Re: Front Hemi Carb
[Re: Sniper]
#2697616
09/14/19 09:51 PM
09/14/19 09:51 PM
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Joined: Jan 2003
Posts: 42,714 Spokane Washington
ScottSmith_Harms
Mr Wizzard
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Mr Wizzard
Joined: Jan 2003
Posts: 42,714
Spokane Washington
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An initial setting might be available but it really needs to be tuned to the combo, which would be unique to your setup.
Dual quad tuning This! The mixture screws on the secondary (front) carburetors are there to provide a little fuel to the forward cylinders to keep them from going lean at idle, this is why they were factory set and plugged off as an anti-tamper measure. Once set they won't usually vibrate out unless someone has modified or damaged the screws or holes making them overly loose.
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Re: Front Hemi Carb
[Re: 3_RTs]
#2697758
09/15/19 01:07 PM
09/15/19 01:07 PM
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Joined: Jan 2003
Posts: 9,436 Blair County,PA
62maxwgn
master
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master
Joined: Jan 2003
Posts: 9,436
Blair County,PA
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Thanks to both of you for your input. The mixture screws on the front carb turn way too easy. At some point in their life, I'm sure they have been adjusted and probably multiple times. I'll follow the write-up on tuning dual quads, and also I'll try to address the looseness issue so that the screws will stay put. Thanks for your help! Which screws are you actually referring to,the ones that go strait into the base that originally had plugs or the angled ones ?
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Re: Front Hemi Carb
[Re: ScottSmith_Harms]
#2698061
09/16/19 12:45 PM
09/16/19 12:45 PM
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Joined: Jun 2014
Posts: 1,381 VA
dragon slayer
pro stock
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pro stock
Joined: Jun 2014
Posts: 1,381
VA
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The Hemi AFB had angled mixture screws, not horizontal ones that later AVS had. Hidden screws only on the front starting in 68 with CAP pollution control.
Maybe semantics, but the screws are the idle mixture screws for getting air fuel ratio at idle and off idle correct for minimum emissions. And while yes the adjustment will effect idle, it is only to a point. Your really shooting for the correct air fuel ratio. Once you have both carbs dialed for idle mixture, you want to set the "idle screw" on the front carb so it is contributing 50 or so RPM to your idle set point. This is the way you get the primary blades of the front carb open some to contribute fuel and air to help prevent starving the front cylinders. Having the "idle mixture screws" correct but blades closed really won't contribute to helping get fuel to the front cylinders.
The 66-67 carbs did not have hidden screws, so both could be adjusted. But starting with 67 CAP only cars, and 68-69 is when the front went hidden and the rear had a lock screw to prevent over adjustment. 70 they went to the plastic cap on the mixture screw to limit adjustment. Must have been a Chrysler method for limiting, as other manufacturer Carter Carbs did not adopt this method. It only had a short run with Chrysler too.
As far as the screws, if they are long enough, you can use a Dremel blade and cut the slot longer. Then spread it some. This will act as a a self locking method to tighten up the idle screws. Or you can use the 30A-80 idle mixture screw which the 66-67 hemi used (and other period AFBs) and is similar to the hidden screw on later carbs. Even the AVS idle mixture screw version is very close. They are #10-32 screws at least for the 66-69 front. I have not actually seen a 70 screw to know for sure, but I assume it is probably the same.
What is your carb Number?
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Re: Front Hemi Carb
[Re: ScottSmith_Harms]
#2698408
09/17/19 11:30 AM
09/17/19 11:30 AM
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Joined: Jun 2014
Posts: 1,381 VA
dragon slayer
pro stock
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pro stock
Joined: Jun 2014
Posts: 1,381
VA
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Scott, I am not Dick Katter. Just a hobbyist.
Bill, What number where those carbs? I did not think 70 carbs had locking idle mixture screws. I have never seen a front hemi with locked idle mixture screw?
Here are some pictures. From what I have seen, FRONT carbs do not have locked idle mixture screws; just hidden starting with 67CAP, but primarily 68 and up Hemi AFBs. 67 CAP AFB, and 68-69 REAR carbs had locked idle mixture screws. I did not think 70-71 carbs did.
The carb on left is 66 4139 Front but C8 date code, this was over counter carb. One on right is 69 4619 with F8 date code. Production carb.
The idle mixture screws are from Left or top the 66 non hidden 30A-80, next is the hidden 69 Front, for comparison one I made from a bent AVS screw 30A-199 or 200 (don't remember which), next is a brass hidden screw from a 68 AVS and last is the hour shaped original REAR 4620 69 hemi screw that is locked with a hex head screw from the side. They are 1/4"-36 thread. While all the others are #10-32 thread.
Picture of the hex lock screw on an original 4620 carb dated J8.
The hidden lock and hidden idle mixture screws would have a lead ball pushed in to close the hole. After adjustment at the factory.
I would like to see a picture of an original 70 and 71 idle mixture screw out of the carb if any one has one?
Last edited by dragon slayer; 09/17/19 01:58 PM.
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Re: Front Hemi Carb
[Re: dragon slayer]
#2698477
09/17/19 02:46 PM
09/17/19 02:46 PM
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Joined: Feb 2003
Posts: 197 Missouri
3_RTs
OP
member
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OP
member
Joined: Feb 2003
Posts: 197
Missouri
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My front carb has the small mixture screws like the ones in dragonslayer's pictures. They turn too easy. That's why I think they may have moved out of adjustment. My front carb is #4742, and it never had the plugs to hide the small mixture screws. I started adjusting the front/rear carbs Air/fuel mixture by turning all four screws out 2 full turns, as stated in the dual four carb setup article mentioned earlier. I hooked up my vacuum gauge to the rear carb to show me the highest vacuum setting, 2 turns is very close to giving me the most vacuum while idling at 900 rpm's. I didn't see where I could hook up the vacuum gauge to the front carb, so I adjusted it to what sounded like the smoothest and best idle. What I have left to do is to make sure that I have 50-100 rpm's from the front carb and then readjust the rear carb idle.
Meanwhile, back to the small front idle screws, I think I will try to find regular mixture screws and use springs on the screws to offer some resistance against movement, at least for the time being.
By the way, I purchased the carbs 17 - 18 years ago from R. D. Katter. The front carb has never had plugs to cover the front mixture screws. Once I know the adjustments are correct, I would like to cover the mixture screws like they are supposed to be, if I can find the correct plugs.
Last edited by 3_RTs; 09/17/19 02:49 PM. Reason: Left out a sentence.
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Re: Front Hemi Carb
[Re: 3_RTs]
#2698523
09/17/19 06:06 PM
09/17/19 06:06 PM
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Joined: Jan 2003
Posts: 9,436 Blair County,PA
62maxwgn
master
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master
Joined: Jan 2003
Posts: 9,436
Blair County,PA
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My front carb has the small mixture screws like the ones in dragonslayer's pictures. They turn too easy. That's why I think they may have moved out of adjustment. My front carb is #4742, and it never had the plugs to hide the small mixture screws. I started adjusting the front/rear carbs Air/fuel mixture by turning all four screws out 2 full turns, as stated in the dual four carb setup article mentioned earlier. I hooked up my vacuum gauge to the rear carb to show me the highest vacuum setting, 2 turns is very close to giving me the most vacuum while idling at 900 rpm's. I didn't see where I could hook up the vacuum gauge to the front carb, so I adjusted it to what sounded like the smoothest and best idle. What I have left to do is to make sure that I have 50-100 rpm's from the front carb and then readjust the rear carb idle.
Meanwhile, back to the small front idle screws, I think I will try to find regular mixture screws and use springs on the screws to offer some resistance against movement, at least for the time being.
By the way, I purchased the carbs 17 - 18 years ago from R. D. Katter. The front carb has never had plugs to cover the front mixture screws. Once I know the adjustments are correct, I would like to cover the mixture screws like they are supposed to be,if I can find the correct plugs
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Re: Front Hemi Carb
[Re: 62maxwgn]
#2698525
09/17/19 06:08 PM
09/17/19 06:08 PM
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Joined: Jan 2003
Posts: 9,436 Blair County,PA
62maxwgn
master
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master
Joined: Jan 2003
Posts: 9,436
Blair County,PA
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My front carb has the small mixture screws like the ones in dragonslayer's pictures. They turn too easy. That's why I think they may have moved out of adjustment. My front carb is #4742, and it never had the plugs to hide the small mixture screws. I started adjusting the front/rear carbs Air/fuel mixture by turning all four screws out 2 full turns, as stated in the dual four carb setup article mentioned earlier. I hooked up my vacuum gauge to the rear carb to show me the highest vacuum setting, 2 turns is very close to giving me the most vacuum while idling at 900 rpm's. I didn't see where I could hook up the vacuum gauge to the front carb, so I adjusted it to what sounded like the smoothest and best idle. What I have left to do is to make sure that I have 50-100 rpm's from the front carb and then readjust the rear carb idle.
Meanwhile, back to the small front idle screws, I think I will try to find regular mixture screws and use springs on the screws to offer some resistance against movement, at least for the time being.
By the way, I purchased the carbs 17 - 18 years ago from R. D. Katter. The front carb has never had plugs to cover the front mixture screws. Once I know the adjustments are correct, I would like to cover the mixture screws like they are supposed to be,if I can find the correct plugs Know where there may be a few !!
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Re: Front Hemi Carb
[Re: Sniper]
#2698919
09/19/19 10:03 AM
09/19/19 10:03 AM
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Joined: Feb 2011
Posts: 211 Halifax, VA.
moparjack44
enthusiast
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enthusiast
Joined: Feb 2011
Posts: 211
Halifax, VA.
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An initial setting might be available but it really needs to be tuned to the combo, which would be unique to your setup. Dual quad tuning Great, simple info. 3 fat thumbs up. >.
K.I.S.S.
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