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I found an Australian site that says Aisan 20/26 had cfm ratings of 134cfm. Pretty small, but actually might be nice for NA dual progressives. I'm looking at getting rid of the secondary air flap. I have arrow in the first pic ( beautiful huh?) going to the step system to help the carb progression to the secondary. The second is of a vacuum secondary and the step jet for progression is just above the throttle plate. The mechanical secondary step jet is considerably higher.( first pic) I think I will try and drill one just above the lower plate after I remove the secondary air flap. Anyone else ever do this???

Just found some explanations of how powervalve and air flap works from a Mazda site. This picture is of a dcg306 mechanical secondary. In it you can see how much more restricted it is with this flap. This would drop the total mm sq by quite a bit. If the pic is to scale, I would guess it obstructs the 26mm venturi to make it a 21-22mm opening. If true, 21mm would drop the secondary cross sectional area down to 346mm sq and the total carb to 664 mm sq. I think it will work great for my application, but also think it would give better top end than a single Weber 32/36.

How about an explanation of the power valve and where fuel goes when the vacuum is reduced enough to open the valve? Is it just straight to the main jet? I'm not sure if Subaru used the secondary air flaps or not, but if so, what was their purpose? I know lots of early Dastuns used a 306dcg which had a weighted arm that controlled a flap just above the secondary throttle plate. Was that to slow the rush of air when you stomped on the peddle? To stop or slow the dead spot from too much air??

I should have added I'm looking for feed back on the purpose of the "secondary air flap" circled in pic. Is it to slow the draw of fuel/air when the second barrel is suddenly opened, to stop the dreaded bog that progressives experience? I thought I would gut that, but since I may be over-carbed anyways, I'll leave it to start with. It might help with lean pops when you slam the throttle down too. Also wondering about the powervalve. Where does the fuel go when the valve opens? Is it just a short cut to the main jet? Again, I'm new, and although I bought two carbs to do this, I haven't laid hands on them so all my knowledge is from surfing here and there. My carb experience is mostly with bikes, cv types. Thanks in advance for any response.

I know this is an old thread, but-- Actually the math is a little flawed comparing carbs because it's venturi size that matters. With a 306 there were different venturis. One was a 20/26 which is what I'm going to attempt this summer. I'm going to try it on a Goldwing, 1200 with bigger valves and solid lifter heads which should allow 10k rpm. The 20 venturi is only 316mm sq and the 26 is 530 mmsq so one Hitachi would be capable of 846 mm sq. The Weber have different venturi options but most 32/36 had 26/27 venturi size, so 530 mm sq on the primary and 572 mm sq on the secondary. So WFO the two Hitachi have 1692 mmsq vs the single Weber 1102 mm sq. for higher rpm it might be better with a built engine that can rev. For me it's this little 20 mm venturis that I think can feed two cylinders better and more directly. I am hoping to elevate them a little to allow a straighter feed, but like Subaru there is still the 90 at the head. Comparing the primaries, the Weber has 530 mmsq feeding four. The Hitachi is 632, ( 316+316 to feed four) not really a huge difference. (16%) I'm new at this, a late in life hobby, so take what I say with a grain of salt followed by tequila. My guess is I won't be fully opening the second barrels. Until it's ready to blow. Love to hear opinions especially from any who've tried it. My reading tells me the progressives are a pita to get tuned. I have committed myself. Will likely get started in May.