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Introduction
In this blog and video, we will show you how to adjust the carburetor of a Tomos A35, the DellOrto SHA. You might notice that your moped isn’t performing well and think this adjustment is necessary. We’ll explain step by step what to do. This process is also demonstrated in the video, covering everything from recognizing carburetor issues to testing and resolving faults.
We recommend ruling out other problems first. Check out our other blogs to ensure that the ignition, gaskets, seals, and other parts are in order, so adjusting the carburetor isn’t done in vain.
When should you adjust a carburetor?
- New parts: Such as a different cylinder, exhaust, or air filter.
- When problems arise: Stuttering, lagging, or difficulty starting often indicate a poor adjustment.
- Spark plug inspection: A black or white spark plug might signal a mixture that’s too rich or too lean.
- Seasonal changes: Cold or warm weather can impact your mixture ratio.
Recognizing carburetor problems
It’s important to quickly recognize and address carburetor issues in your moped. A poorly adjusted carburetor can lead to serious problems, such as overheating or even engine seizure. Here are some common signs that your carburetor needs adjustment. NOTE! Make sure all other issues with your Tomos are resolved first.
1. Difficult starting: If your engine is hard to start, especially when cold, this might indicate a mixture that’s too lean or too rich.
2. Poor throttle response: If the engine hesitates or lags when you accelerate, it might mean the carburetor is out of adjustment.
3. Black or oily spark plug: This indicates a rich mixture. Not all fuel and oil are burned, leaving residue on the spark plug.
4. High temperature/overheating: A lean mixture can cause the engine to overheat, potentially leading to overheating or even seizure.
5. Excessive smoke from the exhaust: If your Tomos produces more smoke than usual, especially blue smoke, it could indicate a rich mixture. Don’t confuse this with a leaking seal.
6. Poor performance: If you notice your engine losing power, accelerating slowly, or not reaching its top speed, an incorrect carburetor adjustment could be the cause.
Carburetor Tools
Adjusting a DellOrto SHA carburetor doesn’t require complicated tools. Here’s an overview of the essential tools and parts you’ll need:
Phillips/flathead screwdriver:
To loosen screws on the carburetor, such as the float bowl and idle screw adjustments.
Spark plug wrench:
To remove and inspect the spark plug for color.
Jet set:
A set of jets in different sizes to adjust the fuel mixture. The jet determines the amount of fuel flowing through the carburetor.
What Jet Size Do You Need?
You can determine if your moped is running too rich or too lean by looking for key symptoms. The fuel mixture (air-fuel ratio) must be just right for optimal performance. Ensure that other problems like air leaks are resolved. Here are some ways to check:
Spark Plug Inspection
One of the simplest ways to see if your moped is running too rich or too lean is by inspecting the spark plug. It takes time for the spark plug color to change, so make sure to ride a few kilometers. NOTE: This method is only effective when using 98(E5) fuel instead of 95(E10).
- Normal mixture: The spark plug should be light brown or coffee brown, indicating good combustion.
- Lean mixture: If the spark plug is white or gray, it means the mixture is too lean (too much air, not enough fuel). This can be dangerous for the engine, as it leads to overheating. A white/gray spark plug might also indicate a faulty coil.
- Rich mixture: If the spark plug is black and oily, the mixture is too rich (too much fuel, not enough air). This often results in smoke, reduced power, and a clogged exhaust.
While Riding
Lean Adjustment
- The engine may hesitate or bog down during acceleration, especially at higher RPMs.
- The moped might overheat quickly, overheat, or even seize.
- Less power than usual, especially at full throttle.
Rich Adjustment
- The engine may stutter or make a "choking" sound, especially at low RPMs. This is also called "four-stroking."
- Excessive exhaust smoke indicates that too much fuel is being burned.
- Power loss, especially when accelerating or at top speed, and often a strong smell of unburned fuel.
Idling
- Lean adjustment: The engine may struggle to idle, frequently stall, or run unevenly at low RPMs.
- Rich adjustment: The engine may idle unevenly, sometimes with a high idle speed and excessive smoke.
Using the Choke While Idling
The choke on your carburetor enriches the mixture, making it easier to start your Tomos when cold. For a DellOrto SHA, it nearly closes the air intake, while for other carburetors like a PHBG, it opens an extra fuel supply. This can help determine if the mixture is too lean. However, this trick isn’t reliable for identifying a rich mixture. Does the engine run better with the choke on? Then the standard mixture is likely too lean, and you’ll need a larger jet.
After every adjustment, take a test ride and continue checking the spark plug to see if the mixture improves.
Carburetor Adjustment Process
Step 1: Side Cover and Fuel Tap
Start by removing the side cover from the frame, if present. Don’t forget to close the fuel tap.
Step 2: Carburetor and Air Filter
Loosen the bolt of the air filter and remove it from the carburetor. Then loosen the bolt on the carburetor that clamps it to the manifold.
Step 3: Float Bowl
Use a screwdriver to remove the float bowl. Be aware that gasoline might be inside the float bowl. Hold it level to prevent fuel spillage. Also, check if the float needle is sealing properly.
Step 4: Inspect and Adjust the Jet
Inspect the jet, which is located at the bottom of the carburetor. The jet is the small gold-colored screw. In an SHA carburetor, there is only one jet. The standard jet size for a 15mm DellOrto SHA is jet 68.
If the Tomos is running too rich, go down one size, and if it’s running too lean, go up one size. Adjust only one thing at a time and test how your moped responds. It’s a process of fine-tuning, requiring patience and care.
Step 5: Reinstall the Jet
Carefully screw the jet back into the carburetor. Be sure not to damage the threads, as this can ruin the carburetor. The jet is made of a relatively soft metal, so it’s easy to damage. Tight is tight!
Step 6: Reassemble the Carburetor
Reassemble everything in the same way you removed it. Make sure the float pin is properly in place, and the float bowl is sealed well.
Repeat these steps patiently until your Tomos runs smoothly.
Conclusion
If you’ve followed all the steps, your Tomos should now run smoothly. If not, there might be another issue at play. Check if a different carburetor might solve the problem. Don’t forget to inspect other parts, such as the spark plug, reed valve, air filter, or ignition, which could also affect your Tomos’ performance.
Still stuck? Don’t worry! We have an extensive collection of videos and blogs diving deeper into specific problems and solutions. From adjusting the carburetor to replacing parts, we’ll guide you step by step. Also, visit our webshop for quality replacement parts to get your Tomos running smoothly again.
Stay patient and thorough; with the right approach, every Tomos can come back to life!
