Starting a 50cc moped or scooter engine is often done manually via the kickstarter pedal. This system is critical, especially when the battery is low or the starter is unable to start a cold engine. Engine series 139QMB, installed on most Chinese mopeds, has a specific design of the starting mechanism, which can fail over time.
Many owners are faced with a situation where, when pressing the pedal, it falls through without resistance, or, on the contrary, gets stuck. Understanding the mechanism and knowing how to troubleshoot the problem yourself will save you time and money. In this article we will analyze all the nuances of the kickstarter, from the principle of operation to complex repairs.
Design and principle of operation of the trigger mechanism
Engine starting system 139QMB is a set of gears and springs that convert the rotational movement of the pedal into rotation of the crankshaft. The key element here is the ratchet mechanism, which ensures that force is transmitted in only one direction. When you press the pedal, the kickstarter drive gear engages a gear on the crankshaft, turning it.
When the pedal is released or when you try to start, when the engine is already running, the return spring returns the mechanism to its original position. If the spring is weakened or broken, the pedal may not return into place. The condition of the splines on the shaft is also important, since their wear leads to the fact that the pedal simply turns idle, without transmitting torque to the motor.
Correct operation of the system depends on the quality of the assembly and the absence of foreign objects in the crankcase. Even small metal shavings can jam the gears. Regularly checking the oil condition and flushing the crankcase helps avoid premature wear of kickstarter parts. Pay attention to how the pedal behaves during a cold start, this is the first indicator of problems.
Main faults and their diagnosis
The most common problem is the pedal spinning without starting the engine. In this case, it is necessary to check the condition of the kickstarter gear and the splined joint on the shaft. It often happens that the splines on the shaft are cut off and the pedal loses traction with the mechanism. It may also be due to a broken return spring that does not disengage the gear.
- 🔍 Spring check: Press the pedal and release it, it should return to its original position with force.
- 🔍 Gear Inspection: When disassembling the crankcase, carefully inspect the teeth for chips or wear.
- 🔍 Shaft check: Make sure the splines on the kickstarter shaft are not worn smooth.
Another common problem is the pedal sticking in the down position. This may occur due to gear damage or debris getting into the mechanism. Sometimes the reason lies in improper installation of gaskets or deformation of the crankcase cover. If the pedal does not return, the engine may be damaged when the starter attempts to start it.
⚠️ Caution: If the kickstart pedal does not return to its original position, do not attempt to start the engine with the electric starter under any circumstances, as this may cause critical damage to the gears and crankshaft.
Diagnostics requires partial disassembly of the engine. You will need to remove the crankcase cover and inspect the internals. Pay attention to the condition of the oil pump, as its gear can also interfere with the operation of the mechanism. Incorrect assembly after repair often causes new breakdowns.
Step-by-step instructions for removal and disassembly
To access the kickstarter mechanism, you must remove the right engine crankcase cover. Before starting work, be sure to disconnect the battery to avoid short circuits. Unscrew the bolts securing the cover and carefully remove it, being careful not to damage the gasket. Inside you will see the flywheel and trigger mechanism.
The next step is to remove the retaining ring that holds the kickstarter shaft in place. Use special pliers or a thin screwdriver to carefully remove the ring. After this, the shaft can be removed along with the gear and spring. Be careful as the spring is under tension and may fly out.
Disassembling the mechanism requires care. Memorize or take a photo of the sequence of installation of gears and springs. This will help you during reassembly. Clean all parts from carbon deposits and old grease before installing new elements. Use a soft cloth and solvent to clean.
- 🛠️ Tools: set of keys, circlip pliers, hammer, screwdrivers.
- 🛠️ Consumables: new crankcase cover gasket, sealant, gear lube.
- 🛠️ Spare parts: kickstarter kit (gear, shaft, spring, circlip).
☑️ Preparing for kickstarter repair
It is important to remove the return spring correctly without deforming it. If the spring has already lost its elasticity, it must be replaced. Installing a new spring requires that the ends be oriented correctly. An installation error may result in the mechanism not working or breaking immediately after startup.
⚠️ Caution: When removing the kickstarter recoil spring, make sure it is firmly secured in your hands, as sudden release of tension may cause injury to your fingers.
What to do if the kickstarter shaft breaks?
If the splines on the shaft are torn off, the entire shaft must be replaced. It is not recommended to try to weld the splines or restore them with a file, as this will upset the balance of the shaft and lead to rapid wear of the new gears.
Replacement of worn parts and selection of spare parts
When replacing kickstarter parts on an engine 139QMB It is important to choose quality components. Chinese spare parts often have different quality, so it is better to give preference to proven brands or original parts. Cheap analogues can quickly break, which will lead to repeated disassembly of the engine.
The main parts that require replacement include the gear itself, the shaft, the return spring and the retaining ring. Sometimes the crankcase cover also needs to be replaced if it has cracks or deformations. When purchasing a kit, make sure that all parts are suitable for your engine model.
| Detail | Signs of wear | Recommendation |
|---|---|---|
| Kickstarter gear | Worn teeth, chips | Replace with a new one |
| Kickstarter shaft | Torn splines, wear | Replace entirely |
| Return spring | Loss of elasticity, stretching | Replace with original |
| Retaining ring | Deformation, cracks | Must be replaced |
Some owners try to restore worn parts, but this often does not have a long-term effect. For example, welding on a gear can lead to overheating and deformation. It is better to install a new part right away to avoid problems in the future. High-quality lubrication during assembly also plays an important role in the durability of the mechanism.
- Lever (pedal on the side)
- Foot (pedal from below)
- Electric starter
- Missing
Correct assembly and adjustment of the mechanism
Assembling the kickstarter mechanism requires precision and adherence to the sequence of actions. First install the return spring onto the shaft, then slide the gear on and secure it with the retaining ring. Make sure the spring engages the special tabs on the housing and shaft. Incorrect installation of the spring is the most common error during assembly.
Before installing the shaft into the crankcase, apply some grease to the splines and axle. This will ensure easy rotation and reduce wear on parts. Carefully push the shaft into place and install the retaining ring. Check that the shaft rotates freely and has no play. After this, you can put on the crankcase cover.
When tightening the crankcase cover bolts, use the recommended torque. Excessive force may cause the lid to deform and seal. Use a torque wrench if available. If not, tighten the bolts in a crisscross pattern, gradually increasing the torque.
- 🔧 Lubrication: Use special gear lubricant, do not use engine oil for assembly.
- 🔧 Tightness: Check the condition of the gasket and apply sealant if necessary.
- 🔧 Control: After assembly, check the operation of the pedal before starting the engine.
Before final tightening the crankcase cover bolts, check that the kickstarter shaft rotates smoothly and without binding, otherwise you will have to disassemble the engine again.
Common repair mistakes and how to avoid them
One of the most common mistakes is using the wrong lubricant. Motor oil is not suitable for assembling the kickstarter mechanism, as it is too thin and washes out quickly. Use a thick lubricant that holds the gears and provides wear protection. This will extend the life of the mechanism.
Another mistake is ignoring the condition of the crankcase cover. If the cover is cracked or deformed, the seal will be compromised, resulting in oil leakage and dirt entering the engine. Always inspect the cover before installation and replace it if necessary. Never try to seal a crack; it's not permanent.
Incorrectly installing the return spring may result in the pedal not returning into place or the mechanism jamming. Make sure the spring is engaged on the correct tabs and is under tension. Errors in spring installation often cause repeated repairs.
⚠️ Attention: If after assembly the kickstarter pedal fails or does not return, do not start the engine until you figure out the cause of the malfunction to avoid breaking the crankshaft gears.
Proper lubrication and careful installation of the recoil spring are key to reliable kickstarter operation on the 139QMB engine.
Prevention of breakdowns and maintenance of the mechanism
Regularly checking the condition of the kickstarter will help avoid sudden breakdowns. Periodically inspect the pedal for play and binding. If you notice that the pedal is working worse, do not put off repairs. Timely replacement of worn parts will extend the life of the engine.
Monitor the level and quality of engine oil. Dirty or old oil can get into the kickstarter mechanism and cause the gears to seize. Change the oil according to the maintenance schedule and use high-quality lubricants. This will also have a positive effect on the operation of other engine components.
Avoid sudden and forceful pressing of the kickstarter pedal, especially if the engine is cold. This may damage the gears and splines. Start the engine smoothly, without excessive effort. If the engine does not start the first time, let it cool slightly and try again.
How often should the lubricant in the kickstarter mechanism be changed?
It is recommended to check the condition of the lubricant every time you change the engine oil. If the lubricant becomes liquid or contaminated, it must be replaced with a new one.
FAQ: Answers to frequently asked questions
In this section we have collected answers to the most popular questions that owners of scooters with an engine have 139QMB when repairing a kickstarter.
Why does the kickstarter pedal spin in one direction?
This is normal for the kickstarter mechanism. The pedal should scroll freely in one direction (when the engine is running) and transfer force to the other (when starting). If it turns in both directions, then the ratchet or gear is broken.
Is it possible to start the engine without a kickstarter?
Yes, if you have an electric starter, you can start the engine with it. However, the kickstarter is an important backup starting method, especially when the battery is dead. Its absence can create problems on the road.
What is the best lubricant to use for a kickstarter?
For the kickstarter mechanism, it is recommended to use thick lithium grease or a special gear lubricant. Motor oil is not suitable as it washes out quickly and does not provide sufficient wear protection.
What to do if the pedal is stuck in the down position?
Do not try to start the engine with the starter. Disassemble the kickstarter mechanism and check the gears and spring for damage. Most often the problem is a broken spring or a stuck gear.
Do I need to replace the entire kickstarter kit or just the gear?
It is better to change the entire set, since wear of one part often leads to wear of others. If you only replace the gear and the shaft or spring is worn, the problem will return very quickly. Complete replacement guarantees reliable operation.
Compliance with recommendations for repair and maintenance of the kickstarter on the engine 139QMB will provide you with a reliable start in any conditions. Regular checks and timely replacement of worn parts will help avoid costly engine repairs in the future. Remember that prevention is always cheaper and easier than eliminating the consequences of a breakdown.