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68 Base Camaro 355 Offy crossram Richmond Super Street close ratio 5 spd ivy gold 92K SoCal car
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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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SY1 is spot on.

And even if you felt you could dial in your cruise with leaning it, you shouldn't do this until you trouble-shoot your idle circuit issue first. Reason being, each of the earlier circuits plays a small role in the next tuning area. So it's important to do all your tuning in order. Main jets won't effect idle circuit, but idle circuit will have a small effect on main jet requirements. Same with secondary jetting. Don't mess with that until your primary jets are set.
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Discussion Starter · #23 ·
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.
View attachment 290328


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.
The confusion on my part is that this was a brand new carb in 1976. It's never been modified, though it has been rebuilt/refurbished twice and never gave me any trouble until It just suddenly went south this past November without any warning. The car would not run. An engine builder nearby suggested the carb was dumping too much raw fuel and I had him rebuild the carb. The car runs great now, but still no idle screw effect. I've read that if the PV is damaged it might be leaking enough fuel to make the idle screws ineffective and it's almost easier to just replace it than to spend time trouble shooting it. That's really the only possible issue/solution to the idle screw issue that I've not addressed. The jet/PV question had nothing to do with my idle issue, but was merely an idea based on the fact that I may have to replace the PV anyways (if it's damaged) and many posts on related threads kept bringing this up as a side-issue to make the car run better once it is back to normal. That's why I wanted to know how to ID what size the stock jets are before I do anything.
 

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The jets will have numbers stamped on them to tell you the size. For aftermarket Holley, they are typically 2 digits. The size is really just a part number and doesn't always equal the orifice diameter in thousands of an inch. Many correlate, (which is the origin of the myth), but it's not always true especially above 66 sizes.

I'd write it down or take a pic so you have it for later. But leave what you have for now until you can drive it with the idle circuit adjustment functioning properly.
 

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Discussion Starter · #25 ·
The jets will have numbers stamped on them to tell you the size. For aftermarket Holley, they are typically 2 digits. The size is really just a part number and doesn't always equal the orifice diameter in thousands of an inch. Many correlate, (which is the origin of the myth), but it's not always true especially above 66 sizes.

I'd write it down or take a pic so you have it for later. But leave what you have for now until you can drive it with the idle circuit adjustment functioning properly.
Thanks, Mike. That's my intention. Since I have to pull it apart to look at the Power Valve anyway I can make some notes then.
 

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Al - Waterloo, Iowa
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I don't see where you have verified the transfer slots are not exposed too much. Also you said the timing doesn't change with the vacuum line to the vacuum advance connected. This makes me believe you have the advance canister connected to ported vacuum. That's not good. If you connect it to manifold vacuum your idle speed will increase and you will need to reduce the idle using the curb idle screw. This will lessen the transfer slot gap and I will bet you can then adjust the idle mixture screws. Unless you remove the carb so you can see the bottom side you won't know for certain if the slots are exposed too much as I, and others who've replied, suspect they are.
 

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One more thing that I don't think was mentioned. If everything checks out, there is a possibility that the screws/seats were damaged if torqued on too hard when adjusting the mixture. For that reason, I always adjust using a screwdriver with small handle held between my thumb and forefinger to lightly twist. If that's happened you'll have to replace the screws and metering block.

And final note, when setting the mixture, always do so with the air cleaner in place. The filter and base can change the airflow significantly.
 

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Discussion Starter · #28 ·
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.
View attachment 290328


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.
Thanks. I've had the carb rebuilt twice now, but unless the guys who did it for me made some custom alterations without telling me, it should be as it came from the factory in 1976. However the first guy was an aging, very automotively-opinionated local drag racer (he ran a 69 Hemi 'Cuda back in the the mid 70's) and I think his mind was getting iffy, so I would never say "no way." Now that I'm thinking about it, I'm starting to wonder if maybe he might have done something like that. Needing a new metering block might not be out of the question.
 

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Well at this point, it’s hard to say without taking it apart on my bench.
I would have already put fresh $10 power valves in it, if they were questionable.

open it up and se what you have and write all the sizes and numbers down.
yiu should be able to find the stock sizes that should be in it on holleys site. Then go from there.
 

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68 Base Camaro 355 Offy crossram Richmond Super Street close ratio 5 spd ivy gold 92K SoCal car
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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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Discussion Starter · #31 ·
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!👍
…AND I am learning g a LOT.
 

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Discussion Starter · #32 ·
Well at this point, it’s hard to say without taking it apart on my bench.
I would have already put fresh $10 power valves in it, if they were questionable.

open it up and se what you have and write all the sizes and numbers down.
yiu should be able to find the stock sizes that should be in it on holleys site. Then go from there.
Thanks. Until recently I didn't realize the PV could be involved in the idle issue. I am learning a lot from everyone.
 

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Discussion Starter · #33 ·
SY1 is spot on.

And even if you felt you could dial in your cruise with leaning it, you shouldn't do this until you trouble-shoot your idle circuit issue first. Reason being, each of the earlier circuits plays a small role in the next tuning area. So it's important to do all your tuning in order. Main jets won't effect idle circuit, but idle circuit will have a small effect on main jet requirements. Same with secondary jetting. Don't mess with that until your primary jets are set. View attachment 290349
Thanks! That is every instructional and helpful.
 

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Al - Waterloo, Iowa
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My understanding is the 6619 is an emissions carb from the early 70's. The mixture screws actually work opposite
of a conventional carb in that they regulate the amount of air, not fuel. So turning them in actually richens the fuel mixture. Google 6619 and you'll get a number of hits to follow.
 

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My understanding is the 6619 is an emissions carb from the early 70's. The mixture screws actually work opposite
of a conventional carb in that they regulate the amount of air, not fuel. So turning them in actually richens the fuel mixture. Google 6619 and you'll get a number of hits to follow.
Nice catch! I had actually forgotten about the reverse-idle cruise carbs.
 

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I found this using a little Google Fu

"The Holley 0-6619-1 is an early 1970's emmissions aftermarket replacement carb.It is a 4160 series that flows approx 600 CFM.These carbs have a fairly lean idle circuit and a relitivly large PVCR (Power valve channle restrictor) that allowed decient power.As assembled from Holley these carbs had a Black secondary diaphram spring.The Black spring is the heaviest spring made and designed to not allow the secondaries to open fully.Changing springs is necessary to allow full secondary opening.

These carbs were never intended as performance carbs.Their idle circuits were designed lean to be easy on emmissions and as such they dont respond well to big camshafts.These carbs ( there were several of this family of carbs 0-6619,0-6909,0-6919 ect ect ) were discontinued several years ago."

It kinda lost me at " 70s emissions carb" and "Never intended as a performance carb" :cry:
 

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Yes, they were designed for emissions, but from the OP description he feels it's still running rich @ idle, so there's enough adjustment available for his setup. And it's likely since someone has gone through it already, the secondary spring has been swapped out to allow quicker opening. If it works, it works. He just needs to turn the screws the right way. :)
 

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Discussion Starter · #39 ·
My understanding is the 6619 is an emissions carb from the early 70's. The mixture screws actually work opposite
of a conventional carb in that they regulate the amount of air, not fuel. So turning them in actually richens the fuel mixture. Google 6619 and you'll get a number of hits to follow.
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.
 

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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.
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 !!
 
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