four corner idle carbs

[flyinlow]

What is the reason for a four corner idle system?

[frederick]

A 4-corner idle system feeds the rear cylinders a little better, allowing a more even distribution.

[Manifold]

The four corner system allows for finer adjustment of the idle circuit. All Holley carbs have a secondary idle circuit. If there are no adjustment screws in the secondary metering block, then it's a fixed idle circuit. If there are adjustment screws, then it's on you to set it up correctly.

[firefighter3931]

Yep, cleaner idle due to adjustability on the secondary venturies. 

Often with big cam profiles and only idle adjustment on the primary venturies the front throttle blades have to be opened up too much to achieve any semblance of an idle. This uncovers the transfer slot and renders the idle circuit useless as the engine is now idleing off the main circuit....and it will idle waaay rich.


Ron

[flyinlow]

So all four blades are slightly open at idle with four mixture screws. As you adjust the idle speed screw ,do all four blades change ?  Are the secondarys still staged if they are mechanical?

Thanks, Craig

[greenpigs]

What about the base plate? If you have a carb that did not have the 4 corner idle originally and add a secondary metering block, will it work properly with the original base plate?

[firefighter3931]

So all four blades are slightly open at idle with four mixture screws. As you adjust the idle speed screw ,do all four blades change ?  Are the secondarys still staged if they are mechanical?

Thanks, Craig

Craig, the mixture screws just adjust for air into the metering block....they do not open/close the blades. That is done with the idle speed screw. On the mechanical secondary carbs with 4 corner idle there is a small set screw that is used to adjust the venturies (rear) into an open/close position but there is not as much movement as with the primary idle speed adjustment. Just enough to render the idle mixture circuit functional on those venturies. 

What about the base plate? If you have a carb that did not have the 4 corner idle originally and add a secondary metering block, will it work properly with the original base plate?

GP, You're better off to start with a carb that originally had 4 corner idle when considering a ProForm main body upgrade. I believe the 2 corner idle baseplate can be modified to accomodate this upgrade. 



Ron

[Manifold]

So all four blades are slightly open at idle with four mixture screws. As you adjust the idle speed screw ,do all four blades change ?  Are the secondarys still staged if they are mechanical?

Thanks, Craig

Craig, the mixture screws just adjust for air into the metering block....they do not open/close the blades. That is done with the idle speed screw. On the mechanical secondary carbs with 4 corner idle there is a small set screw that is used to adjust the venturies (rear) into an open/close position but there is not as much movement as with the primary idle speed adjustment. Just enough to render the idle mixture circuit functional on those venturies. 

I'm digging the concept of this secondary throttle shaft adjusting screw from QFT for double pumper carbs. http://www.summitracing.com/parts/QFT-63-2/ They're much more affordable than the Holley version: http://www.summitracing.com/parts/HLY-26-137/ I wish such an animal existed for the VS carbs. I got a used Holley SA which had been "adjusted" and had to guess at where the secondaries needed to be set - I didn't guess right and needed to open them more while it was mounted on the car. It was a pain!