Choosing an intake path for Honda dual Keihin CV carburetors (D14A1 and related dual-carb D-series) is about airflow control, filtration, heat, and fuel calibration: not only induction noise.

Option A: modified stock airbox (preferred street baseline)#

Do:

• Delete the exhaust-heat feed into the box after warm-up
• Remove the internal restrictor flap
• Install a quality panel filter (confirm dual-carb box fitment; K&N E-series panels are often cited)
• Optionally drill extra box inlets once the above are done
• Route cooler air from the grille / wing when fabricating ducts
• Keep the factory mesh out of the way of a good panel element - do not stack mesh + tights as a “filter”

Avoid:

• A single cone only on the factory snorkel (little benefit; can add restriction)
• Mesh or fabric stretched tightly over open carb mouths - more restriction than it looks

This path keeps debris protection and makes cold-air ducting easier. Many stronger street dual-carb cars stay on a drilled box plus panel filter rather than open stacks.

Stock dual-carb layout with factory airbox still fitted (Gizmo).

Winter: several owners put the airbox back on for wet/cold months even if they run open filters in summer.

Option B: velocity stacks (trumpets) with foam or ram socks#

Stacks sit on the carb mouths. They smooth the entry, raise air speed at part load (venturi effect), and can use standing-wave / length effects to move the torque band. They only make power if the fuel system matches the new airflow.

D14 dual carb velocity stacks (honda-hardy).

Trumpets from above (honda-hardy).

Airbox off, carb mouths open for stacks or filters (Gizmo).

Open dual Keihin throats after airbox removal (Gizmo).

Plumbing when the airbox comes off#

Filters on stacks#

• Foam socks / ram socks stretch over the trumpet mouth (Pipercross-style and similar).
• Trumpets sized for other sidedrafts (e.g. some 45 mm Weber/Dellorto trumpets) are sometimes adapted - verify mouth OD and sealing.
• Open OEM mesh alone is not adequate long-term filtration.

Stack diameter (community street notes)#

On dual-carb GLs running open stacks long-term (with and without socks):

Those are seat-of-pants reports on dual-carb D14/D15 cars, not controlled dyno deltas. Length still matters: short ~50 mm stacks accentuate top-end character; longer runners trade that for midrange. Always re-jet and re-balance after changing trumpet size.

Fueling requirements with stacks#

• Re-set idle mixture and balance after fitting.
• Expect to revisit needles (D15B / CRX needles often add fuel at part throttle) and main jets.
• Cold-air ducting matters: open stacks with the bonnet down mainly ingest hot bay air unless you feed them.
• Optional stiffer slide springs if the engine goes rich as soon as the slides hit the stops (opposite of blind spring-cutting).

Open stacks without jetting: what goes wrong#

Community dyno on a dual-carb 1.4 with stacks, spring cuts, and lean idle CO (~0.2%) showed roughly ~75 bhp, recovering to ~86 bhp when fuel was added. High-power dual-carb D-series street cars on the same forums often run drilled stock boxes + K&N, not trumpets - because mixture control is easier.

Relative intake comparison (built dual-carb D15, not stock D14)#

On a sorted dual-carb CRX with ported head/manifold work, controlled dyno fan, ~7600 rpm limit:

Use this only as a relative lesson: open area can free top end when the engine and fueling already support it; a stock D14A1 will not jump into that power band from socks alone.

Decision guide#

Related work order#

1. Rebuild and balance carbs on the stock box
2. Exhaust / header
3. Decide airbox vs stacks with AFR data
4. Only then change springs, needles, or jets for the new airflow