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Discussion Starter · #41 ·
Drilling/threading lower hole IFR in the meter block does not accept the flanged brass so #6 set screws is a necessity.

It’s true pre drilled brass is a whole lot easier to achieve desired finished hole. And solid 3/16” brass #6 set screws will require a larger C-bore prior to hole drill so the thickness of the hole depth is not too thick. The flow thru the C-bore may affect flow but it can be filed off flush.
Apparently, you have done the modification to lower the position of the IFR's in the metering block. Did it make a noticeable difference in idle quality?
 

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Apparently, you have done the modification to lower the position of the IFR's in the metering block. Did it make a noticeable difference in idle quality?
That was the latest change IFR move top to bottom. I was getting a pulsing at lower speeds. So I figured it was worth a try since so many guys say the performance of lower IFR’s was superior at providing and even flow. That was the only modification since last summer. We will see how it goes.

Prior to these “newer” carburetors be available everything was lower IFR’s.

On a another car I just changed my 327’s Holley 3810 from .031 stock IFR’s to .025 w/#6 set screws they are all lower position. Also changed out meter plate to meter block. Setting up idle was instantly better with adjustment screws.

I will suggest a 3 or 4 flute #6 tap after tap drilling for threading into meter blocks. They definitely center better than typical 2 flute tap. Also new sharp tap and drill since the meter blocks are soft material. A bit of lube helps to. You will also need a bottom tap if using 3/16” long set screws.

You don’t want to ruin a good meter block. One ruined block is enough to convince one of the need to be diligent and patient.........

Need to wait till “normal driving weather” to test out both cars, it’s snowing now. Issue with north east one has to be patient for good weather.
 

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Discussion Starter · #43 ·
That was the latest change IFR move top to bottom. I was getting a pulsing at lower speeds. So I figured it was worth a try since so many guys say the performance of lower IFR’s was superior at providing and even flow. That was the only modification since last summer. We will see how it goes.

Prior to these “newer” carburetors be available everything was lower IFR’s.

On a another car I just changed my 327’s Holley 3810 from .031 stock IFR’s to .025 w/#6 set screws they are all lower position. Also changed out meter plate to meter block. Setting up idle was instantly better with adjustment screws.

I will suggest a 3 or 4 flute #6 tap after tap drilling for threading into meter blocks. They definitely center better than typical 2 flute tap. Also new sharp tap and drill since the meter blocks are soft material. A bit of lube helps to. You will also need a bottom tap if using 3/16” long set screws.

You don’t want to ruin a good meter block. One ruined block is enough to convince one of the need to be diligent and patient.........

Need to wait till “normal driving weather” to test out both cars, it’s snowing now. Issue with north east one has to be patient for good weather.
Yeah! I'm on hold for the weather also. We got 4" of snow last night! This is the latest I remember ever getting snow! It is almost 50 now and melting fast.
I am very interested in lowering the ifrs. Am I correct that the lower hole that gets drilled and tapped is the hole that you can see the end of the idle mixture screw in. There are 2 holes there that you can see the IMS in. The one we are talking about is the one at the bottom of the slot, Correct? It looks like a very delicate operation. Not sure these old eyes and shaky hands can do that anymore.
 

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I recently bought a range of IFRs, air bleeds and main jets for my tuning experiment. I pin measured all the IFRs, Air bleeds, and main jets. Every one was tight with the correct sized pin, but the next size smaller slid easily into them. So all were within .001.
After determining that I was not going to use the .025 IFRs, I did try drill them out with an .031 bit. It produced a .032 hole. I decided then to use only predrilled jets and pin measure every one.
I bought a 30 day pill organizer for storage of all the little pieces. Labeled everything. Makes a great tuning kit.
The .001” difference for idle air bleeds or main air bleed should not be an issue. BUT for IFR brass the .001“ is too much variation. IFR Is way more sensitive to hole size changes. A range of .006” is about all you are working with to tune the idle circuit on a street car. IFR brass you want dead on the money size wise. Tight with the correct size pin is what you want. A pin .001” smaller should be loose while a .0005”/.001” bigger should not go in.

Mic each pin prior to checking brass as the wrong size pin is easy to mistake for the correct one.

if you are drilling your own brass be sure to multi steps in drill size. Final sizing s/b in .001” steps. The finer the drill cuts the more precise the finish on the hole will be.
 

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Discussion Starter · #46 ·
Ah ha! I had the wrong hole. I'm still learning. That one doesn't appear to be quite as tricky. I have an old 3310 parts carb. I took it apart to see if I could practice on it. It does have the low restrictor. Just drilled .031, not a jet. I don't think that would make a difference. I'm going to give it a try on the old one.
Thanks for the pic and info.
 

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I think those top holes are the siphon holes and you don't want to close those off or mess with them. I know some people do but I have never once ever messed with those and you don't want to mess with what is there for a reason. As far as what tap to use I use the flute spiral tap and never had to use a bottom tap in my metering blocks and I have always came out just fine. All I do is a few turns at a time and then blow it out with compressed air and then screw in my brass set screw and see how far it goes in and how many more turns I need to go. I go back and forth till I get it flush to the gasket will fit over with out any issues. The 3/16 length works fine and have done at least ten plus blocks with that method and never failed once.

I myself have not noticed a difference by going from one spot to the other between the cast blocks with the mod and the billet blocks leaving them at the top. If I am going to drill anything then yes I always put them at the bottom. I am getting ready to setup a 600 Quick fuel brawler which basically is the 600 slayer but with a set of side hung fuel bowls here when I get it as my Holley I have on there runs fine but its a used carb with some older parts and the blocks are only a year old and I modified them for brass allen screws and they work good but I am going to just sell it to off set the cost and start from new with my new build coming here in about a month or so depending on when my Father can get it build.

I could do it but my health is just poor right now I can hardly bend over and mess with a carb and adjust it for very long and other things. But I will have to calibrate it with idle feed restrictor change that I know since I will be even milder with my up coming 350 build with only the change being the short block and all else the same. I will be a point down in compression and loose 27 cubic inches and some breathing but I am needing things easier for future use if I have a future. Keep letting us know how your stuff comes out I will be curious the changes and how it makes and how mine will come out.

Hopefully mine will come with the right air bleeds as the last one I got was completely off. They gave me .063 high speed air bleeds and .070 secondary idle air bleeds on a 2 corner idle setup lol. Mains would have been so lean at wide open throttle it would have been horrible as they should be around a normal .028 to .031 on average depending on jetting and other factors.
 

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Discussion Starter · #48 ·
I came across this thread. Relocating the Holley Idle Jet. This afternoon I drilled and tapped the old 3310 block for the lower IFR restrictors. Wasn't really a problem.
I have a couple of questions on the depth. The set screw should bottom where the top of the screw completely exposes the hole that runs on an angle towards the center in that same depression?Correct? Using an 1/8" screw, that's how deep I set them. So I could just see the bottom of the angled hole. I see that eric32 said he only sinks the ifr screw until it is below gasket level, but uses a 3/16" screw. Is this far enough or did I misunderstand? I'm not sure what that angled hole does? I bought 1/8 and 3/16 so I could use either one.
I haven't drilled any set screws yet. 57Nomad mentioned counterboring. Is this necessary with an 1/8" screw? What size and depth counterbore?
I kind of hate to go to drilled jets as mentioned by 2001Blazer4X4, but there is no other choice with the low restrictor.
Sorry for all these questions. Now I just have to work up the courage to drill into $150 worth of metering plates on my 600! Those old 3310 cast metering blocks weigh about twice what the new billet blocks do. I assume the tapping will go easier in the new blocks.
Now that the snow is gone, I'm going to drive it for a while with the 28 IFRs and 70 IABs to get a better feel for what it is doing before making any changes. The temperature changing 50 degrees day to day is going to make it difficult to tune anyway.
I always get points for taking the wife for a ride. I think I will give her a pad and pencil and make her my data logger.;)

Something I have seen that puzzles me . Start the engine cold. Electric choke is on. Idling on high idle, then not touching the pedal. AFR will start at 14.5 or so. As the choke opens up you can watch the AFR go rich at 12.5 - 13. I would have thought that it would have worked just opposite to that.
Something else. This engine seems to be cold blooded anyway without the exhaust crossover in the intake, but it seems that it starts and warms quicker with the richer ifr's in it. Is this a thing? Thanks guys.
 

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The only thing withe the depth that you have to worry about it not to go to far to where you can't have a stopping point for the brass set screws to tighten up on. I just go down as far enough to where the top part of it sits about flush give or take a hair so the metering block gasket sits over it with no issues and it about flush with the metering block but its not precise like a pressed bleed is. And yes there is a hole at the side of it that should be an emulsion hole and when you drill the pressed in ones out you will see it more but you don't have to worry if it shows a little more or less then the other side at least it has never been a problem for me and I usually get both sides within 99 percent of each other in matching height a I always go the same amount of turns.

The trick is to do so many turns at a time then back all the way out and while turning you go half a turn then back out half a turn then go back to where its tight and turn some more then back out half a turn then proceed to tap more threads. That helps to get the material to come out and make the best threads possible and also the zinc blocks are harder then aluminum blocks so it takes a little more in the feel when you do it. The most critical thing is to only do so much at a time and then blow it out and take your brass allen screw which I do believe it takes a 1/16 allen wrench and turn it in and see how far it goes and then you will not about how many more turns of the tap you need to go till it till sit flush with the metering block. You basically want it to go in far enough to clear the gasket but not so far that you go to far in and tap down to far and you can't get the brass set screw to tighten up and juts spin all the way through.

Trust me its not hard at all. I was scared the first time I did it but I was successful the very first time and am a 100 percent success rate and if I can get that I am sure you can sine I have only that tapping experience my whole life time. I have never tapped anything else wise over the years. The 1/8 inch size which is what I will use in the future since its less material to drill through, will be my choice and will still take the same principal and you also won't be digging in as far but you won't get any difference between either size as far as that goes. The 3/16 size just goes down 1/16 of an inch further.
 

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The C-bore is only a means of removing brass material so the the bleed hole is not to thick. See the thickness (length) of the brass set screw will affect the passage of air/fuel thru the small (approx .025” to .031”) hole in the set screw. Thinner equals more flow and thicker lesser flow due to the longer restriction time as fuel/air passes thru oriface.

By using a shorter #6 set screw of 1/8” vs 3/16” there is less material between cup point and bottom of hex and possibly no C-bore would be necessary. I’ve only experience with 3/16” long #6 brass set screws with a cup point. By the way the center of the cup point helps center pilot drill.
 

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Discussion Starter · #51 ·
I was just kinda crudely comparing a QFT restrictor to an 1/8" set screw, it appears the orifice depth is very close. Within a few thou. So probably no counterbore required.
I had the 600 apart this morning. It has been running pretty lean on idle circuit cruise, 15-1800 rpm with the 28 IFRs. I decided to try 30's and if it makes the idle way too rich, I will try leaning with the IABs which ae 70 right now. That magic setup is in there somewhere. All you gotta do, is find it.
So new setup is
IFR.....30
APDN......31
IAB......70
WOT with any combo I have tried has never been lean, so I am not too worried about that yet. The only time it ever sees WOT is when I'm testing. With the 700 r4's OD, the engine never sees enough rpm to get on the main circuit unless I am standing on the throttle.
The weather looks like it will settle in a couple of days for better results.
 

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You also could leave the .028” IFR in and change the IAB to .066”. Both methods send you in the same direction of less air more fuel. Just the volume is lower scale. I do not know which is a better solution but based on the low rpm you run at less volume might be a benefit.
 

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Discussion Starter · #53 ·
Areas for Adjustment on a QFT SL600VS:
1. Curb idle (T Slot)
2. Secondary idle
3. Idle metering screws
4. Idle feed restrictor
5. Idle air bleed
6. Main jets
7. High speed bleed
8. Power valve
9. Power valve channel restrictors
10. Vacuum secondary opening point
11. Choke
12. High idle

Did I miss any?

There must be trillions of different combinations of these 12 adjustable areas. What do you reckon the odds are of hitting that "perfect" combo?

I'm waiting for the roads to dry out this morning. In the meantime, I have had it running in the shop while adjusting the IMS and idle speed. With the 30 IFRs I am only at 3/8 turn out on the IMS, but I believe that this is the best idle quality out of any IFR I have tried. Also the best idle vacuum reading I have had @16. AFR is better @ 13.2. Seems to take fuel well. Starts better. Not quite as cold natured. The proof will be in the test drive. I'm hoping for no stumbles on transition and slightly richer AFR @ cruise.

I've a long way to go. I'm still adjusting on IFR's and IAB's and IMS's.

You also could leave the .028” IFR in and change the IAB to .066”. Both methods send you in the same direction of less air more fuel. Just the volume is lower scale. I do not know which is a better solution but based on the low rpm you run at less volume might be a benefit.
Yep. That's another way to go. I'll probably try that too. I'm getting pretty quick at changing IFRs and IABs.
 

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The perfect combo is out there. Will you hit it 100%........ no, but chances are you’ll get something very acceptable to yourself.
 

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Here's the other issue to consider. Air density changes throughout the day/year as temperature, barometric pressure, and humidity varies. The more dense the air is, the more oxygen it contains and the more fuel it takes to get an optimal A/F ratio. Precise adjustment with a computer and sensors can be made automatic, as it has been made automatic on new vehicles. But with a carb on the street you have to do what it needs to run good all the time. Better to err slightly on the rich side when setting it up, than too lean. That way if you get into some really good air, then it won't give problems. Adjusting an optimized race carb for weather conditions is much simpler than a carb for the street since you are only concerned about full throttle performance - main/secondary jet change and you are good to go. But not so for a street carb with all circuits optimized for a specific air density - better to be slightly rich, or tune during the best air that you will be running in.

As an aside, note that your engine will give proportionally more power in good, denser air where there is more oxygen to burn more fuel (provided fuel mix is still good).

Here's a site that gives air density forecast data (historical too) for tracks all around the country. With a change in the weather, today the air density varies 6% in my area. Typically see total changes as much as 15% or more over the course of the year. Difficult to be precise with the weather changing all the time.
Weather forecast: Osceola Dragway
 

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Discussion Starter · #57 ·
Here's the other issue to consider. Air density changes throughout the day/year as temperature, barometric pressure, and humidity varies. The more dense the air is, the more oxygen it contains and the more fuel it takes to get an optimal A/F ratio. Precise adjustment with a computer and sensors can be made automatic, as it has been made automatic on new vehicles. But with a carb on the street you have to do what it needs to run good all the time. Better to err slightly on the rich side when setting it up, than too lean. That way if you get into some really good air, then it won't give problems. Adjusting an optimized race carb for weather conditions is much simpler than a carb for the street since you are only concerned about full throttle performance - main/secondary jet change and you are good to go. But not so for a street carb with all circuits optimized for a specific air density - better to be slightly rich, or tune during the best air that you will be running in.

As an aside, note that your engine will give proportionally more power in good, denser air where there is more oxygen to burn more fuel (provided fuel mix is still good).

Here's a site that gives air density forecast data (historical too) for tracks all around the country. With a change in the weather, today the air density varies 6% in my area. Typically see total changes as much as 15% or more over the course of the year. Difficult to be precise with the weather changing all the time.
Weather forecast: Osceola Dragway
Yep! I see guys with air density meters at the sprint car races all the time nowadays. As you say, race carb tuning is a lot easier. I never messed with more than main jets as long as it ran good 4500 - 7500 rpms. I am well aware of weather changes and tuning problems. I am hoping that tuning for 50 degrees and then seeing what the difference is at 75 degrees or higher, I can start forming an opinion about what will work well in both temperatures.
I really appreciate all you guy's thoughts.
It looks like the roads are about dry. Time for a drive.
 

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Discussion Starter · #58 ·
Back from the test drive. It is 50 degrees, windy and sunny. Engine stumbles around until completely warmed up. Doesn't want to idle. Once fully warm, that all goes away and runs great. Transitions well. Idle steady @13.2 AFR @ 16" vac @800 rpm. 1800 rpm or less cruise 14.5 -16 AFR @ 16" vac. That cruise afr and vac is running with the wind. It's pretty windy today. When I turned around to come back on the same road, into the wind, 1800 rpm cruise became 12.0 AFR @ 14" vac. I expected a change but not that much. I think that speaks to the vertical flatness of my Model T and the wind catching visor. Might have to think about some holes in it. A stiff crosswind didn't lower the AFR, but it does buffet the car about pretty good.
I'm going to drive it a while longer like this, but I liked this change so far.
 

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Discussion Starter · #59 ·
Had the car out again today for some more testing. Even though it is 80 degrees today, the AFR readings didn't change much at all from 50 degrees.
I think I am going to leave the idle transition circuit with the 30 IFR;s and 70 IAB's and call it good for now. Time to move onto the mains and power valve.
Today the 3000 rpm cruise in 3rd gear (1:1) is way rich in the 10-11 range. It has 68/74 jets in it now, stock. PV is 6.5. PVCR are .052. WOT is in the 11-12 range. I think I will try 66/72 main jets and see where that ends up.
 

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Discussion Starter · #60 ·
Wow. If I didn't know that the main jets aren't supposed to affect idle and transition AFR, I would say that they definitely do. The jet change from 68/74 to 66/72 leaned the idle to almost 14, the transition and the 1800 rpm cruise to 15.5 - 17. WOT is still rich in the low 11's. There is also a very, very slight hesitation at part throttle acceleration. 3000 steady cruise gives 12:1. Still very rich, but better. Didn't notice any surging. I'm not sure if I could lean out the mains anymore without going back and fattening up the IFR if the next step leaner produced similar results. Again going to drive it and think about it for awhile before the next move.

I was able to get (4) 6-32 x 1/8" set screws drilled for 028 and (4) 030's drilled to my satisfaction and may go after lowering the IFR's.
 
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