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[Datsun 1200 encyclopedia]

Mikuni PHH Jetting

From Datsun 1200 Club

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Category: Fuel System


Main Jets

You cannot say what jets to use until you know which venturies you will be using. Venturi selection is the first step in tuning. 32 mm venturis are a bit large for an A12 so won't run perfect. For 6400 rpm I recommend 29mm venturis for excellent top-end power with good low speed response. Or for 6000 rpm use 27mm for stock A12 to get best response.

Or, bolt the Mikuni PHH40s on, adjust them, and see how it runs. It should run OK. Then you can begin tuning them to meet the needs of your engine.

For baseline jetting:

  • 32mm venturi fit 128 main jets and 180 air jets
  • 29mm venturi, 116 main jets, 165 air jets.

This will set the air/fuel ratio for cruising (non-accelerating mode)

Fine Tuning

Next comes the hard part - tuning for acceleration and low speed running. This is where each modified engine will be different: idle jets, power valves, emulsion tubes, accelerator pump adjustments, and possibly juggling the main/air jet combo (larger main + smaller air or vice-verse ).

Example

Dual PHH40s on 78.5 mm bore A14 with ported H89 head with 38x29 ports and 45mm inlet manifold.

NOTE: It ran too rich

Carburetors: Twin Mikuni PHH 40
Venturis: 32 mm See Carburetor Sizing
pilot jets: 57.5
main air jets: 180 
main bleed pipes: T 
main jets: 140
pump nozzles: 40
starter jets: 130
pump nozzles: 40
mixture settings: 2, 2-1/8, 2-1/4, 2

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