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Rotax Carburettor

VHSB 34 – Rotax (all classes)

The VHSB 34 is homologated in Rotax Max Challenge from Micro to DD2. It is a float/needle-type carburetor, so float height, jets and needle/seat are the key parameters you can tune within the regulations.

Maintenance / Service

Cleaning & disassembly: Remove jets, needles, needle valve and filter. Use carb cleaner and compressed air – be careful with small holes and fine channels.
Float height: Small changes have a big impact. As a guide: 23.0 mm = leanest, ~23.5 mm = medium, 24.0 mm = richest.
Needle & seat: Check for wear, sealing and fit. Copy previous measurements when reassembling to maintain your baseline. Needle radius/height is critical from mid to full throttle.
Jets: Keep a selection of main jets available. Adjust to temperature, air pressure and humidity. Typical jet ranges depend on class and conditions.
Air bleed / idle screw: Start with manufacturer’s baseline. Fine-tune for stable idle and crisp throttle response.

Tuning for different conditions

Cold weather / high air density

Cold, dense air requires more fuel → try a larger main jet. You can also set float height slightly richer.

Hot weather / low air pressure

Hot, thin air often makes the engine run too rich → try a smaller main jet, and ensure the air screw isn’t turned out too far.

High humidity

Moist air carries less oxygen → engine can lose power. Give it a touch more fuel and check exhaust color and throttle response.

Long straight / full throttle

If top speed drops or feels flat, the main jet may be too small or the needle too lean → go richer or raise the needle.

Acceleration ½–¾ throttle

If there’s a hesitation in mid-range, the needle clip may be in the wrong position → adjust clip or choose another needle profile.

Troubleshooting

Problem	Possible cause
Engine stalls at low throttle / poor idle	Idle jet or mixture too lean, air screw set incorrectly, or low float height (fuel level too low).
Flat spot between throttle positions (¼ → ¾)	Needle clip too high/low, giving the wrong fuel amount in mid-range, or unsuitable needle profile.
Lack of top speed / weak at full throttle	Main jet too small, needle too lean, restricted air filter, low float height, or weather changes making the mix lean.
Excessive smoke / black exhaust / rich running	Main jet too large, overall mixture too rich, or ignition problems.
Intermittent misfire / jerky throttle	Air leak, partially clogged jets, worn needle or leaking gaskets. Can also be caused by unstable fuel supply (bubbles/leaks).

Remember: Measure – don’t guess. Record jet/needle/float setup and weather for every session. Small, systematic changes = faster troubleshooting.