secondary flap: what distance for full CFM ?

[Frank400]

Regarding the opening of the secondary air flap, how far open does it have to be to get the maximum airflow capacity of the carb ?  I have Cliff's book, I know how to adjust them, and I've read his recommendations, but I'd like to know which distance yields which CFM ?

[blazer74]

Recipe 3 shows 1.240 max opening. Ranges from 1.330 to 1.240 from recipe's 1 to 3 .

Page 118 shows a good pick of the measurement. Pages 107 108 explain it.

[Frank400]

I've read that part of the book, but I just read something else on the internet (I know it's not necessarily true because it's on the internet) but I thought it deserved a few different opinions:

    "AIR VALVE ADJUSTMENT
A full 90 degreeing opening can inhibit fuel flow from the tubes.  By 90 degrees I'm talking about the rear portion of the flap, the front will go past 90 degrees on a correctlyset up carb.  The very best q-jets, Ram Air, Super Duty and Ho models were set so that the secondary flaps leading edge is 1.130" to the edge of the opening in the airhorn when fully open.  Going beyond this can cause a lean spot as it effects how well fuel is pulled from the tubes and the lower edge of the flap nearly blocks the openings".

    That was taken from this webpage:http://www.mako.com.au/buick/html/qjet_tips.htm

[Cliff Ruggles]

No need to go any further than 1.240" as shown in the book for any reason.  I have the opening angle set on my own carb at 1.315", and it supports nearly 600hp. 

I see folks grinding the stop clear down to bring the lower portion of the flaps at or past 90 degrees to supply engines that don't make 300hp.  Less here is better than too much, when it comes to full open angle of the air flaps......Cliff

[Schurkey]

Aside from ultimate air flow through the carb, is there any benefit to having the secondary throttles under- or over-center so as to direct mixture forward or backward in the plenum?

I'm thinking that if a carb mounting pad is offset forward or backward on the engine--or has unequal-flowing runners--directing airflow/fuel flow with the throttle blades MIGHT be of some value for power, even if it mildly decreased CFM.

[Cliff Ruggles]

You pretty much have to tune on the dyno or at the track to find the perfect open angle for your particular set-up.

I've also heard that allowing the huge throttle plates to go over or under center helps on some set-ups as well, but I've never found any power there in all the testing I've done.....Cliff

[garybohannon]

Some Buick guys say their stock and Edelbrock dual plane intakes run lean on #1 cyl. and a little over center helps feed fuel that direction. A 5 to 7 degree forward pitch probably hurts nothing since qjets dump more in back than front.
Some also Buick guys "stagger jet" the driver side primary up 1 or 2 jet sizes to help that #1 problem.
Never found any dyno or detailed documented proof.
However, on a single plane intake, distribution problems are solved with "turtles" or porting.

[garybohannon]

My above comments referred to the Throttle Plates, not the air valve.
Air valves... A Pontiac guy said that an air valve gives "diminishing returns" if opened more than 80 degrees. Straight down or "fully open" is not recommended. May obstruct fuel nozzles and cause serious lean out at upper rpm's.
Cliffs 1.315" setting should be good enough. Stop there.