Customer service
YouTube
Adjusting a Tomos A35 DellOrto SHA carburetor
Want to adjust the carburetor on your Tomos A35? In this guide, we explain how to check a DellOrto SHA carburetor, how to recognize symptoms of a lean or rich mixture, and how to choose the correct jet step by step.
A correct carburetor setup helps your Tomos start better, accelerate cleaner, and run at a safer temperature. Before changing jets, first rule out other problems such as air leaks, incorrect ignition timing, worn gaskets, or bad seals. Otherwise, you may keep tuning the carburetor while the real problem is somewhere else.
When should you adjust a carburetor?
You do not need to adjust the carburetor for no reason. Most of the time, carburetor tuning is needed after changing your setup or when your Tomos clearly starts running differently than normal.
| Situation | Examples | Why adjust it? |
|---|---|---|
| New parts | Different cylinder, exhaust, or air filter | The engine may receive more or less air, which often requires a different jet size. |
| Poor starting or stuttering | Hesitation, bogging, poor throttle response, or stalling | These symptoms can point to a mixture that is too lean or too rich. |
| Spark plug inspection | Black, wet, white, or grey spark plug | The spark plug color can help show whether the mixture is roughly correct. |
| Seasonal changes | Cold weather, hot days, or seasonal temperature changes | Temperature and air density can affect the air-fuel ratio. |
Recognizing carburetor problems
A poorly adjusted carburetor can cause annoying and sometimes serious problems. These include hard starting, loss of power, excessive smoke, overheating, or even engine seizure. The table below helps you recognize the most common symptoms.
| Symptom | Possible cause | What should you check? |
|---|---|---|
| Difficult starting | Mixture too lean or too rich | Check spark plug color, choke behavior, jet size, and possible air leaks. |
| Poor throttle response | Wrong jet size or air filter setup | Test whether the engine responds better with a different jet size. |
| Black or oily spark plug | Mixture too rich | Too much fuel may be entering the mixture, or the engine is not burning cleanly. |
| White or grey spark plug | Mixture may be too lean or there may be another issue | Check jet size, air leaks, and ignition. A lean mixture can be dangerous for the engine. |
| Overheating | Often caused by a lean mixture | Stop riding hard and check the setup before causing engine damage. |
| Excessive exhaust smoke | Rich mixture or possibly a leaking seal | Do not assume smoke is always a carburetor issue. Also check the seals. |
Symptoms of a lean setup while riding
- The engine hesitates or bogs during acceleration, especially at higher RPM.
- The moped heats up quickly and can overheat or seize.
- You have less power than normal, especially at full throttle.
- The engine may seem to run better when the choke is used.
Symptoms of a rich setup while riding
- The engine stutters or sounds like it is drowning in fuel.
- The engine starts four-stroking, especially at low RPM or part throttle.
- More smoke than normal comes from the exhaust.
- You notice power loss during acceleration or at top speed.
What jet size do you need?
On a DellOrto SHA carburetor, the mixture is mainly adjusted with the jet. The jet determines how much fuel flows through the carburetor. For a 15mm DellOrto SHA, jet size 68 is often used as a starting point, but the correct size depends on your cylinder, exhaust, air filter, and overall engine condition.
Spark plug inspection
One simple way to judge the mixture is by checking the spark plug. Ride a few kilometers first so the spark plug color can show something about combustion. This method works best with 98 E5 fuel. With 95 E10, spark plug color is often less reliable.
| Spark plug color | Meaning | Action |
|---|---|---|
| Light brown or coffee brown | Usually good combustion | Your setup is probably heading in the right direction. |
| White or grey | Can indicate a lean mixture | Check for air leaks and try a larger jet if needed. |
| Black and oily | Can indicate a rich mixture | Check the air filter, choke, and try a smaller jet if needed. |
Choke test while idling
The choke enriches the mixture. On a DellOrto SHA, it almost closes the air intake. If your Tomos runs better when the choke is on, the normal mixture is often too lean and you probably need a larger jet. This trick is mainly useful for recognizing a lean mixture, not for perfectly diagnosing a rich mixture.
Carburetor tools
You do not need complicated tools to adjust a DellOrto SHA carburetor. With a few basic tools and a good jet set, you can already do most of the work.
| Tool or part | What is it used for? |
|---|---|
| Phillips or flathead screwdriver | For the float bowl, idle screw, and other small carburetor screws. |
| Spark plug wrench | For removing and checking the spark plug color. |
| Jet set | For fine-tuning the fuel mixture with different jet sizes. |
DellOrto SHA carburetor adjustment process
Once you have checked the basics and know that you want to work on the jet, you can remove the DellOrto SHA carburetor and inspect or replace the jet
Step 1: side cover and fuel tap
Start by removing the side cover from the frame if your Tomos has one fitted. Then close the fuel tap before loosening the carburetor.
Step 2: remove the carburetor and air filter
Loosen the bolt on the air filter and remove the air filter from the carburetor. Then loosen the bolt that clamps the carburetor to the intake manifold.
Step 3: remove the float bowl
Use a screwdriver to remove the float bowl. Keep the float bowl level, because there may still be fuel inside. While you are there, check whether the float needle still seals properly.
Step 4: inspect the jet
The jet is located at the bottom of the carburetor. On an SHA carburetor, it is the small gold-colored screw. Because a DellOrto SHA uses one main jet, this jet is the most important part for adjusting the mixture.
Step 5: replace the jet
If your Tomos is running too rich, usually go down one jet size. If your Tomos is running too lean, usually go up one jet size. Screw the jet in carefully. The material is soft, so do not force it. Tight is tight.
Step 6: reassemble and test
Reinstall the carburetor, float bowl, and air filter. Make sure the float pin is seated correctly and that the float bowl seals properly. Then take a test ride and check again how your Tomos responds.
Conclusion
A Tomos A35 with a correctly adjusted DellOrto SHA carburetor starts better, runs cleaner, and responds more strongly to the throttle. Work calmly, change only one jet size or part at a time, and test after every adjustment.
If your Tomos still does not run properly, look beyond the carburetor. Also check the spark plug, reed valve, air filter, ignition, and possible air leaks. With a solid basic setup and patient tuning, almost every Tomos can be brought back to life.
Back to topFAQ: Tomos A35 DellOrto SHA carburetor adjustment
What jet do I need for a Tomos A35 with a DellOrto SHA?
That depends on your setup. For a 15mm DellOrto SHA, jet size 68 is often used as a starting point, but the correct size depends on your cylinder, exhaust, air filter, and engine condition. Always test carefully and change only one jet size at a time.
How do I know if my Tomos is running too lean?
A lean Tomos can hesitate, run hot, lose power, or run better with the choke on. A white or grey spark plug can also point to a lean mixture, but always check for air leaks and ignition problems too.
How do I know if my Tomos is running too rich?
A rich Tomos can stutter, four-stroke, smoke heavily, or accelerate poorly. A black and oily spark plug can also point to a rich mixture.
Can I adjust a DellOrto SHA with a mixture screw?
On a DellOrto SHA, the mixture is mainly adjusted with the jet. The idle screw mainly controls how far the throttle slide stays open at idle. For the main running setup, use the correct jet size.
Should I test ride after every jet change?
Yes. Take a test ride after every adjustment and check how the engine responds. Then inspect the spark plug again. This prevents you from changing too many things at once and losing track of what improved or worsened the setup.
