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Hello, I have a Holley 0-82751SA on a 327 with a 294 cam [email protected] on a 104 LSA. 26 degrees initial timing, 10 degrees mechanical, I didn’t have a chance to check how much the vacuum advance is pulling but I only have 5” of vacuum at idle. I’m using manifold vacuum for the vacuum advance. Stick shift car. I have a brand new 2.5 power valve installed. I had to open the secondaries a little so I wouldn’t have to open the primaries open so much at idle and pull fuel from the transition slot. It was open too much and there was vacuum from the ported port at idle which let me know I wasn’t on the idle circuit. I got that situation fixed but the thing is running EXTREMELY rich. So rich I don’t feel comfortable taking it anywhere because it loads up. It starts up cold with a higher idle and then once it runs for a few minutes it’ll go down because it’s just consuming so much fuel. I don’t have control over my mixture screws. The idle air restrictors are not removable and I heard about sticking some wire in there to restrict the amount of fuel but it makes more sense for me to try to change out my idle air bleeds to a larger size since they are removable at the top of the carb. I currently have 71’s and I was thinking about going to 75’s, do you think this will allow me to gain control of my mixture screws? Thanks.
 

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Changing out your air bleeds won't be enough especially with that big of a cam. Air bleed changes need to be at minimum a .004 size change up or down in order to get any change in the curve if you need it leaner or richer. You might need to lock out your timing and move your vacuum advance to timed port and install a vacuum advance limiter on your distributor depending on what style your using.

It tells me you still have too much transfer slot exposure if you don't have any idle mixture screw adjustment. What rpm at idle are you shooting for? A cam that size will need at least a 1200 rpm idle give or take a hair on the rpm. I had a cam way smaller then that and I had to idle it at around 1100 rpm and it would not go any lower.

On your carburetor its pretty sad for the cost of it that Holley can't put changeable idle feed restrictors in the blocks and does your blocks have the four emulsion ports or five? I don't know for sure on the HP carbs as I did have one before but I put it together from scratch by myself and the metering blocks I used had only three emulsion ports. I read that some of the HP carb models come with a 4 or 5 emulsion ports on each side of the metering block and though that won't affect the idle circuit it will effect the entire fuel curve on the jets and main air bleeds and from what I have read on them for cruising with the extra emulsion ports it can be a very rich cruise and can be a bear to tune and get good mileage.

A lot of times folks will block a few of the ports on both blocks to help out with that part. On the idle circuit on your metering blocks you can drill out the pressed in idle feed restrictors and then tap it for 6-32 brass allen cup screws and get you a drill bit gauge set from ebay for the cheap and you can make your own sizes. With your big of a cam you will have to do a trial and error and really know your advance tuning on Holley carbs and might want to get the book by David Vizard as it gives you a lot of information on how each circuit works on the carb and some good knowledge on how you can advance tune things.

Or if you don't feel comfortable doing the metering block mods which are actually not hard at all, you can get a set of Quick fuel metering blocks and install them and they already have the screw in idle feed restrictors and also power valve channel restrictors as well. On you primary transfer slot exposure you want to keep it square and no more then around .020 to .040 at the most. Also when you try to equal out the transfer slots to show on the front and rear, a lot of times the rear transfer slot sits up higher in the rear on some Holley carbs and if you try to equal them sometimes it can create an imbalance between the front and rear and cause some off idle issues as the rears will be open more then the primary side and not allow proper transfer slot exposure.



 

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Discussion Starter · #3 · (Edited)
Changing out your air bleeds won't be enough especially with that big of a cam. Air bleed changes need to be at minimum a .004 size change up or down in order to get any change in the curve if you need it leaner or richer. You might need to lock out your timing and move your vacuum advance to timed port and install a vacuum advance limiter on your distributor depending on what style your using.

It tells me you still have too much transfer slot exposure if you don't have any idle mixture screw adjustment. What rpm at idle are you shooting for? A cam that size will need at least a 1200 rpm idle give or take a hair on the rpm. I had a cam way smaller then that and I had to idle it at around 1100 rpm and it would not go any lower.

On your carburetor its pretty sad for the cost of it that Holley can't put changeable idle feed restrictors in the blocks and does your blocks have the four emulsion ports or five? I don't know for sure on the HP carbs as I did have one before but I put it together from scratch by myself and the metering blocks I used had only three emulsion ports. I read that some of the HP carb models come with a 4 or 5 emulsion ports on each side of the metering block and though that won't affect the idle circuit it will effect the entire fuel curve on the jets and main air bleeds and from what I have read on them for cruising with the extra emulsion ports it can be a very rich cruise and can be a bear to tune and get good mileage.

A lot of times folks will block a few of the ports on both blocks to help out with that part. On the idle circuit on your metering blocks you can drill out the pressed in idle feed restrictors and then tap it for 6-32 brass allen cup screws and get you a drill bit gauge set from ebay for the cheap and you can make your own sizes. With your big of a cam you will have to do a trial and error and really know your advance tuning on Holley carbs and might want to get the book by David Vizard as it gives you a lot of information on how each circuit works on the carb and some good knowledge on how you can advance tune things.

Or if you don't feel comfortable doing the metering block mods which are actually not hard at all, you can get a set of Quick fuel metering blocks and install them and they already have the screw in idle feed restrictors and also power valve channel restrictors as well. On you primary transfer slot exposure you want to keep it square and no more then around .020 to .040 at the most. Also when you try to equal out the transfer slots to show on the front and rear, a lot of times the rear transfer slot sits up higher in the rear on some Holley carbs and if you try to equal them sometimes it can create an imbalance between the front and rear and cause some off idle issues as the rears will be open more then the primary side and not allow proper transfer slot exposure.



I can drop the idle down low no problem. I had it idling at 750 and took the carb off to see how much of the transfer slot is exposed and it was .025. Like I said, I opened the secondaries up a little instead of drilling the butterflies. I’m using an MSD 8360 distributor. If I can lower the idle and not have too much transfer slot exposure on my primaries then why do I have to lock the centrifugal advance and use the ported advance? Thanks.
 

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Wait, back away a minute.
Four corner idle and mechanical secondaries and you mentioning drilling the blades tell me you don't have a good grasp on whats going on here. That would be a real good way jack up stuff.

First of all, start with the idle mix screws about 3/4 from seated on four. Set the transfer slots about .025 showing, aim for around 1000 rpm. Try to keep the all four corners the same transfer slot exposure with the secondaries SLIGHTLY more sometimes. Floats need to be in the middle of the sight glass assuming 7 psi pump.

You have a really long, slow cam, (I'll argue the way wrong cam to begin with) not enough compression, not enough timing, and a generic metering curve on a generic carb from Holley. I'm not at all surprised it smells rich. It's probably not gonna get a whole lot better without reworking the idle feeds in the metering blocks.
Do you have a decent coil?
 

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Discussion Starter · #5 ·
Wait, back away a minute.
Four corner idle and mechanical secondaries and you mentioning drilling the blades tell me you don't have a good grasp on whats going on here. That would be a real good way jack up stuff.

First of all, start with the idle mix screws about 3/4 from seated on four. Set the transfer slots about .025 showing, aim for around 1000 rpm. Try to keep the all four corners the same transfer slot exposure with the secondaries SLIGHTLY more sometimes. Floats need to be in the middle of the sight glass assuming 7 psi pump.

You have a really long, slow cam, (I'll argue the way wrong cam to begin with) not enough compression, not enough timing, and a generic metering curve on a generic carb from Holley. I'm not at all surprised it smells rich. It's probably not gonna get a whole lot better without reworking the idle feeds in the metering blocks.
Do you have a decent coil?
Already did what you mentioned. Mixture screws not working and I’m not saying it just smells rich. It’s so rich it’s smoking. I mentioned drilling but I did not say I drilled anything.
 

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Discussion Starter · #7 ·
Fair enough......
Whats the rest of the engine specs....ALL of em.....all the details please.

030 over flat tops
.041 quench
64cc double hump heads ported and polished
1.6 ratio rockers
10.3:1 compression
Edelbrock performer rpm manifold
5.7 rods
294 duration, 250 @.050 lift, 104 LSA
MSD 8360 distributor
MSD blaster 2 coil
30lb. flywheel
1 5/8 headers
 

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Do you have a drill bit gauge set by chance? If you do you need to measure what size your idle feed restrictors are and also with your big cam your not going to be able to calibrate your carb to have a real lean mixture and be able to run good because with low vacuum and poor signal going to the carb and exhaust reversion which comes with bigger rough idle cams and with a ton of overlap, it will always require a richer mixture versus a way smaller camshaft. I am guessing you have at least a .033 or slightly bigger idle restrictors in your metering blocks front and back.

All you can do with that part is first try to maybe add more initial timing and verify how much your vacuum advance is adding and see if adding more will help with the idle. Johnsongrass1 is right that your engine is pretty small for such a big cam and with low vacuum it won't be super easy to get a very clean tune especially with the specs of the grind. I had a Comp cam Big Mother Thumper hydraulic roller cam once which specs at 243/[email protected] with a tight lobe separation angle and it only gave me 6 inches of vacuum in park with an automatic transmission and it was a pain to get tuned but would idle at around 1100 rpm but it took me a little trial and error and even when I finally had it tuned with a good idle it was still on the rich side as anything less it would not like it and not run well.

That is the downfall with bigger cams and even worse on smaller cubic inch builds is they are not very friendly for street manors in my opinion and don't cruise well like builds with milder cams and less overlap but that will vary from person to person on there experience and opinion on that.
 

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Discussion Starter · #9 ·
Do you have a drill bit gauge set by chance? If you do you need to measure what size your idle feed restrictors are and also with your big cam your not going to be able to calibrate your carb to have a real lean mixture and be able to run good because with low vacuum and poor signal going to the carb and exhaust reversion which comes with bigger rough idle cams and with a ton of overlap, it will always require a richer mixture versus a way smaller camshaft. I am guessing you have at least a .033 or slightly bigger idle restrictors in your metering blocks front and back.

All you can do with that part is first try to maybe add more initial timing and verify how much your vacuum advance is adding and see if adding more will help with the idle. Johnsongrass1 is right that your engine is pretty small for such a big cam and with low vacuum it won't be super easy to get a very clean tune especially with the specs of the grind. I had a Comp cam Big Mother Thumper hydraulic roller cam once which specs at 243/[email protected] with a tight lobe separation angle and it only gave me 6 inches of vacuum in park with an automatic transmission and it was a pain to get tuned but would idle at around 1100 rpm but it took me a little trial and error and even when I finally had it tuned with a good idle it was still on the rich side as anything less it would not like it and not run well.

That is the downfall with bigger cams and even worse on smaller cubic inch builds is they are not very friendly for street manors in my opinion and don't cruise well like builds with milder cams and less overlap but that will vary from person to person on there experience and opinion on that.
The top of the idle feed restrictor is 0.0585 but 3/16” down in the pressed in restrictor is only about 0.033.
 

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Carb is too big, cam is WAY to big, Coil is crappy.

Anyway, you gonna need to spin too 8500 for the cam to be work but the heads won't support the RPM so you have some reconfiguration to do.
First, I'd idle it up to around 1200, Lock the timing down to 38, Put a LOT of gear in it.
As far as the carb goes, it need adjustable idle feed restrictions and the transfer slots moved to make the best of it and then you'll find the carb likely going lean at RPM anyway.
Simply put, The vacuum signal to the carb is poor and it's compromising your tuning abilities.
 

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Discussion Starter · #11 ·
Carb is too big, cam is WAY to big, Coil is crappy.

Anyway, you gonna need to spin too 8500 for the cam to be work but the heads won't support the RPM so you have some reconfiguration to do.
First, I'd idle it up to around 1200, Lock the timing down to 38, Put a LOT of gear in it.
As far as the carb goes, it need adjustable idle feed restrictions and the transfer slots moved to make the best of it and then you'll find the carb likely going lean at RPM anyway.
Simply put, The vacuum signal to the carb is poor and it's compromising your tuning abilities.
sounds good. Back to the drawing board.
 

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Discussion Starter · #12 · (Edited)
Changing out your air bleeds won't be enough especially with that big of a cam. Air bleed changes need to be at minimum a .004 size change up or down in order to get any change in the curve if you need it leaner or richer. You might need to lock out your timing and move your vacuum advance to timed port and install a vacuum advance limiter on your distributor depending on what style your using.

It tells me you still have too much transfer slot exposure if you don't have any idle mixture screw adjustment. What rpm at idle are you shooting for? A cam that size will need at least a 1200 rpm idle give or take a hair on the rpm. I had a cam way smaller then that and I had to idle it at around 1100 rpm and it would not go any lower.

On your carburetor its pretty sad for the cost of it that Holley can't put changeable idle feed restrictors in the blocks and does your blocks have the four emulsion ports or five? I don't know for sure on the HP carbs as I did have one before but I put it together from scratch by myself and the metering blocks I used had only three emulsion ports. I read that some of the HP carb models come with a 4 or 5 emulsion ports on each side of the metering block and though that won't affect the idle circuit it will effect the entire fuel curve on the jets and main air bleeds and from what I have read on them for cruising with the extra emulsion ports it can be a very rich cruise and can be a bear to tune and get good mileage.

A lot of times folks will block a few of the ports on both blocks to help out with that part. On the idle circuit on your metering blocks you can drill out the pressed in idle feed restrictors and then tap it for 6-32 brass allen cup screws and get you a drill bit gauge set from ebay for the cheap and you can make your own sizes. With your big of a cam you will have to do a trial and error and really know your advance tuning on Holley carbs and might want to get the book by David Vizard as it gives you a lot of information on how each circuit works on the carb and some good knowledge on how you can advance tune things.

Or if you don't feel comfortable doing the metering block mods which are actually not hard at all, you can get a set of Quick fuel metering blocks and install them and they already have the screw in idle feed restrictors and also power valve channel restrictors as well. On you primary transfer slot exposure you want to keep it square and no more then around .020 to .040 at the most. Also when you try to equal out the transfer slots to show on the front and rear, a lot of times the rear transfer slot sits up higher in the rear on some Holley carbs and if you try to equal them sometimes it can create an imbalance between the front and rear and cause some off idle issues as the rears will be open more then the primary side and not allow proper transfer slot exposure.



Locked the timing and moved the vacuum advance to ported which raised the vacuum at idle to about 7-8. Waiting on the brass screws to deliver but what size do you think I should start with so I can work my way up and not have to keep drilling new screws. Thanks.
 

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If your talking about your idle feed restrictors then I would start with .031 to start off with. I believe with your cam being so big that .028 would be to lean as I have done many Holley carbs and I had a decent sized cam but tame compared to yours and .028 was way to lean on the idle circuit and I had 12 plus inches of vacuum and yours would need a richer mixture with all the overlap and stuff and low vacuum. Make sure you have it equal front and back. As far as air bleeds goes I would put them back at around the .070 stock size and you will want to mess with idle restrictors and adjustments first before anything else.

You will then want to start of with all four idle mixture screws about 3/4 of a turn out to start on all four and make sure you butterfly setting is correct on the front and make sure to make notes on how many turns you can go from start before your transfer slot becomes past square and more then .30 to .040. You want to make any changes to the primary idle screw setting such as say it can go 3/4 past contact when they first open and have around .020 transfer slot exposure. You can normally go an estimate of a 1/4 turn past the .020 transfer slot exposure before its exposed too much. So you would write down 1 full turn is the max amount you can go and you want to keep that setting marked down on hand and every time you would have to adjust it then make a note of doing so while adjusting your idle mixture screws.

On those 4 corner idle deals they are super sensitive to adjusting if your idle circuit is within the proper window of what the engine needs and you will want to adjust your idle mixture screws only an 1/8 of a turn at at time because it won't take much if the idle feed restrictors are the proper size for it to adjust to what it needs but its a trial and error deal and you will be listening to how the engine is reacting and also you can use a vacuum gauge but don't shoot for highest vacuum as it will end up making it to lean but once you were to reach the highest vacuum point then you would want to turn it back an 1/8 of a turn richer as an estimate.

It can vary from build to build on what it will need. Be warned though your carb will always seem rich to an extent because of the size of your cam and all of its overlap and low vacuum signal. That is the nature of the beast when working with these size of cams.
 

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Discussion Starter · #14 ·
If your talking about your idle feed restrictors then I would start with .031 to start off with. I believe with your cam being so big that .028 would be to lean as I have done many Holley carbs and I had a decent sized cam but tame compared to yours and .028 was way to lean on the idle circuit and I had 12 plus inches of vacuum and yours would need a richer mixture with all the overlap and stuff and low vacuum. Make sure you have it equal front and back. As far as air bleeds goes I would put them back at around the .070 stock size and you will want to mess with idle restrictors and adjustments first before anything else.

You will then want to start of with all four idle mixture screws about 3/4 of a turn out to start on all four and make sure you butterfly setting is correct on the front and make sure to make notes on how many turns you can go from start before your transfer slot becomes past square and more then .30 to .040. You want to make any changes to the primary idle screw setting such as say it can go 3/4 past contact when they first open and have around .020 transfer slot exposure. You can normally go an estimate of a 1/4 turn past the .020 transfer slot exposure before its exposed too much. So you would write down 1 full turn is the max amount you can go and you want to keep that setting marked down on hand and every time you would have to adjust it then make a note of doing so while adjusting your idle mixture screws.

On those 4 corner idle deals they are super sensitive to adjusting if your idle circuit is within the proper window of what the engine needs and you will want to adjust your idle mixture screws only an 1/8 of a turn at at time because it won't take much if the idle feed restrictors are the proper size for it to adjust to what it needs but its a trial and error deal and you will be listening to how the engine is reacting and also you can use a vacuum gauge but don't shoot for highest vacuum as it will end up making it to lean but once you were to reach the highest vacuum point then you would want to turn it back an 1/8 of a turn richer as an estimate.

It can vary from build to build on what it will need. Be warned though your carb will always seem rich to an extent because of the size of your cam and all of its overlap and low vacuum signal. That is the nature of the beast when working with these size of cams.
Ok, I never changed the original .071 idle air bleeds but I brought a pack of blanks in the event I may need them. My screws won’t be here until Tuesday unfortunately. I’ll do the mod and install .031 restrictors like you instructed and if it’s still a tad rich you’re not suggesting that I got to .028 since it will be too lean so would you want me to change an air bleed? I’m just thinking of all scenarios so when I’m performing the tests I can make adjustments and not have to wait for a response back on the forum. I appreciate all of your help.
 

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Big cams with tight LSA’s really screw with idle mixture. Some of this can be subdued with less split in intake to exhaust duration. You can fiddle the rocker ratio mix of 1.6 on intake or exhaust with 1.5 or reversed. The head chambers can play in this as well, open chamber heads play into this where there is no dividing beak between the intake and exhaust valve. The exhaust system especially the header and collector lengths can mess with this.

What I’m talking about is reversion playing between intake and exhaust where both impulse and mass is pulsing through the co-open exhaust and intake valves during overlap. This is always a big issue when LSA’s get under 110 degrees and get worse the smaller the angle gets as this is indicative of more time (duration) and lift being devoted to both valves being open at the same time as the piston transitions from exhaust stroke through TDC on into the intake stroke.

The carb really doesn’t care whether pulses and mass movement is going into or out of the carburetor that device adds fuel in either direction. So for a mass of air going in it adds fuel making that mass an air fuel mixture. When reversion forces of pulse wave or a mass flow of mixture is pushed outward through the carb it gets another dose of fuel. Then the next time this air fuel mixture is drawn in it gets another dose of fuel. This seesaw just keeps richening the mixture and you quickly find that you have no idle mixture control.

Hence you hear engines with carbs and big cam that use a tight LSA these cams when idling being throttle blipped all the time to keep them cleaned out.

If this isn’t a racer that needs a cam like this the shortest distance between where you are and a decent idle is a cam change to one with an LSA of 110 or more. The other option is going back to my first paragraph and start experimenting with different headers, intake, rocker ratio combinations and heads with different combustion chamber shapes and port sizes.

Think of the engine as it being a pipe organ with spark plugs.

Bogie
 

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You want to just try one step at a time is what I recommend. Always start off with stock air bleeds when messing with the idle as it will have a way bigger effect on the idle and off idle before the mains will even activate. Because I have a lot of experience with Holley carbs and many different engine combos from bone stock to wild but nothing as wild as yours but was close with a comp cam big mutha thumpr hydraulic roller shaft but it had really crappy vacuum of only about 6 inches in park.

it required a very rich setting and calibration on my Holley double pumper I had on it. I had a 350 cubic inch build versus your 327 which might play a tad bit into things but I doubt a lot though but I am not an expert like Bogie and Johnsongrass1 and a handful of others on here. The last biggest hydraulic roller cam I tuned with was a 276/280 228/[email protected] 5??/5??( can't remember exact numbers) lift and a 110 lsa and I had around 14 inches of vacuum in park and I had a Holley 650 double pumper and converted it to a 4 corner idle and I had .028 idle feed restrictors front and rear and .074 or .078 idle air bleeds and it ran exceptionally well and it was a destroked 377 cubic inch Dart SHP build (4.155 bore x 3.48 stroke) and that cam was way milder then yours and I know with the very low vacuum and smaller cubic inch size that your carburetor won't run with a .028 idle feed restrictor and would end up having the throttle blades opened up too much to the point you would maybe have to drill holes in the butterflies in the front or even the rear in order to keep the transfer slot square.

There is a balance of having the right idle feed restrictors and to a smaller extent the correct range of idle air bleeds in relation to proper butterfly adjustment along with idle mixture screw adjustment and all those parts have to be very close in order to get the right idle the engine needs without being to far out on one area or the other and it takes a lot of time and practice from knowing what your engine wants and now what direction you need to go. I always start with a basic baseline and try and see how it does and reacts to adjustment on the mixture screws and also what it takes to get it to idle at the correct rpm that it needs and also keeping the primary idle speed screw in the correct transfer slot range and also having not to have the secondary side open way more then the primary side and doing the very upmost best to not have to drill any holes in the butterflies and your camshaft is knocking at that door with it being so big and such low vacuum.

Bogie explained the reason as to why those carbs will always be rich with big cams as there is so much scientific stuff involved with all that stuff that I can't explain like he can. I would say it might not run very well with .031 but at least try and see how things do as that would be the leanest starting point I would work with if it where me with what I know. I have read a gob of forums over the last 14 year on folks and Holley carbs being rich on the idle circuit and I would write down there engine specs and also all the changes they did with there carbs and kept them in a library and over time with those notes and information and also I have like 6 Holley books on tuning and I have had to many builds over the last ten plus years to even remember how many I had and I learned my craft and also from others on here who have helped me learn how to do such tuning.

I am still learning even after 14 years and my recent build has thrown me a big curve ball that I have not been able to figure out yet the problem but I am getting very close though as its a situation I have never seen before and too much to type here but hopefully this well be a helpful experience with help from others on here that you will learn the fine art of advance tuning 101 that is more then what about 75 percent of carb tuners never learn to do on Holley style carburetors.
 

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Discussion Starter · #17 · (Edited)
You want to just try one step at a time is what I recommend. Always start off with stock air bleeds when messing with the idle as it will have a way bigger effect on the idle and off idle before the mains will even activate. Because I have a lot of experience with Holley carbs and many different engine combos from bone stock to wild but nothing as wild as yours but was close with a comp cam big mutha thumpr hydraulic roller shaft but it had really crappy vacuum of only about 6 inches in park.

it required a very rich setting and calibration on my Holley double pumper I had on it. I had a 350 cubic inch build versus your 327 which might play a tad bit into things but I doubt a lot though but I am not an expert like Bogie and Johnsongrass1 and a handful of others on here. The last biggest hydraulic roller cam I tuned with was a 276/280 228/[email protected] 5??/5??( can't remember exact numbers) lift and a 110 lsa and I had around 14 inches of vacuum in park and I had a Holley 650 double pumper and converted it to a 4 corner idle and I had .028 idle feed restrictors front and rear and .074 or .078 idle air bleeds and it ran exceptionally well and it was a destroked 377 cubic inch Dart SHP build (4.155 bore x 3.48 stroke) and that cam was way milder then yours and I know with the very low vacuum and smaller cubic inch size that your carburetor won't run with a .028 idle feed restrictor and would end up having the throttle blades opened up too much to the point you would maybe have to drill holes in the butterflies in the front or even the rear in order to keep the transfer slot square.

There is a balance of having the right idle feed restrictors and to a smaller extent the correct range of idle air bleeds in relation to proper butterfly adjustment along with idle mixture screw adjustment and all those parts have to be very close in order to get the right idle the engine needs without being to far out on one area or the other and it takes a lot of time and practice from knowing what your engine wants and now what direction you need to go. I always start with a basic baseline and try and see how it does and reacts to adjustment on the mixture screws and also what it takes to get it to idle at the correct rpm that it needs and also keeping the primary idle speed screw in the correct transfer slot range and also having not to have the secondary side open way more then the primary side and doing the very upmost best to not have to drill any holes in the butterflies and your camshaft is knocking at that door with it being so big and such low vacuum.

Bogie explained the reason as to why those carbs will always be rich with big cams as there is so much scientific stuff involved with all that stuff that I can't explain like he can. I would say it might not run very well with .031 but at least try and see how things do as that would be the leanest starting point I would work with if it where me with what I know. I have read a gob of forums over the last 14 year on folks and Holley carbs being rich on the idle circuit and I would write down there engine specs and also all the changes they did with there carbs and kept them in a library and over time with those notes and information and also I have like 6 Holley books on tuning and I have had to many builds over the last ten plus years to even remember how many I had and I learned my craft and also from others on here who have helped me learn how to do such tuning.

I am still learning even after 14 years and my recent build has thrown me a big curve ball that I have not been able to figure out yet the problem but I am getting very close though as its a situation I have never seen before and too much to type here but hopefully this well be a helpful experience with help from others on here that you will learn the fine art of advance tuning 101 that is more then what about 75 percent of carb tuners never learn to do on Holley style carburetors.
I measured with my pin gauge and found out that my idle feed restrictors were .028. Just for a test, I drilled them out and installed 6-32 screws and drilled them to .026 which seemed to clean the idle but I have to give the car a lot of gas to bring the RPMS up because it will hesitate and try to stall if I let the clutch out. I have the same off idle hesitation if I mash the throttle that I did before but it’s no longer smoking at idle. I did not touch the stock 71 idle air bleeds. Plates are square.
 

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For one thing on a Holley style carb and a four corner setup many things can happen that can throw a tune out the window. The first thing that comes to mind is that if you had .028 idle feed restrictors the first time then in my past situations when I had a big cam if the idle feed restrictors were to small and the idle circuit was to lean for the said build then it would have me end up with the transfer slot being exposed to much in order to get a correct rpm setting for the idle.

Now that is a situation that I have dealt with many times even with small camshafts that was not very radical at all. I have learned the last two years that what normally works in tuning carbs sometimes won't always make sense in certain circumstances in certain builds. Take mine for instance. Its a basic 350 Dart SHP build with 9:1 compression and a mild entry level performance cam 268/272 220/[email protected] 510/510 lift 114 lsa. I have Dart pro 1 platinum 200cc heads ported and bowl blended and cleaned up real well and using an Edelbrock rpm performer intake.

I had this build put together last year and had a tuning issue with a Holley 600 vacuum secondary carburetor that since day one would run pig rich and I could not figure out why as the normal changes in the calibration was not changing anything and it kept running rich no matter what and it should have not been working like that. In previous builds changing out the idle feed restrictors in such small sizes made a world of different either rich or lean depending on what direction I needed to go.

On this particular Holley 600 vacuum secondary carb it was an older model that had a different base plate compared to the current ones used on there vacuum secondary carburetors of the 600 sized variety such as the 80457 and 1850. I used the old style Holley in the past one time on a 350 similar to my current one but not with decked out heads and it had a slightly bigger cam with a lot more overlap. It worked excellent on that build and not many changes had to be done but I knew nothing about tuning them back then.

My Father was the one who installed it and all he did was have to set the screws and things worked good for the month I used it. My spark plugs were nice and clean and showed a good burn back in 2008. It was his carb as I bought a brand new Holley 670 street avenger and it was a horrible experience and ran horrible out of the box and had a defect on the first one and the second one ran but was very lean in the way it drove.

I figured that out after many years of learning the differences in the older carbs versus the newer ones. Anyways long story short I could not get my idle mixture screws to adjust no matter where I would set that carb and could not even kill my engine it was running so rich. I could even close up the transfer slot and it would still run pig rich and no amount of changing the idle feed restrictors and even the idle air bleeds would hardly make any difference.

I had taken the carb apart and ran through my ignition and all things else and could not figure out what the problem was. I had to then think outside of the box because what normally works is not working here and I need to figure out what is the difference in the older Holley 600 Holley vacuum secondary compared to the current ones. Holley will revise things and tweak the calibration on them as years goes on but how much is something I can't give an honest answer on.

I had the old one and looked it over and I knew that Holley had slightly richer air bleeds on the idle part of things and a hair richer in the main jets but that won't effect the idle part of things on the main jets being slightly richer versus the idle air bleeds which would affect things.

The older Holley had like .064 primary idle air bleeds and .029 in the rear. They leaned it out to around .078 primary and .048 secondary in the current ones. They moved the pcv port from the side to the rear in the change in the later model ones. I looked it over and over and then noticed I had to have 1 1/2 turns on the primary idle screw to get a square transfer slot of about .020. The newer ones only took 3/4 of a turn.

So I took out a dial caliper and the older Holley base plates had shorter transfer slots compared to the newer ones which were longer. Also the rear transfer slots were longer on the older ones and shorter on the newer ones. That right there turned out to be the first big problem causing me my issues for some reason I can not explain and most likely never will except its one of those weird things that normally should not be that big of an issue but on this build it is.

I put on the same carb main body and metering blocks etc with the newer style base plate and bam I can now get a very close to normal carb to tune and adjust like it normally should. It was all because of the transfer slot size difference and length of the base plates that was causing a big chunk of my problems. I was working on my tune but I have another issue of my intake gaskets that keep breaking seal on only the front coolant ports and start to weep in short order.

I am down because of surgery I am getting done soon but will be back at it come June. I have another mystery issue that whenever I let off my throttle it will sputter from 1900 down to 1000 rpm then it clears up and is smooth again. I never had anything like that before and no changing the air bleeds so far or idle feed restrictors in my carb have helped to even make a change in it. I think there is something wrong with my custom camshaft grind as I have had a lot of horrible running experiences of it in two builds. Sorry for the long post but its just to share that sometimes weird out of the normal things can happen with carbs compared to what normally would not be that way for a tune.

The best thing is to adjust around and make sure your not fouling your plugs and things are running clean and see how it drives and make sure your transfer slots are correct and go from there.
 

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Discussion Starter · #19 · (Edited)
For one thing on a Holley style carb and a four corner setup many things can happen that can throw a tune out the window. The first thing that comes to mind is that if you had .028 idle feed restrictors the first time then in my past situations when I had a big cam if the idle feed restrictors were to small and the idle circuit was to lean for the said build then it would have me end up with the transfer slot being exposed to much in order to get a correct rpm setting for the idle.

Now that is a situation that I have dealt with many times even with small camshafts that was not very radical at all. I have learned the last two years that what normally works in tuning carbs sometimes won't always make sense in certain circumstances in certain builds. Take mine for instance. Its a basic 350 Dart SHP build with 9:1 compression and a mild entry level performance cam 268/272 220/[email protected] 510/510 lift 114 lsa. I have Dart pro 1 platinum 200cc heads ported and bowl blended and cleaned up real well and using an Edelbrock rpm performer intake.

I had this build put together last year and had a tuning issue with a Holley 600 vacuum secondary carburetor that since day one would run pig rich and I could not figure out why as the normal changes in the calibration was not changing anything and it kept running rich no matter what and it should have not been working like that. In previous builds changing out the idle feed restrictors in such small sizes made a world of different either rich or lean depending on what direction I needed to go.

On this particular Holley 600 vacuum secondary carb it was an older model that had a different base plate compared to the current ones used on there vacuum secondary carburetors of the 600 sized variety such as the 80457 and 1850. I used the old style Holley in the past one time on a 350 similar to my current one but not with decked out heads and it had a slightly bigger cam with a lot more overlap. It worked excellent on that build and not many changes had to be done but I knew nothing about tuning them back then.

My Father was the one who installed it and all he did was have to set the screws and things worked good for the month I used it. My spark plugs were nice and clean and showed a good burn back in 2008. It was his carb as I bought a brand new Holley 670 street avenger and it was a horrible experience and ran horrible out of the box and had a defect on the first one and the second one ran but was very lean in the way it drove.

I figured that out after many years of learning the differences in the older carbs versus the newer ones. Anyways long story short I could not get my idle mixture screws to adjust no matter where I would set that carb and could not even kill my engine it was running so rich. I could even close up the transfer slot and it would still run pig rich and no amount of changing the idle feed restrictors and even the idle air bleeds would hardly make any difference.

I had taken the carb apart and ran through my ignition and all things else and could not figure out what the problem was. I had to then think outside of the box because what normally works is not working here and I need to figure out what is the difference in the older Holley 600 Holley vacuum secondary compared to the current ones. Holley will revise things and tweak the calibration on them as years goes on but how much is something I can't give an honest answer on.

I had the old one and looked it over and I knew that Holley had slightly richer air bleeds on the idle part of things and a hair richer in the main jets but that won't effect the idle part of things. They moved the pcv port from the side to the rear in the change in the later model ones. I looked it over and over and then noticed I had to have 1 1/2 turns on the primary idle screw to get a square transfer slot of about .020. The newer ones only took 3/4 of a turn.

So I took out a dial caliper and the older Holley base plates had shorter transfer slots compared to the newer ones which were longer. Also the rear transfer slots were longer on the older ones and shorter on the newer ones. That right there turned out to be the first big problem causing me my issues for some reason I can not explain and most likely never will except its one of those weird things that normally should not be that big of an issue but on this build it is.

I put on the same carb main body and metering blocks etc with the newer style base plate and bam I can now get a very close to normal carb to tune and adjust like it normally should. It was all because of the transfer slot size difference and length of the base plates that was causing a big chunk of my problems. I was working on my tune but I have another issue of my intake gaskets that keep breaking seal on only the front coolant ports and start to weep in short order.

I am down because of surgery I am getting done soon but will be back at it come June. I have another mystery issue that whenever I let off my throttle it will sputter from 1900 down to 1000 rpm then it clears up and is smooth again. I never had anything like that before and no changing the air bleeds so far or idle feed restrictors in my carb have helped to even make a change in it. I think there is something wrong with my custom camshaft grind as I have had a lot of horrible running experiences of it in two builds. Sorry for the long post but its just to share that sometimes weird out of the normal things can happen with carbs compared to what normally would not be that way for a tune.

The best thing is to adjust around and make sure your not fouling your plugs and things are running clean and see how it drives and make sure your transfer slots are correct and go from there.
That makes sense. Yeah I was a little confused when I saw the small IFR’s. I had to open the curb idle screw 1 turn past contact to get it to idle at 750 so I knew the transfer slots were exploded too much. I’ll try going to .031 IFR’s and let you know how it does.
 

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Also on another note don't always think its running super rich all the time as with big cams it can throw the common symptoms of it running rich at idle when its not necessarily always the carb as the overlap of the camshaft and exhaust reversion and other factors will play into things as well and make you think its pig rich and too fat when its not.

When I was tuning for my build of many years ago with the comp cam big mother thumper hydraulic roller cam with 243/[email protected] and the low vacuum of about 6 inches in park, it smelled rich and would make your eyes burn if you were around it long enough. I started off with a lean starting point but did not go by that and I went by how my engine ran and also reading my plugs and would run it in many different situations and tweak things one spot at a time and I had normal changes happen for me and I went up a few sizes at a time on the idle feed restrictors until I had the idle being able to give me a good adjustment range on the mixture screws along with keeping my transfer slot square and my plugs would not foul out and looked good.

I could never get them being a good tan color but it was a light grayish color and that was ten plus years ago and with gasoline being refined and changed over time its had become a lot more harder to read plugs since they are for fuel injected vehicles nowadays and not the same gas of yesteryear especially with ethanol in it. That is also where an o2 air fuel ratio gauge will come in handy be that also is something you can't always go for the leanest number as well as each engine will differ on what it will want to run with.

Camshaft timing and the opening and closing of the valves will also play a big role into things and that is something I know the basics of but can't make a cool post lke Bogie and other guys on here can.
 
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