Carburetor idle air bleeds

[astikhossw]

Do any of you guy's know if the idle air bleeds on the secondary side perform a bleed function on a vacuum secondary carb.,The reason that i need to know is that i am going try out my QF 780 carb. next week and it is idling to the point of stalling (lean)and i had no control of the idle mixture screws,so i notice that the air bleeds on the secondary side were smaller than the primary side so i swapped them over and now i have some control of the mixture screws and it idles alot better without stalling.

[Tom Goldman]

The secondary idle circuit pulls a small amount of fuel at idle but nowhere near the amount of the primaries.
I would call Quickfuel's tech dept. ,they are really good guys to deal with , and see if they recommend going larger on the primary idle feed restriction to allow more fuel in the idle circuit ,along with the smaller air bleeds.
You may need more fuel than just going smaller on the air bleeds can provide, especially on the off idle transition to full throttle.

[astikhossw]

Thanks Tom,I'll do just that.

[jmarkaudio]

If it's a 4 corner idle carb the idle circuits should be mirrored including bleeds. If the secondary has a metering plate or is a 2 corner idle it won't. The SSR 780's I had through my hands were 4 corner idle.

[Tom Goldman]

If its the approved S/SS NHRA replacement carb , I believe it can only have the primary idle circuit adjustable , and has a non adjustable secondary idle circuit .
Not 100% sure on that , but pretty certain.

[jmarkaudio]

As delivered the secondary is adjustable with short mixture screws, they must be plugged once in competition. They supply lead plugs, much like the emission carbs had the idle screws plugged off. Not real sure why, it offers no real advantage having 4 corner idle.

[Jeff Lee]

Originally Posted by jmarkaudio

As delivered the secondary is adjustable with short mixture screws, they must be plugged once in competition. They supply lead plugs, much like the emission carbs had the idle screws plugged off. Not real sure why, it offers no real advantage having 4 corner idle.

That is correct. I wonder what thought process went into "we'll give them this Holley replacement but hinder them by making the secondary mixture screws adjustable just once"
I guess they wanted to make sure your not driving through the pits at a proper a/f mix if you drive different elevation tracks?

[astikhossw]

Jeff did you have any idle issues with your QF carb.if you did how did you cure the problem, i've seen your dyno video.By the way are you going to make the division race next week.

[Tom Goldman]

Originally Posted by jmarkaudio

As delivered the secondary is adjustable with short mixture screws, they must be plugged once in competition. They supply lead plugs, much like the emission carbs had the idle screws plugged off. Not real sure why, it offers no real advantage having 4 corner idle.

We know that , but this is NHRA were talking about .
Their reasoning is the OE didn't have it so neither can you .

[Jeff Lee]

Originally Posted by astikhossw

Jeff did you have any idle issues with your QF carb.if you did how did you cure the problem, i've seen your dyno video.By the way are you going to make the division race next week.

The engine was never much below 1500 on the dyno so I don't know what it will be like around 900 or so RPM. Finally installing engine today so we can get final pinion angle factored in and final weld the axle tubes to perches. Spent over a day completely redesigning / machining all parts related to the Browell TO bearing collar to get all parts inside the bell working right. All that stuff ads up to a lot of work!
I will be at Vegas this week. Hopefully Wednesday for TNT