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X30 Carburettor

Guide to Your X30 Carburetor

Tillotson HW-27A Carburetor – Springs Explained

The inlet spring determines the pop-off pressure, i.e. the fuel pressure at which the needle valve opens. This directly affects how much fuel the engine receives at low and mid throttle.

Lighter spring = lower pop-off = richer mixture at low/mid RPM (more fuel, stronger bottom-end response, risk of flooding).
Heavier spring = higher pop-off = leaner mixture at low/mid RPM (cleaner throttle response at high revs, but weaker acceleration off corners).

The standard spring for the X30 HW-27A is 37 g, while the Mini Swift normally uses 26 g.

26 g Spring (Part No. 24-B323X)

Effect: Lowest pop-off pressure. Needle opens very easily, providing a rich mixture. Strong punch out of slow corners, but high risk of over-fueling (“bogging down”) in warm conditions or on high-grip tracks.
Typical use: Standard for Mini Swift. Rarely used on X30 except in very specific conditions where extra low-end enrichment is needed.

31 g Spring (Part No. 24-C296X)

Effect: Slightly lower pop-off than the standard 37 g. Provides a richer low-end and better throttle pickup.
Typical use: Sometimes chosen in restricted classes (e.g. TAG Restricted) or on low-grip tracks where acceleration out of corners is more important than peak RPM cleanliness.

37 g Spring (Part No. 24-B299X) – X30 Standard

Effect: Balanced setting between bottom-end and top-end. Provides stable pop-off around 0.75 bar (≈10.9 psi) and reseal around 0.55–0.60 bar (≈8.0–8.7 psi).
Typical use: Standard configuration for X30 Junior/Senior. The best all-round spring; always the baseline for tuning.

42 g Spring (Part No. 24-C298X)

Effect: Higher pop-off than standard. Makes the engine leaner in the low/mid range, cleaning up throttle response on high-grip or fast circuits. May cause hesitation if the low-speed needle is not enriched accordingly.
Typical use: Useful in hot weather or on high-traction tracks where the engine tends to run too rich at corner entry.

46 g Spring (Part No. 24-C297X)

Effect: The heaviest spring, producing the highest pop-off pressure. Strongly leans out the low/mid range. Can sharpen the engine at high RPM but requires careful tuning of the low-speed needle to prevent bogging.
Typical use: Specific high-RPM applications. Rarely recommended for general racing unless conditions demand a very lean bottom-end.

Practical Notes

Always measure pop-off with a calibrated gauge after changing springs – do not rely on spring weight alone.
Carburetor fulcrum height (lever height) also influences pop-off and must be correct.
Membrane condition affects consistency – old or stiff diaphragms can cause misleading results.
Start with 37 g, then adjust needles first. Only move to a different spring if you cannot achieve the desired throttle response with screw adjustments.

Important

When you apply full throttle, the butterfly valve must open completely vertical in the carburetor bore.

If it only opens 90–95%, there will still be a “shadow” in the airflow. This reduces the amount of air entering the engine → the mixture becomes too rich → the engine loses top speed and pulls poorly.

If the throttle does not open fully, it can feel as if the engine “dies out” on the straight, even though you have your foot flat on the pedal.

If the butterfly opens too far (past vertical), it will actually disturb the airflow and reduce performance.