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Oh, good grief. I took the carb off, got my throttle adjusted to show the the transfer slots correctly. Switched my vacuum advance to the manifold vacuum (it was on ported vacuum), got it all put back together and I was going to try retuning it tomorrow—now this monkey wrench. Other than the reversed mixture screws, does this make any difference to all the advice I've been absorbing? But thanks, it HAS to help knowing which carb I actually have.
Well I learned something new, thanks guys. Never new the emissions carbs mixture screws worked that way. When I built my carbs recently I was studying all of Holley's vac secondary spring offerings and could not for the life of me understand WTH they ever bothered to even offer the black spring for. Now it makes sense, they never intended to ever fully open secondaries on black spring carbs, EVEN on a BBC.
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Discussion Starter · #42 ·
No difference in tuning. Just reverse turning of the mixture screws. If your motor is very far from stock this may be the wrong carb for you but I kind of doubt it. Happy tuning !!
Thanks, but in terms of the idle screws, after watching video after video, I was instructed (on a carb I didn't actually have) to start with them 1 - 1 1/2 turns out from seated and gradually work them back inwards. Where do I start with these backwards screws? All the way in?

47 years I've been tuning this car wrongly.
 

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Little is known about tuning these, and yours has been gone through which means it's difficult to say what someone did to it internally. Since you feel it's running rich (usually it's hard to get one of these rich at idle), someone could have modified the air bleeds already.

Turning the screws will be adjusting the air amount rather than fuel. But fully seated (least amount of air in this case) is still clockwise, so that should be your starting reference point. Since you didn't notice a difference from 1 1/2 turns out to fully clockwise, I would still start at the 1 1/2 turns out from seated, and then keep going counterclockwise (thus leaning the mixture) in increments until you reach highest vacuum reading.
 

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Thanks, but in terms of the idle screws, after watching video after video, I was instructed (on a carb I didn't actually have) to start with them 1 - 1 1/2 turns out from seated and gradually work them back inwards. Where do I start with these backwards screws? All the way in?

47 years I've been tuning this car wrongly.
You turn the screws which ever way it runs better. If you turn them in and it runs worse go the other way lol.
 

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Thanks, but in terms of the idle screws, after watching video after video, I was instructed (on a carb I didn't actually have) to start with them 1 - 1 1/2 turns out from seated and gradually work them back inwards. Where do I start with these backwards screws? All the way in?

47 years I've been tuning this car wrongly.
This carb would have originally had limiter caps on the mixture screws. That was typical of all emissions carbs. Here is Holley's instructions for one of their current emissions carb. I don't know if it's exact for your carb but it makes sense to me as a starting point.
Link to the full manual :

Copy & paste from the manual :
GENERAL INFORMATION: Idle Mixture Adjustment: The idle mixture of your new Holley carburetor has been pre-set at the factory to give the best idle while meeting emission requirements. It should not be necessary to adjust the idle mixture screws beyond the stops of the plastic limiter caps. This carburetor is equipped with a “reverse” idle system. Screwing the idle adjustment needles IN, “RICHENS” the mixture. Backing the screws OUT, “LEANS” the mixture. This system has two peculiar characteristics: A. It is frequently impossible to “kill” an engine by leaning the mixture. Likewise, it is often difficult to make an engine “lope” by richening the mixture. B. It is possible to adjust the idle mixture within the range of the limiter caps. The mixture needles affect the delivered fuel mixture up to approximately 45 mph. Fuel economy for low speed can be improved in most, but not all cases, by a leaner adjustment of the needle. Trial and error is the best approach. CAUTION! Adjusting the mixture screws beyond the rich stop of the limiter cap will cause the engine to emit illegal amounts of air polluntants. Adjusting them beyond the lean stops may result in engine damage due to lean misfire. If the limiter caps are removed for any reason, the idle mixture is best set with an exhaust gas analyzer, available in most dealer garages and many independent garages. The analyzer should read .2-.6% CO using the following steps: A. With the engine at operating temperature, set the idle speed to the manufacturer’s specifications. B. Connect the exhaust gas analyzer. For vehicles equipped with a catalytic converter, the analyzer probe must be upstream of the catalyst. C. Turn both idle mixture screws clockwise (rich) until they lightly seat (See Fig. 1). Reset the idle to correct speed. D. Equally turn both screws counter-clockwise (lean) in 1/4 turn increments. Reset the idle to correct speed. E. Note the exhaust gas analyzer reading. Repeat Step D until the reading is .2-.6% CO.
 

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Discussion Starter · #46 ·
Little is known about tuning these, and yours has been gone through which means it's difficult to say what someone did to it internally. Since you feel it's running rich (usually it's hard to get one of these rich at idle), someone could have modified the air bleeds already.

Turning the screws will be adjusting the air amount rather than fuel. But fully seated (least amount of air in this case) is still clockwise, so that should be your starting reference point. Since you didn't notice a difference from 1 1/2 turns out to fully clockwise, I would still start at the 1 1/2 turns out from seated, and then keep going counterclockwise (thus leaning the mixture) in increments until you reach highest vacuum reading.
Well, I've done everything I can without opening the carb up. I started at 1 1/2 turn as you suggested, but the very best result pulls almost 17" (16.8 ish) very steady vacuum at ~600 steady rpm, in drive — at a mere 3/4 of a turn out from seated. ( It drives really well and accelerates smartly but the idle is a bit jittery when sitting at a stop sign.) The timing is unknown since I only have the stock timing tab, but with the vacuum advance hooked to manifold vacuum it's advanced about 2x the amount the tab would indicate. That would be, what, about 34º BTC? Retarding the spark makes the engine stumble.

I read the attached article about the plastic adjustment limiter caps on the screws. Those went missing about 40 years ago.

Turning the idle screws any further out, the vacuum starts dropping and it runs gradually rougher. Much further out (only 1 1/2 to 2 turns out) and it won't run at all. Strangely, it idles very well at 600 rpms when fully warmed up (hot) just a bit cooler than that (after sitting for an hour) and it idles about 100 rpm slower and a bit rougher until it gets hot again.

I'm beginning to think getting a new (factory refurbished) carb might be a simpler but more expensive fix—unless you guys think the PV could still be an issue or some other internal tweak might be the ticket. (Could the 103k/mile distributor be an issue?)

Thanks for all your help, I really, really appreciate it, but I'm beginning to feel like a pest. I know enough to keep my baby going for 47 years, but I think I'm reaching my diagnostic limit here.
 

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I simply bring my carbs down the street to have them rebuilt if I don't have time. Two generations of experience he has. Never had an issue. If any tuning is needed after that he does it for you. Done dozens of carbs for me and many for a friend who owns over 50 cars. His wife is also gearhead. Everyone around here brings their carbs to him. Kurt at Carbs r Us in Thomaston CT. Dual quads TRI powers. They all run mint.
 

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I simply bring my carbs down the street to have them rebuilt if I don't have time. Two generations of experience he has. Never had an issue. If any tuning is needed after that he does it for you. Done dozens of carbs for me and many for a friend who owns over 50 cars. His wife is also gearhead. Everyone around here brings their carbs to him. Kurt at Carbs r Us in Thomaston CT. Dual quads TRI powers. They all run mint.
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Well, I've done everything I can without opening the carb up. I started at 1 1/2 turn as you suggested, but the very best result pulls almost 17" (16.8 ish) very steady vacuum at ~600 steady rpm, in drive — at a mere 3/4 of a turn out from seated. ( It drives really well and accelerates smartly but the idle is a bit jittery when sitting at a stop sign.) The timing is unknown since I only have the stock timing tab, but with the vacuum advance hooked to manifold vacuum it's advanced about 2x the amount the tab would indicate. That would be, what, about 34º BTC? Retarding the spark makes the engine stumble.

I read the attached article about the plastic adjustment limiter caps on the screws. Those went missing about 40 years ago.

Turning the idle screws any further out, the vacuum starts dropping and it runs gradually rougher. Much further out (only 1 1/2 to 2 turns out) and it won't run at all. Strangely, it idles very well at 600 rpms when fully warmed up (hot) just a bit cooler than that (after sitting for an hour) and it idles about 100 rpm slower and a bit rougher until it gets hot again.

I'm beginning to think getting a new (factory refurbished) carb might be a simpler but more expensive fix—unless you guys think the PV could still be an issue or some other internal tweak might be the ticket. (Could the 103k/mile distributor be an issue?)

Thanks for all your help, I really, really appreciate it, but I'm beginning to feel like a pest. I know enough to keep my baby going for 47 years, but I think I'm reaching my diagnostic limit here.
Ken we have all been where you are at right now, it is frustrating for an onwer, especially someone who hss had the car nearly 5 decades and hand their hands on every part over that time. Take a minute to celebrate that, you've been through a ton over the years with this car, amazing. Just got thrown a curveball with this one, but with everyones help I certainly learned some things on this. Most of us are anxious to see this playout. Throw a power valve at it and let us know results.

A new or recertified/refurbished Holley 600 off the Holley web site is about $230 and 1 year warranty. Nobody would blame you though for ditching the emissions carb.
 

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I simply bring my carbs down the street to have them rebuilt if I don't have time. Two generations of experience he has. Never had an issue. If any tuning is needed after that he does it for you. Done dozens of carbs for me and many for a friend who owns over 50 cars. His wife is also gearhead. Everyone around here brings their carbs to him. Kurt at Carbs r Us in Thomaston CT. Dual quads TRI powers. They all run mint.
I rebuild my own since I was 14 or take them to a local shop that's been doing them for 40 years here in Vegas.

How does your post or mine help the OP?

The OP has been researching, learning and, in fact, educating us all a little. I would have tossed in the towel long ago and bought anew one. Lol
 

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I rebuild my own since I was 14 or take them to a local shop that's been doing them for 40 years here in Vegas.

How does your post or mine help the OP?

The OP has been researching, learning and, in fact, educating us all a little. I would have tossed in the towel long ago and bought anew one. Lol
Yep, never stop learning, this is why we build things.
 

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On the air horn:
LIST-6619-1
2446

On the primary metering block:
7492
66191
Ken attached pics from Holley book for your carb. Some of the p/n are unusual propbably due to the emissions carb had some unique parts. The secondarymetering plate isnt listed. Your model is a 4160 style that doesnt have a full metering block or jets as we are used to them in 4150 carbs. The metering plate fills the function of the jets. The suffix, I think yours is a 39, tell you what size metering feeds it has, but again its has an odd prefix so not sure if it correlates to a 71 jet size like a standard prefix-39 would. The blue line is your carb.
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Discussion Starter · #52 ·
Ken attached pics from Holley book for your carb. Some of the p/n are unusual propbably due to the emissions carb had some unique parts. The secondarymetering plate isnt listed. Your model is a 4160 style that doesnt have a full metering block or jets as we are used to them in 4150 carbs. The metering plate fills the function of the jets. The suffix, I think yours is a 39, tell you what size metering feeds it has, but again its has an odd prefix so not sure if it correlates to a 71 jet size like a standard prefix-39 would. The blue line is your carb. View attachment 290420
View attachment 290419
Thank you! That's an amazing archive of info. Just when I was beginning to understand carb engineering and tuning—a little—and beginning to formulate a strategy to get it working correctly I find MY carb is from another planet and works very differently. Almost nothing you guys taught me or learned from tuners on video applies. Does it even HAVE a power valve, or jets? Who knows?


Ken we have all been where you are at right now, it is frustrating for an onwer, especially someone who hss had the car nearly 5 decades and hand their hands on every part over that time. Take a minute to celebrate that, you've been through a ton over the years with this car, amazing. Just got thrown a curveball with this one, but with everyones help I certainly learned some things on this. Most of us are anxious to see this playout. Throw a power valve at it and let us know results.

A new or recertified/refurbished Holley 600 off the Holley web site is about $230 and 1 year warranty. Nobody would blame you though for ditching the emissions carb.
 

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Discussion Starter · #53 ·
Thank you! That's an amazing archive of info. Just when I was beginning to understand carb engineering and tuning—a little—and beginning to formulate a strategy to get it working correctly I find MY carb is from another planet and works very differently. Almost nothing you guys taught me or learned from tuners on video applies. Does it even HAVE a power valve, or jets? Who knows?
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Thanks. It runs pretty good right now (Still smells a little rich) so I can let sleeping dogs lie if I want… but the nagging question is: I've gone this far, should I keep experimenting just for grins to see what happens? Or go ahead and buy a new carb that I can understand and see if the performance has been "throttled back" all these years, so-to-speak? After all, the secondaries have NEVER opened all the way with that black spring in there. (Think of all the tickets I could have gotten?) I could change the PV, and maybe the secondary spring and see what happens. It would be a lot cheaper. Decisions, decisions…
 

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Discussion Starter · #54 ·
Thank you! That's an amazing archive of info. Just when I was beginning to understand carb engineering and tuning—a little—and beginning to formulate a strategy to get it working correctly I find MY carb is from another planet and works very differently. Almost nothing you guys taught me or learned from tuners on video applies. Does it even HAVE a power valve, or jets? Who knows?
Thanks. It runs pretty good right now (Still smells a little rich) so I can let sleeping dogs lie if I want… but the nagging question is: I've gone this far, should I keep experimenting just for grins to see what happens? Or go ahead and buy a new carb that I can understand and see if the performance has been "throttled back" all these years, so-to-speak? After all, the secondaries have NEVER opened all the way with that black spring in there. (Think of all the tickets I could have gotten?) I could change the PV, and maybe the secondary spring and see what happens. It would be a lot cheaper. Decisions, decisions…
 

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Thank you! That's an amazing archive of info. Just when I was beginning to understand carb engineering and tuning—a little—and beginning to formulate a strategy to get it working correctly I find MY carb is from another planet and works very differently. Almost nothing you guys taught me or learned from tuners on video applies. Does it even HAVE a power valve, or jets? Who knows?
Think a lot of us learned things from this I never knew their was a reverse idle circuit carb. I think I read Holley saying all their carbs use power valves and yours may be a 2 stage? Again never messed with those. Also "always" and "never" are forever removed from my volcabulary. I should have banished those words long ago LOL.
 

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Think a lot of us learned things from this I never knew their was a reverse idle circuit carb. I think I read Holley saying all their catbs use power valves and yours may be a 2 stage? Again never messed with those. Also "always" and "never" are forever removed from my volcabulary. I should have banished those words long ago LOL.
“May, Should, mostly , Normally, usually,etc…”
Are “normally “ the correct answers on multiple choice tests.
are , will, always, etc…”
Normally = false.

test taking 101.
 

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Discussion Starter · #57 ·
OK. One more question for my personal gang of instructors: I have not given up on my quest to get this 600 cfm 4160 working even better by doing some internal surgery—it's become a bit of a challenge—but it has also been suggested to just go ahead and get a refurbished 4150 that we all understand better.

In my search for those, at the Holley web site, there are even several versions of those: 600 cfm Street Warriors, 670 cfm Street Avengers, regular old 650 cfm 4150s, and several sizes of refurbished ("Hall of Fame?") 4160's. (I'm wondering if those 4160's are the same design as my antique emissions-friendly 4160 or have they been upgraded somehow to be considered "Hall of Fame" worthy?

Most all of these are "not available" but they'll "let me know when they are available."

I can/will call Holley directly but do any of you have experience with these other versions of 4150's I'd never heard of? (good or bad?) Since some of those larger carbs are currently available, is a 650, 670 or 750 too much carb for my internally stock 350/300? They also offer refurbished Brawlers, Quick Fuel Slayers, and Demons. Any opinions on those? Thanks.
 

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A 570-680cfm would be fine for your application as long as it has vacuum secondaries (VS).

The Holley 4150s with VS have changeable jets with metering block on the secondary side, whereas the 4160 uses a "jet plate" in place of the metering block/jets. Although you can buy different jet plates to tune the secondary side, I prefer to get the 4150s to be able to change the jets. So one of those, or their other models that has a metered block.
(Note, because all mechanical secondary (MS) Holley's came as 4150s and many VS used the 4160 model, some people assume if it's a 4150 is automatically a mechanical, which is always true.

I'd get one with the quick-change spring design that doesn't disturb the spring seal by unscrewing the entire cap. Those can easily tear when removing and then you need to replace that as well.

I like downleg boosters over straight boosters for better signal. Annular boosters are a little bit better for atomization and signal that downleg, but I think only the Truck Avenger has annular.

Things like 4-corner idling, tuneablescrew-in air bleeds, or screw-in idle restriction are well beyond what you'll need to tune a mild 350 build and would just add complexity. Probably fine if they come that way, you just won't mess with those features (except the four-corner idling of course would still need adjustment).

Don't forget to select our preferred choke-type. Also, Summit site sells Holley refurbished carburetors as well. They may have more selection sitting in the warehouse.

Don't know much about the different QFT, Brawler, etc. QFT was started by former Holley employees (then was purchased back). Demon prior to Holley buying them, had some quality issues back before they went chapter 11 (great designs, but they needed to focus on the production quality). I imagine that got resolved when Holley bought them. Maybe check the reviews on Summit of the different models.
 

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Discussion Starter · #59 · (Edited)
A 570-680cfm would be fine for your application as long as it has vacuum secondaries (VS).

The Holley 4150s with VS have changeable jets with metering block on the secondary side, whereas the 4160 uses a "jet plate" in place of the metering block/jets. Although you can buy different jet plates to tune the secondary side, I prefer to get the 4150s to be able to change the jets. So one of those, or their other models that has a metered block.
(Note, because all mechanical secondary (MS) Holley's came as 4150s and many VS used the 4160 model, some people assume if it's a 4150 is automatically a mechanical, which is always true.

I'd get one with the quick-change spring design that doesn't disturb the spring seal by unscrewing the entire cap. Those can easily tear when removing and then you need to replace that as well.

I like downleg boosters over straight boosters for better signal. Annular boosters are a little bit better for atomization and signal that downleg, but I think only the Truck Avenger has annular.

Things like 4-corner idling, tuneablescrew-in air bleeds, or screw-in idle restriction are well beyond what you'll need to tune a mild 350 build and would just add complexity. Probably fine if they come that way, you just won't mess with those features (except the four-corner idling of course would still need adjustment).

Don't forget to select our preferred choke-type. Also, Summit site sells Holley refurbished carburetors as well. They may have more selection sitting in the warehouse.

Don't know much about the different QFT, Brawler, etc. QFT was started by former Holley employees (then was purchased back). Demon prior to Holley buying them, had some quality issues back before they went chapter 11 (great designs, but they needed to focus on the production quality). I imagine that got resolved when Holley bought them. Maybe check the reviews on Summit of the different models.
Hey, thanks for the well reasoned and detailed input Mike. I appreciate it. With all the criticism the 4160 emission-design has gotten on this board, and others, I wonder if the "newer" 4160's that are being inducted into the "Hot Rod Magazine Speed Parts Hall of Fame" have been redesigned to a more "modern"/traditional configuration eliminating the goofy stuff like the reverse idle screws?
 

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Discussion Starter · #60 ·
A 570-680cfm would be fine for your application as long as it has vacuum secondaries (VS).

The Holley 4150s with VS have changeable jets with metering block on the secondary side, whereas the 4160 uses a "jet plate" in place of the metering block/jets. Although you can buy different jet plates to tune the secondary side, I prefer to get the 4150s to be able to change the jets. So one of those, or their other models that has a metered block.
(Note, because all mechanical secondary (MS) Holley's came as 4150s and many VS used the 4160 model, some people assume if it's a 4150 is automatically a mechanical, which is always true.

I'd get one with the quick-change spring design that doesn't disturb the spring seal by unscrewing the entire cap. Those can easily tear when removing and then you need to replace that as well.

I like downleg boosters over straight boosters for better signal. Annular boosters are a little bit better for atomization and signal that downleg, but I think only the Truck Avenger has annular.

Things like 4-corner idling, tuneablescrew-in air bleeds, or screw-in idle restriction are well beyond what you'll need to tune a mild 350 build and would just add complexity. Probably fine if they come that way, you just won't mess with those features (except the four-corner idling of course would still need adjustment).

Don't forget to select our preferred choke-type. Also, Summit site sells Holley refurbished carburetors as well. They may have more selection sitting in the warehouse.

Don't know much about the different QFT, Brawler, etc. QFT was started by former Holley employees (then was purchased back). Demon prior to Holley buying them, had some quality issues back before they went chapter 11 (great designs, but they needed to focus on the production quality). I imagine that got resolved when Holley bought them. Maybe check the reviews on Summit of the different models.
One more question: I've seen replacement metering blocks that supposedly can convert a 4160 into a 4150-style carb with jets and everything. Worth a try?
 
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