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Since you had the same issue before carb was rebuilt verify your throttle linkage is adjusted properly and moves without any binding. Back the idle speed screw all the way off the stop and make sure the primary butterflies are fully closed. Also make sure secondary butterflies are fully closed. If secondaries are not fully closed they have a seperate stop screw you can adjust by taking the carb off intake and flipping it over. You will see it in the base plate.

Another way to verify engine is running on idle circuit only is with engine warmed up and choke off put a vacuum gage on the ported vacuum port. This is the port the distributor vacuum advance is advised by manufacturer to be connected (mfr advises, but many use full manifold port, so make sure it is the ported vacuum port you measure for this test). If your ported vacuum doesn't read zero the main circuit is active when it shouldn't be.

A few things can cause this but as already stated usually butterflies are open too far exposing the transfer slots in the base plate bores. Bad or damaged power valves can also cause this. If you had backfiring through intake/carb during your problems the power valve may have been damaged, particularly on older Holley carbs.
 

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Transfer slots become exposed when the idle speed screw is turned in too far causing the throttle plate to open more. Eventually the plates open far enough to expose the tranfer slots and you begin transitioning from the idle circuit to the main circuit and idle mixture screws are no longer able to be effective.

Floats set too high can also cause idle mixture screws to be ineffective. Check your floats are set correctly on your float bowls you should have brass float level sight plugs. Should be able to remove them and just see the fuel level at bottom of hole without it running out of the holes. Does your carb have 2 or 4 idle mixture screws? Some have "4 corner idle" as it is referred to when primary and secondary fuel blocks have idle mixture screws.

So linkage is good, check float levels, check vacuum at idle when you get your new gage, make sure your timing is set like Ben suggested, then check your idle vacuum full manifold port and ported vacuum ports. Disconnect and plug PCV vacuum hose and check if any effect.
 

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Pic shows an 830 annular booster Holley baseplate. You can see the transfer slots on all 4 bores with throttle plates open. You can also see the secondary throttle plate stop screw I mentioned in my original response. Small straight slot screw driver adjusts it after removing carb. This is an old NASCAR legal carb. All 4 throttle plates have small hole drilled to allow additional airflow at idle. Your carb, like most, probably has solid plates.
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• 1969 350/300 internally stock.103,000 miles
• Rebuilt 4150 Holley 600 cfm
• New points, condenser, coil, and cap
• Compression—all cylinders test 150-155 except one (#5) at 145
• Float height at primaries and secondaries adjusted to correct levels.
• No vacuum leaks detected
• DWELL 29º
• Timing 14º BTDC (I'm not 100% positive the distributor vacuum canister does anything. No change at idle with vacuum connected to canister or without. Is there a way to test this?)
• Idle 600 rpm in drive (engine will actually idle as low as 375 rpm—not well, but will run without dying.)
• Manifold vacuum steady 20 in park, steady 17 in drive while idling at 600 rpm.
• Ported vacuum 0 at idle
• Car idles smoothly, accelerates quickly without any lag or misfires, and pulls strong to the yellow line—best in years
• Exhaust still smells mildly rich, idle screw adjustment does nothing

I have not yet removed the carb, I wanted to do everything else, short of that, first. Using an isolated bright light shining down into the manifold thru the wide open carb secondaries I can see a sliver of light when primaries are slightly opened, but when tested, it appears that the primary butterflies are fully closed with the throttle at rest...but I'd like to try that test once more.

All that to say that everything appears correctly adjusted but STILL the the exhaust smells slightly rich and the idle mixture screws have absolutely no affect on idle or vacuum even when screwed all the way in.

One theory suggested is a damaged power valve, possibly from the previous backfires.

Any suggestions?
Main jets, according to Holley, have no effect on idle circuit. So I dont think thats your issue. Since its almost always too much transfer slot exposed double check that. Then try disconnecting PCV hose from carb base or whatever port its connected to and plug the port then see if mixture screws have any effect. Finally take your primary fuel bowl off and check your power valve, I would just repace it, probably a 6.5 is what you will find it to be. Holley stamps single number on flat of hex bade, 6 indicates a 6.5, 8 indicates 8.5, ect. Make sure you use the correct power valve gasket for the type valve you have. There are 2 styles, rectangular hole (referred to as picture window PV) uses standard round gasket. Small circular hole valve uses a circular with 3 standoffs designed on inside diameter. If valve is bad take carb off, turn upside down and if 1992 or newer carb make sure when built or rebuilt someone didnt remove the anti-blowout check ball. If missing you need to replace it to prevent damage to another new PV. If carb older than 92 Holley makes a kit toodify your base plate or buy a whole new baseplate with the blowout protection.
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If no damage, all okay. Someone may have drilled out the idle feed supplies in your metering block. Mixture screws dont set the mixture, they only adjust the volume of fuel. The metering block has 4 supplie feeds, 2 for main circuit, 2 for idle circuit. The main body has air bleeds seen through top. The idle air bleeds and metering block idle feed restriction actually establish idle mixture. Sometimes guys try drilling these out to correct situation where someone is trying to use a street carb on a race engine richening the idle mixture. If thats your case a new metering block will solve this.

A short cut would be a brand new or Holley reburbished carb, about 60% the cost of a new Holley. But you know what you have and Holley makes them run and look like new.
 

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Ken I am with you. Guy might have had good intentions but maybe he thought if taking a drill to restrictos or feeds helped his drag car it may work for you too. Never good idea to start drilling on a carb. The magazine used to be full of article like that in the 70s & 80s 😒

We did discuss the transfer slots in post #3 & some others and making sure you have no vacuum on the ported vacuum port. But I wasnt as clear as Alan to make sure not to use that port for distributor. I like his wording best, follow his advice use manifold vacuum, not the port on side of metering block, cap it off.

Mikes advice to pull the mixture screws out and inspect them for any damage is a good one too.

Regarding jets, what model # is on your airhorn? I can tell you what it came with originally. Most 600 vac secondary Holley had 65 in primary and 6.5 power valves.

Lots of good ideas from everyone!👍
 

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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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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 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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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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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?
Hi Ken. I was impressed with the 4160 #80457 street warrior. But if you want to use the factory Holley dual inlet lines yes you need a 4150. I have 2 warrior 80457 4160s heavily modified to 4150s, center hung float bowls, modified primary throttle shafts, chokes removed, QFT vs adjustable actuators, light yellow vs spring, special secondary float bowls that Holley never provided a part number for and say they werent intended for sale as a piece part and a number of other things. But I only did this to build my cross ram project. My only other option was spending 3k-5k a piece for two very unique factory carbs. So I found another way.

You can buy a refurbished 4160 and convert to 4150 with Holley kit 34-13, but this only gives you half of what you need.
If you really want a 4150 just buy it as a 4150. You know its right out of the box.

My project has not been fun and its not done yet. Car was meant to be driven and enjoyed not sit in a garage for 2 years. Instead I study engineering data and search for parts that dont exist, are simply not what vendor says they are, (heart who?), or never assigned a part number so cant be sold.

Spring for a refurbished 600 or 650 4150 you will not be sorry.
 

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Thanks! The more I looked into it the more I was/am leaning toward just getting a refurbed 4150. The reason I was even thinking of the custom alteration of my, or another, 4160 into a 4150 is because the refurbed 4150's are not very available on the Holley or Summit sites...and I don't trust the eBay offerings. When I saw that there was such thing as a 4160 to 4150 kit, I thought I'd see if anyone on here had done so—I got two opposite opinions. LOL. I appreciate your kind remarks and all your help.
Holley updates their refurbished carbs site regularly, so dont get discouraged, just keep checking for the carb you want.
 

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HA. Another mystery. Here is what I found when I started looking at converting my 4160 to a 4150-ish carb. THIS is the metering block I found that has the exact same number #7482 and also stamped 6619-1 for a reverse idle carb like that which is on my current carb. We thought mine had a "metering plate," but, unlike what we were thinking, this DOES have changeable jets and and power valve. I either need to take mine apart just for fun or find a 4150, which is looking like the best option right about now. smh. View attachment 290558
Ken the 4160 uses a metering plate on the secondary. 4160 secondary has no metering block that just has the float bowl attached directly to the main body. If you take the float bowl off the secondary side you will find the metering plate and since thete is no metering block there is no provision for a PV. Snap a shot of the metering plate when you find it and maybe I can find the listing for what jets it is equivalent to. 👍
 

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I found a refurbed 4150 (670cfm) on the Holley site. (Electric choke, VS, quick change secondary spring thing) It's only $335 or something. (Do you think that would be too big?) Since it runs a little rich right now—there's also a 570cfm for $314. (Too small?) I'm still tempted to experiment with the one I got, though, just for fun. I've read that Holley usually jets the primaries too large, the PV too low, and the secondary spring too stiff. However right now it runs best when the idle is adjusted almost as rich as it can be. So I'm wondering if a couple sizes smaller primary jets and it might not run at all. But I'm thinking adding a little higher number PV and a less-stiff VS spring just to see if it might run a little peppier? (Purple or orange?) That'd be cheaper than $335.
I would think that 670 would be a good carb for your 350. Throttle bores are the same as the 600 warrior and it worked great for me. Plus it has quick change VS spring cover, 4 corner idle mixture screws, electric choke which I really miss (simple hook up to your fuse block). Dont know what trans you currently have but if it is an auto 700R you could need another $70 worth of hardware to hook up the linkage according to Holley, which they have. No its not too big, ventury bores being 600 venturis, the purple VS spring installed will not fully open secondaries until 6950 rpm, which I dont think you are going to see. The yellow or yellow short spring will both fully open around 5700 if you so choose to try later. Plus Holley says you get the kit of VS optional springs with this carb if I read that correctly.

CFM estimated
Engine CID x max rpm/3456 x Ve = cfm potential

Ve is volumetric efficency. Holley considers most hi perf engine = 85%, full modified racing engines 95%, smog motor low perf 75%.

So 350 cid x 6000 rpm/ 3456 x .85 Ve = 516 cfm. I assumed 6K redline, 350 hyd flat tappet cam street motor. Remember the purple spring is never going to fully open second secondaries for another 950 rpm beyond that and doesnt start opening until 1915 ish. Assuming the secondary actuation is linear, and I dont know it is or not, at a 6K redline at WOT (wide open throttle) your secondaries will only 80% open. Which would mean in theory to me 20% of cfm capabilty is left on the table, so 670 x .80% = 536 cfm @6K & WOT w/ purple spring.

Maybe some others can chime in here to confirm or correct.

If this is correct 536 cfm is what you can expect to deliver and 516 cfm is what Holley formula feels is required for 350 w/6K redline & .85 Ve. So seems like a good fit.
 

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Wow. You know your stuff. You should charge for the "carb school" training I'm getting. With a 100k mile motor, (tho' I have good compression on all) I'll bet my motor is less than 85%, and I'm not going above 5500, (I don't need to blow this thing up at this point) and I'll go with a weaker spring as you suggest…so, if the formula at 6000 and .85 says 516 why not just go with the 570cfm? (The formula at 5500 and .80 ve gets me 445cfm.) I'm not disagreeing with you, just trying to learn. Thanks.
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I really like the 580 HR that ilikeike suggested. I wished I had known about these when I started this crossram project rabbit hole.

I know you were looking at the refurbed, they are cheaper, but dont include all the features this one has. If it were me I would go with Ilikeike's HR 580. It has everything you could ever want for your application. Think of the grief you wont have to go through upgrading anything.

Upgraded my 600s to center hung & installed QFT tunable VS actuator housings on both carbs. According to QFT these will fine tune the VS system after selecting the optimum spring to get you in ballpark. Mine runs but yet to drive and dial all in. Crossram kind of sucked the joy out of this car, so taking a break. Dont let that happen to your 47 yr old passion. HR 580 it and enjoy the ride!
 

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Just for fun, I'll throw this into the mix: I have the original Rochester 7029202 spread bore that came on the car. It's been sitting in a box since I took it off the car back in 1976, so I had it rebuilt at the same time I had the 4160 rebuilt. I'm curious, how does that carb compare to all these Holleys we're discussing. Just from looking at it, I get the feeling that they are a bit of a different animal to deal with.
Well I had one on a 74 Z28 I owned back in 1979. I knew nothing about quadrajets and still dont. But I took the car to a local guy that knew about them and it worked reall well when he was done with it. I know some swear by them, some swear at them. If you find someone that knows how to set them up it would be very cool to have another original piece on that gem!
 

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It could be a 642 stamping. Holley's jets sold as aftermarket were typically 2 number stampings. However, for OE applications they would have even finer increments designated by 3 numbers to differentiate. Since your carb was intended to be an emission replacement, this could likely be the case. So your jets would be slightly richer than 64s, but still leaner than a 65s.
Mike thanks for the post. My Holley book showed 642 jets for Ken's emission carb. I had never seen a 3 digit Holley jet number but your post explains it and makes sense. One thing I have learned through this topic is that emission carb is a little different animal from a standard 4160.
 
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