptable range for a cold engine.

We recommend you do a google search to find the correct compression readings for your scooter and compare them to your reading.

How to Fix Low Compression on a Moped or Scooter

Ok, so you’ve performed a compression test and found that you have low compression. Quite often, the easiest step in this situation is to buy and install a new piston and cylinder kit specific to your bike. If you are not the most mechanically minded person in the world, this is a job that is best left to a professional.

We will guide you through the basic process in this article, but bear in mind that this is just a guide as every scooter will be slightly different (PS. this job on a 4-stroke is more complex than a 2-stroke).

Cylinder and Piston Replacement (2-stroke)

Tools and Materials Needed

• New piston and cylinder kit suitable for your 50cc moped
• Basic hand tools (socket set, screwdrivers, pliers)
• Torque wrench
• 2-stroke oil
• Clean rags

Step-by-Step Process

1. Safety First: Make sure the moped is off, the battery is disconnected, and the moped is stable (ideally on a center stand) before you begin.
2. Access the Engine: Remove any body panels or parts necessary to access the engine. This will typically involve removing a number of bolts or screws.
3. Remove the Exhaust: Detach the exhaust from the cylinder. You’ll likely need to unscrew a few bolts. Be sure to keep track of all bolts and small parts.
4. Remove the Cylinder Head: Unbolt the cylinder head from the top of the cylinder. Once the bolts are removed, you should be able to lift the cylinder head off.
5. Remove the Cylinder: Carefully slide the cylinder up and off the piston. Be careful not to damage the piston or connecting rod.
6. Remove the Piston: Remove the circlip from one side of the piston pin, then push the pin out and lift the piston off the connecting rod. Be very careful not to drop anything into the engine.
7. Clean the Gasket Surfaces: Clean any old gasket material off the cylinder head and engine block. Be careful not to scratch or gouge these surfaces.
8. Install the New Piston: Apply a small amount of 2-stroke oil to the new piston pin, then carefully slide the new piston onto the connecting rod. Insert the pin and secure it with the new circlip.
9. Install the New Cylinder: Lightly oil the inside of the new cylinder and the new piston rings. Carefully slide the new cylinder down onto the piston, making sure the piston rings are correctly positioned in their grooves.
10. Install the Cylinder Head: Install a new head gasket, then bolt the cylinder head onto the cylinder. Use a torque wrench to tighten the bolts to the manufacturer’s specifications.
11. Reattach the Exhaust: Bolt the exhaust back onto the cylinder, and tighten it securely.
12. Reassemble the Moped: Reinstall any body panels or parts that you removed.
13. Check Everything: Double-check all your work, make sure all bolts are tight, and ensure there are no parts left over.
14. Start the Moped: Turn on your moped. It might take a bit longer to start due to the new parts. Pay close attention to how it runs and immediately shut it off if anything seems wrong.

https://www.youtube.com/watch?v=bUrlKLeGgjo

Cylinder and Piston Replacement (4-stroke)

Tools and Materials Needed

• New piston and cylinder kit suitable for your 50cc 4-stroke moped
• Basic hand tools (socket set, screwdrivers, pliers)
• Torque wrench
• Engine oil
• Clean rags
• Feeler gauges

Step-by-Step Process

1. Safety First: Ensure the moped is off, the battery is disconnected, and the moped is stable (ideally on a center stand) before you begin.
2. Access the Engine: Remove any body panels or parts necessary to access the engine. This will typically involve removing a number of bolts or screws.
3. Remove the Exhaust: Detach the exhaust from the cylinder. You’ll likely need to unscrew a few bolts. Remember to keep track of all bolts and small parts.
4. Remove the Valve Cover: Remove the valve cover on the top of the cylinder head to access the valves.
5. Set the Engine to Top Dead Center (TDC): You’ll need to rotate the engine so that the piston is at the top of its stroke (TDC). This is usually done by turning a bolt on the flywheel.
6. Unhook the Timing Chain: With the engine set at TDC, unhook the timing chain from the camshaft sprocket.
7. Remove the Cylinder Head: Unbolt the cylinder head from the top of the cylinder. Once the bolts are removed, you should be able to lift the cylinder head off.
8. Remove the Cylinder: Unbolt the cylinder from the engine block. Then, carefully slide the cylinder up and off the piston. Be careful not to damage the piston or connecting rod.
9. Remove the Piston: Remove the circlip from one side of the piston pin, then push the pin out and lift the piston off the connecting rod. Be very careful not to drop anything into the engine.
10. Clean the Gasket Surfaces: Clean any old gasket material off the cylinder head and engine block. Be careful not to scratch or gouge these surfaces.
11. Install the New Piston: Apply a small amount of engine oil to the new piston pin, then carefully slide the new piston onto the connecting rod. Insert the pin and secure it with the new circlip.
12. Install the New Cylinder: Lightly oil the inside of the new cylinder and the new piston rings. Carefully slide the new cylinder down onto the piston, making sure the piston rings are correctly positioned in their grooves. Bolt the cylinder onto the engine block, using a torque wrench to tighten the bolts to the manufacturer’s specifications.
13. Install the Cylinder Head: Install a new head gasket, then bolt the cylinder head onto the cylinder. Use a torque wrench to tighten the bolts to the manufacturer’s specifications.
14. Reattach the Timing Chain: With the engine still at TDC, reattach the timing chain to the camshaft sprocket. Ensure that the timing marks align.
15. Adjust the Valves: With the engine reassembled, you’ll need to adjust the valve clearances. This is usually done using feeler gauges and following the specifications in your service manual.
16. Reattach the Exhaust: Bolt the exhaust back onto the cylinder, and tighten it securely.
17. Reassemble the Moped: Reinstall any body panels or parts that you removed.
18. Check Everything: Double-check all your work, make sure all bolts are tight, and ensure there are no parts left over.
19. Start the Moped: Turn on your moped. It might take a bit longer to start due to the new parts. Pay close attention to how it runs and immediately shut it off if anything seems wrong.

Timing Issues

Alright, now that we’ve covered issues related to fuel, air, and compression, it’s time to tackle another crucial part of your moped’s engine: spark timing. Spark timing is about when the spark plug fires during the engine’s cycle. It’s like the conductor of an orchestra, making sure everything happens at the right moment. If the timing is off, your moped may not start or run properly, even if it’s getting fuel, air, and has good compression.

What is Spark Timing?

When you kick-start your moped, you’re setting in motion a sequence of events that leads to the engine running. This sequence happens in what’s called a “cycle,” and it’s broken down into stages – intake, compression, power, and exhaust. The spark plug’s job is to ignite the fuel-air mixture in the power stage. But this can’t just happen at any time – it needs to be precisely timed to get the most efficient burn and the best performance from your engine.

What can Cause Timing Issues?

1. Faulty Ignition Components: This is rare, but will be the easiest fix. Components like the CDI box (Capacitor Discharge Ignition) or ignition coil, when faulty, can lead to timing issues. If the CDI box or the ignition coil is not able to provide the necessary electrical charge at the right time, it can cause the spark to occur at the wrong time.
2. Incorrectly Set Timing: If the timing has been manually adjusted in the past and set incorrectly, this can definitely cause timing issues. Timing could be adjusted during a repair, engine rebuild, or performance tuning.
3. Wear and Tear on Mechanical Parts: In some older engines, the ignition timing is controlled mechanically, for example with a distributor. Wear and tear on these mechanical parts can lead to inaccurate timing.
4. Damaged Flywheel Key: The flywheel is a part of the engine which helps control the timing of the spark. It has a small piece called a “key” which keeps it locked in the correct position on the crankshaft. If this key is damaged (sometimes called “sheared”), it can cause the flywheel to move out of position, which can mess up the spark timing.
5. Electronic Control Unit (ECU) Problems: Some modern mopeds may use an ECU, also known as an engine control unit, to control the ignition timing. If there’s a problem with the ECU, it could cause timing issues.

How to Fix Timing Issues

1. Fixing Faulty Ignition Components: If the CDI box or ignition coil is faulty, replacing them with new ones is the most straightforward solution. Ensure the new parts are compatible with your moped model. Once installed, these components should restore proper spark timing.
2. Correcting Incorrectly Set Timing: If the timing was set incorrectly, it will need to be adjusted. This usually involves a timing light. The exact method depends on the moped model, so it’s best to consult the service manual or take the bike to a professional.
3. Addressing Wear and Tear on Mechanical Parts: If there’s significant wear on parts like the distributor or other timing mechanisms, replacing or servicing these parts might be necessary. It’s a good idea to consult a professional mechanic for this as it can be a complex job.
4. Replacing a Damaged Flywheel Key: If the flywheel key is sheared or damaged, it will need to be replaced. This typically involves removing the flywheel, replacing the key, and then reinstalling the flywheel. It’s a fairly straightforward job, but you may need a special tool (a flywheel puller) to do it.
5. Resolving Electronic Control Unit (ECU) Problems: ECU problems can be challenging to diagnose and fix. Sometimes, resetting the ECU (by disconnecting and then reconnecting the battery, for example) might help. If the ECU is faulty, it may need to be replaced. Due to the complexity and cost of ECUs, it’s a good idea to get a professional diagnosis before replacing one.

Frequently Asked Questions

Conclusion

Alright, we’ve covered quite a bit of ground in this guide. You now have an understanding of the primary reasons why your 50cc moped might not start despite having spark and fuel, which include issues with air supply, compression, and spark timing. We’ve explained how you can diagnose and potentially fix these problems.

Remember, it’s essential to approach each step methodically. Troubleshooting an engine issue can sometimes be a process of elimination. Start with the most obvious and easiest checks, and move on to more complex ones.

Most importantly, don’t be afraid to ask for professional help if you’re feeling unsure about any steps or if the fixes require tools and skills you don’t have. It’s better to be safe than sorry, especially when you’re dealing with a vehicle you rely on for transport.