Dual Quad 428 CJ

[Khobra]

Originally Posted by DanEC

Jerry's advise is generally the text book guide to selecting power valves. Not sure what happens if a lower range one is used than needed - possibly a rich condition.

Are you carbs sensitive to idle mixture adjustments and the primary throttle blades resting around the middle area of the little idle transition slots in the bores? Or are they resting at the top of the slots at idle?

And I assume you are tuning the idle mixture and speed on both carbs equally?

Normally I would think you are just fighting the characteristics of a fairly big cam if those duration figures are at .050 inches lift, but if you can pull 12 inches of vacuum at idle I think you should be able to get an outcome that isn't obviously rich. But even my 231/236 @ .050 cam with my dual quads is not something I let idle inside my garage for long.

Dan and Undy,
Thanks for your input. Yes, idle mixture screws are somewhat sensitive but not like they should be. The throttle plates are nearly closed and yes, both carbs adjusted equally. Spoke with a very knowledgeable individual at the race track today and I think he is on the right track. I need to increase the size of the idle air bleed restrictions. This will be my next step. I will drill out the current restriction and tap and install a screw in type.

Undy, did you have to play with this on your carbs?

My linkages are bang on, the power valves are 6.5. In the past, on other applications, I have drilled small holes in the primary throttle plates which did help a huge amount but I don't want to go that direction in this case if I don't have to. I would rather take the air bleed restriction route.
Hopefully I can do this sometime this coming week.
Will keep you posted.
BTW undy, what is your initial and total advance that you're running on your engine? I'm at 16 initial and 36 deg. total.

[DanEC]

Originally Posted by Khobra

Dan and Undy,
Thanks for your input. Yes, idle mixture screws are somewhat sensitive but not like they should be. The throttle plates are nearly closed and yes, both carbs adjusted equally.

I have drilled small holes in the primary throttle plates which did help a huge amount but I don't want to go that direction in this case if I don't have to. I would rather take the air bleed restriction route.

The following may be a wild goose hunt as possibly I'm taking your comment out of context, but if your throttle plates are nearly closed you don't need to drill holes in your primary plates. Usually that was done on engines with a big cam and low vacuum because the primary throttle plate had to be opened excessively to get the engine to idle and they end up above the idle transfer slots in the bores - basically taking the idle mixture adjustment circuit out of commission.

You seem to have the opposite condition for some reason (throttle plate possible closed too far). Also, although I have drilled the throttle plate on one Holley before - it was before I discovered that most if not all 4-bbl Holleys have an adjustment screw on the secondary throttle plates. You have to pretty much remove the carb to access it because it's on the underside of the base plate. It's there for two reasons - first, to slightly crack the throttle blades open so they don't become wedged in the bores and won't open, and two, on engines with big cams and low vacuum the secondary throttle plates can be opened slightly more to pass more air at idle, allowing the primary throttle blades to be closed to where they rest in the middle of the idle transfer slots and the idle mixture screws will again function.

If you have a pretty healthy cam and your primary plates in the carbs are excessively closed you might experiment with increasing your idle speed or pulling the carbs and see if the rear secondary plates need to be closed slightly.

[Khobra]

Originally Posted by DanEC

The following may be a wild goose hunt as possibly I'm taking your comment out of context, but if your throttle plates are nearly closed you don't need to drill holes in your primary plates. Usually that was done on engines with a big cam and low vacuum because the primary throttle plate had to be opened excessively to get the engine to idle and they end up above the idle transfer slots in the bores - basically taking the idle mixture adjustment circuit out of commission.

You seem to have the opposite condition for some reason (throttle plate possible closed too far). Also, although I have drilled the throttle plate on one Holley before - it was before I discovered that most if not all 4-bbl Holleys have an adjustment screw on the secondary throttle plates. You have to pretty much remove the carb to access it because it's on the underside of the base plate. It's there for two reasons - first, to slightly crack the throttle blades open so they don't become wedged in the bores and won't open, and two, on engines with big cams and low vacuum the secondary throttle plates can be opened slightly more to pass more air at idle, allowing the primary throttle blades to be closed to where they rest in the middle of the idle transfer slots and the idle mixture screws will again function.

If you have a pretty healthy cam and your primary plates in the carbs are excessively closed you might experiment with increasing your idle speed or pulling the carbs and see if the rear secondary plates need to be closed slightly.

Dan,

I will start from scatch to make sure that all your points have been addressed. I may have made some mistakes. Will update you when I have conclusive results'

[undy]

Originally Posted by Khobra

Dan and Undy,
Thanks for your input. Yes, idle mixture screws are somewhat sensitive but not like they should be. The throttle plates are nearly closed and yes, both carbs adjusted equally. Spoke with a very knowledgeable individual at the race track today and I think he is on the right track. I need to increase the size of the idle air bleed restrictions. This will be my next step. I will drill out the current restriction and tap and install a screw in type.

Undy, did you have to play with this on your carbs?

My linkages are bang on, the power valves are 6.5. In the past, on other applications, I have drilled small holes in the primary throttle plates which did help a huge amount but I don't want to go that direction in this case if I don't have to. I would rather take the air bleed restriction route.
Hopefully I can do this sometime this coming week.
Will keep you posted.
BTW undy, what is your initial and total advance that you're running on your engine? I'm at 16 initial and 36 deg. total.

No, I gave QuickFuel all my engine specs; max RPM, displacement, cam profile and target HP. They custom built the pair. They were perfect as delivered.

16 degrees initial with 38 total.