
Originally Posted by
duskylim
Dear Sir:
I'm very surprised by your comment that a Mikuni carb has worse fuel economy vs an Aisan.
Having grown up with and tuned both carbs I can definitely say the Mikuni when properly tuned will give better economy - both city and highway driving.
The Aisan will give you more top-end, full-bore power - for several reasons.
1) The Mikuni uses a 21 mm primary main venturi with a 28 mm primary throttle plate. The Aisan has a 23 mm primary main venturi and a 28 mm primary throttle plate.
2) the Mikuni uses a 27 mm secondary main venturi with a 32 mm secondary throttle plate, the Aisan has a 28 mm secondary main venturi and a 32 mm secondary throttle plate.
So as you can see from airflow alone the Aisan will outflow the Mikuni.
However, due to the smaller venturi throats, the Mikuni will produce a stronger suction signal and faster airflow - which results in better response and fuel atomization both of which mean better fuel economy.
This is particularly important in the primary system - because you drive on this system 80 to 90 percent of the time - you really only use the secondary for a long time at highway speeds.
3) the Aisan has a triple secondary venturi while the Mikuni has only a double secondary venturi - so even with the larger airflow the Aisan secondary should still produce a good suction signal and atomization.
4) the Mikuni's emulsion tubes are long and thin, occupying little space in the emulsion well, meaning they are RICH TUBES.
The Aisan's emulsion tubes are short and fat, occupying a lot of space in the emulsion well - meaning that they are LEAN TUBES.
However, the Mikuni emulsion tubes have air holes all along their length, greatly weakening the suction signal - leaning them out.
The Aisan's emulsion tubes have holes only at the very bottom (to let gas in) and very small ones at the top (to let air in), - this design mimics a rich tubes behavior.
5) the Aisan's secondary throttle is actuated by a mechanical linkage and opens full when floored - while the Mikuni's secondary is vacuum-actuated.
I have much experience tuning both on our Galant Sigma and my friend's Lancer GSR.
I in fact had twin downdraft Mikuni's and Weber 32/36 DGAV's.
The Mikuni carb is closer to the Weber in the idle circuit, throttle plate and accelerator pump design.
The Aisan (3K, 4K, 5K) carb is quite different.
Best Regards,
Dusky Lim