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Increasing the IAB size should lean the lower rpm (off idle) more than the cruise rpm. Might have to go back to the bigger stock IFRs To get cruise right again.

Easy does it on the sizes. Worth a try. You can always put it back.

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Discussion Starter · #22 ·
Increasing the IAB size should lean the lower rpm (off idle) more than the cruise rpm. Might have to go back to the bigger stock IFRs To get cruise right again.

Easy does it on the sizes. Worth a try. You can always put it back.

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Like you say. Easy enough to try and I can always put it back. How many steps bigger and should the IABs remain the same in all 4 corners?
I have 3 different QFT carbs, SL-600-VS, SL-750-VS, and an SS-850 DP mechanical secondary. The 850 runs real rich on the idle circuit, and the 750 is getting converted to 4 corner idle also, so I ordered a range of IFRs and Squirters. I didn't order any air bleeds, though. I guess I will need to get a range of bleeds also.
 

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I can tell you from experience that changing the idle feed restrictors and getting them within range first is the very most important then before changing air bleeds as you will get a bigger change for the ifr change then an air bleed change. When working with air bleeds you go up or down .04 of a size at a time. From my research on tuning with air bleeds that is an area that is the last to mess with on the idle circuit to get the finest final spot to get it to be the best it can be. Just changing the ifr size by .01 is a double digit change in the percentage of change in the fuel flow but I don't know the math on that without looking at my book. Changing air bleeds does not have that much of an effect like that with just a .004 change at a time though.

That is why I don't mess with air bleeds unless I can change them without having to drill and tap and then using brass allen screws to make what ever size I want. I have only had to change the idle air bleeds about two different times to lean out my idle and that was just from a .70 up to a .078 to get a fine tuned idle calibration.

There is also emulsion tuning in the metering blocks and that is an area I have not messed with as that there will effect the fuel curve quite a bit all over the place and it takes a lot more experience and definitely will need an O2 gauge like your using but its something I have never had to do for a nice street build and yours is not super radical to warrant something like that. I know your wanting to get a magic number of the 14.7 ish number for your engine but you can't always get that with a performance build as they like a richer mixture vs a bone stock motor.

Its just part of the game. When you go bigger with the cam size and higher flowing heads then you have to start to sacrafice other things to get what your looking for.The more radical the engine is the more fuel it will want.

On the idle air bleeds at least I have ranged from .070 to .074 or .078 for the most possible lean mixture. I always tune with the idle feed restrictors and the pump circuit and the idle mixture screws at first before I go to messing with air bleeds. Once I get to where I have what my engine needs then I will try with the air bleeds if they are changeable but on my 600 Holley vacuum secondary I customized on my truck I just left the air bleeds alone and got it right on the money without having to change any of them. I use experience and plug reading and also looking through my files I have kept over the years with many different engine combos and read what all they did with there carbs and kept a library to compare things.

I once had a guy my Father knew that had a Demon carburetor that was the old variety which he got at a parts swap meet and the thing was so out of whack he ran it on a 351 windsor in a late 90's mustang with a manual transmission and the engine had a pretty radical motor and he wanted a good double pumper that would just work. My father told him about me and I took a 650 Holley double pumper and built him a carb from scratch and did new metering blocks and customized them with changeable idle feed restrictors and did some other changes to the calibration to what it was out of the box according to the specs of his engine and I gave it to him and he knows nothing about them and my father adjusted it and set it up for him and let me just say he was so please he swore he ran like fuel injection.

His old carb was all messed up and the old Barry Grant double pumpers which his was an 850, had a style of metering blocks that could not be worked on like the holley ones to be able to change the restrictor size. When I made his 650 double pumper I went by his cam size and some other factors and set it up within a ballpark range and once he had it on he no longer had a pig rich idle and also could tromp on it in all conditions and no bogs or stumbles. I use the old school methods and stuff and use a lot of many builds over the years to kind of have an idea of what a certain engine would want and I get pretty close for a good starting range without having to do a ton of tuning and changing this and that and pulling out your hair.

Your 600 or 750 or even you 850 if I read that write as I did not go back will all give issues until you give your engine what it wants and sometimes your O2 gauge will not give you a correct reading all the time as exhaust reversion and cams with more overlap can mess up with the fuel mixture burn and what its reading. If your cam is correct with the seat to seat duration of 60 ish degrees of overlap, trust me you engine won't run lean like you can get away with on a way milder cam with less overlap. Even mine which has only 42 degrees of overlap would not run with .028/.028 ifr size front and back and it would buck and surge at low speeds because its a 377 cubic inch build unlike my previous 350 and because of that one thing it takes a richer mixture compared to a 350 with all about the same specs minus the cubic inches and a point more in compression.


Like I said I had some bigger sized idle feed restrictors in one of my old 350 builds that had a pretty decent sized cam and I had a rich enough idle on it to give my engine what it wanted and I thought it was just to much but anything less it would run horrible so I gave it what it wanted and then after getting it fine tuned to the leanest I could get it to where it would just start to have some idle problems I went just a step up and bam it was right on the money and I got 19 mpg cruising at 3000 rpm at 60 mph and also driving around town with a 3500 stall convertor with a turbo 350 and 3:73 rear gears I could get around 10 or a hair more mpg in an old 86 chevy s10.

My plugs were clean and mixed with some tan and a faint amount of black but the cam was a choppy idle and was at a 1000 rpm in park and only had 12 inches of vacuum. The O2 gauge is not the end all means of setting things up but just a extra tuning device to get you where your engine wants to be at. Keep working at it but if it runs good even though you might think its to rich you might not ever get the leanest number you want because of your build and what it needs more so then what your looking for in a number.

I hope that does not sound mean and please don't take it like that as I try to go lean on my stuff too but at times no matter how much I try to keep it lean, at the end of the day I have to give it what it wants to run right and strong. You will be a great tuner by the time you get it all ironed out. Also one last note there is some folks that say moving the idle feed restrictor to the bottom of the block vs having it at the top will create a more stable idle circuit tune and keep the air fuel ratio more stable and you would have to tap the bottom of the metering blocks to take a brass allen screw and then drill them to size for whatever size idle feed restrictors you need. You would no longer be able to use the regular quick fuel idle feed restrictors since the bottom of the blocks are not made for those.

I have had mine at both spots and had not noticed any difference in my builds over the years but in certain cases it can help. With the idle feed restricto being above fuel level and mixing with the incoming air in the emulsion part of the metering block it can cause the air and fuel mixture to go up and down depending on certain conditions during low speed driving and I don't know all the information about it but you can do a google search on idle feed restrictor location and read up on what some folks have done with that.
 

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This is not my information but just what I read before and how much truth to it can be debated but take it as an idea.

If moving the idle jet to the top of the block improves throttle response it indicates there was a need for a more aggressive accelerator pump.

Anybody who has the opinion a high idle jet is good is not working with any data systems which supply accurate AFR information or they don’t understand the following information.

Locating the Idle Jet below float level is more important than many people realize because cruising at a low RPM and steady speed the fuel delivery is nearly 100% through the idle circuit (T-slot), and, because intake vacuum oscillates rapidly over a wide range, with the jet above float level the AFR oscillates a lot, as much as 2 AFR or more. Smooth idle, clean sparkplugs and oil, and good fuel economy, all depend on a consistent idle-low speed circuit.

An un-damped vacuum gauge will clearly show engine manifold pressure (vacuum) oscillates constantly and as valve timing becomes more radical the increased overlap causes larger pressure swings and more intense oscillation.

An un-damped gauge clearly shows engines are not a steady flow device. The intake pressure (vacuum) is oscillating with each intake suction stroke and overlap reversion pulse. Because passages and cavities in carbs and manifolds have individual resonances at particular frequencies, the resultant effects are more pronounced at some engine speeds than others. Because air is compressible, it is elastic, it can rebound like a spring and as a result it bounces and reverses direction in small passages in carbs.

With a high (above float level) idle jet, air can easily get upstream behind the jet when the idle fuel is pushed backwards by the pulsing.

When the jet is submerged, the air cannot get back upstream of it as easily because the much greater viscosity of the liquid fuel (than air) on both sides of the jet impedes the backflow and the air cannot get back through the jet unless the flow is reversed for a longer period of time than the frequency of the pulsations.

Another undesirable phenomenon caused by the high idle jet is the larger oscillations in the idle circuit causes a pumping effect on the main well that results in the booster nozzle dripping in spurts and blobs before it would normally start to flow if the venturi air flow were the only excitation and the idle circuit flow was more regular. With a submerged jet the metering reversion isn’t as severe or does not occur at all unless the pressure reverses for a longer time period than the manifold pressure (vacuum) pulses at a cruise.

This submerged idle jet phenomena was thoroughly studied and well understood 95 years ago during aircraft carburetor research for World War One.






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Discussion Starter · #25 ·
Hi Eric!
I know I am being nit picky. This engine runs great. No issues at all. If I didn't have this AFR gauge, I would quit tuning on it and be happy as a pig in...doodoo.;)
I am almost 71 years old. I spend 16 hours a day most every day in my well appointed shop. Puttering with this or that. I don't want to build anymore whole cars. I still have 2 finished hotrods. Well... finished is a strong word. So I spend my time detailing and upgrading. Right now carbs are on my mind. Gives me something to play with. I like doing the experimenting. Also gives me a reason to go for a ride.
What I like about these QFTs is they are so adjustable. I can make a change, go for a ride, mull over the results and decide if the change was good or not. Change it back if not.
I enjoy the learning. I raced late model dirt cars for a long time. Always on alcohol. I never messed with the carbs. I bought carbs from the name boutique carb builders through my engine builder. I stuck them on and they always performed. In the last few weeks I have learned more about carbs than in the previous 50 years
 

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Don't feel bad I get nitpicky with my stuff as well and love to mess with things and especially with carburetors as I even have one as a display that I plan on to never use as its just like a hotrod sign on the wall or a cooking grill that looks like a motor lol but mine is a new Holley 600 that is all nice and shiny and I just put it up for display to look at like a trophy lol. I know how it is as I have spent days and hours over the years and I love to know how to do things and how they operate and always try to learn new things while my health will allow me.

I had cancer at 36 years old back in 2012 so I had 35 rounds of radiation and 9 rounds of chemotherapy over a 7 week period for the rads and 6 months for the chemo and it was to my head and neck and I did receive some damage to my brain and stuff and I have lost a lot of some of my cognitive skills and struggle with memory and certain things and some days are better then others. I have been out of work since then and don't function very well at times and its a hard life dealing with things and I can't even remember peoples names like I used to. To much to list here but I do with what I still got as best as I can but that is one reason I am detuning my engine and making it a lot milder cause as the time goes forward I might not be able to do the things like I used to even though I am only 45 because radiations can even effect you many years lator on in life and its getting harder and harder for me to do basic tasks anymore with out getting scatter brained and forgetting where I left off and where I put my tools at etc.

That is why I am having my Father building this for me as I can't afford to make a minor mistake that can destroy my build. I can do some minor things but not a lot of the bigger stuff anymore. I am hoping before I get to bad that I can rebuild a 350 turbo although with my Fathers help though to watch over me to at least say I was able to do one in in life before my time is up. Keep at it my good man.
 

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Discussion Starter · #27 · (Edited)
This is not my information but just what I read before and how much truth to it can be debated but take it as an idea.

If moving the idle jet to the top of the block improves throttle response it indicates there was a need for a more aggressive accelerator pump.

Anybody who has the opinion a high idle jet is good is not working with any data systems which supply accurate AFR information or they don’t understand the following information.

Locating the Idle Jet below float level is more important than many people realize because cruising at a low RPM and steady speed the fuel delivery is nearly 100% through the idle circuit (T-slot), and, because intake vacuum oscillates rapidly over a wide range, with the jet above float level the AFR oscillates a lot, as much as 2 AFR or more. Smooth idle, clean sparkplugs and oil, and good fuel economy, all depend on a consistent idle-low speed circuit.

An un-damped vacuum gauge will clearly show engine manifold pressure (vacuum) oscillates constantly and as valve timing becomes more radical the increased overlap causes larger pressure swings and more intense oscillation.

An un-damped gauge clearly shows engines are not a steady flow device. The intake pressure (vacuum) is oscillating with each intake suction stroke and overlap reversion pulse. Because passages and cavities in carbs and manifolds have individual resonances at particular frequencies, the resultant effects are more pronounced at some engine speeds than others. Because air is compressible, it is elastic, it can rebound like a spring and as a result it bounces and reverses direction in small passages in carbs.

With a high (above float level) idle jet, air can easily get upstream behind the jet when the idle fuel is pushed backwards by the pulsing.

When the jet is submerged, the air cannot get back upstream of it as easily because the much greater viscosity of the liquid fuel (than air) on both sides of the jet impedes the backflow and the air cannot get back through the jet unless the flow is reversed for a longer period of time than the frequency of the pulsations.

Another undesirable phenomenon caused by the high idle jet is the larger oscillations in the idle circuit causes a pumping effect on the main well that results in the booster nozzle dripping in spurts and blobs before it would normally start to flow if the venturi air flow were the only excitation and the idle circuit flow was more regular. With a submerged jet the metering reversion isn’t as severe or does not occur at all unless the pressure reverses for a longer time period than the manifold pressure (vacuum) pulses at a cruise.

This submerged idle jet phenomena was thoroughly studied and well understood 95 years ago during aircraft carburetor research for World War One.






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That is interesting. I just looked at a metering block. I think it would be tough to do because of the idle mix screw. I'll bet somebody has done it though. The concept makes perfect sense though.
Edit: I went back through your posts and see where you had suggested using a brass set screw. Boy, it would still be a chore with that idle screw coming in right below it. I guess if it got messed up it would only cost a metering plate, not a whole carb body.
 

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Like you say. Easy enough to try and I can always put it back. How many steps bigger and should the IABs remain the same in all 4 corners?
I have 3 different QFT carbs, SL-600-VS, SL-750-VS, and an SS-850 DP mechanical secondary. The 850 runs real rich on the idle circuit, and the 750 is getting converted to 4 corner idle also, so I ordered a range of IFRs and Squirters. I didn't order any air bleeds, though. I guess I will need to get a range of bleeds also.
Aren't we working on the 600 VS? Sounds like your ideal will be with IFRs between 0.29 - 0.31 and slightly bigger IABs, size TBD. I would do all 4 corners the same. There will be an IAB size that will give you proper cruise mix for each IFR size. But the off idle mix should be leaner as the IAB size increases. I think 2 sizes at a time on IABs is recommended. Good luck.

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Discussion Starter · #29 ·
Yes. We are working on the 600. I just have 2 more carbs waiting in the wings. HA. Maybe I can mess up all three of them! The SS-850 is on my 383. Pig rich at idle. But that engine is a whole different animal. 11:1 CR, 210 Profiler Heads, Hi rise air gap, Straub HR cam. Approx. 500hp. In a 2000# car, it can get exciting.. The 750 is what my 383 started with. I'm going to convert it to 4 corner idle like the 600 and may stick it back on the 383. I won't start on any of that though until I get the 600 sorted.
 

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Discussion Starter · #30 · (Edited)
OK. Back to tuning the 600.
Leaving the 28 IFRs and changing the IABs from 70 to 74. This seemed to lean the idle very little. Still about 12.4, but it did lean the 1800 rpm cruise to 15-17 the peg buried once in a while at 18. It runs just fine once fully warm. No stumbles. Can't hear any pinging. WOT is still 12.8 - 13:1. But those are awfully high numbers at cruise and it didn't lean the idle.
Just got back from the test drive and am noodling over what step to take next. With this last test showing little gain in leaning the idle, I am not sure the idle could be leaned any more without blowing the cruise AFR right out of the water. I don't want to do it, but is this where guys end up drilling the throttle plates to lean up the idle?
Right now it looks like the 28 IFRs with the 70 IAB's is the best setup. Right where I started!
 

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OK. Back to tuning the 600.
Leaving the 28 IFRs and changing the IABs from 70 to 74. This seemed to lean the idle very little. Still about 12.4, but it did lean the 1800 rpm cruise to 15-17 the peg buried once in a while at 18. It runs just fine once fully warm. No stumbles. Can't hear any pinging. WOT is still 12.8 - 13:1. But those are awfully high numbers at cruise and it didn't lean the idle.
Just got back from the test drive and am noodling over what step to take next. With this last test showing little gain in leaning the idle, I am not sure the idle could be leaned any more without blowing the cruise AFR right out of the water. I don't want to do it, but is this where guys end up drilling the throttle plates to lean up the idle?
Right now it looks like the 28 IFRs with the 70 IAB's is the best setup. Right where I started!
The idle mix itself itself is just the 4 mixture screws. Turn them in to lean the idle. Yes, right where you started with the carb may be the best. If you want to play some more......

Go to 72's on the IABs - or go to 29 or 30s on the IFRs and keep the 74 IABs - either should richen the off idle and cruise AFR just a tad. But your choice of IAB should determine where the emphasis of the richening will be.

Looks like you're very close. Super easy does it. No way should you have to drill throttle plates with a mild cam (even using a hot cam, with the 4-corner idle).
 

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Discussion Starter · #32 ·
Well, I slept on it last night. This is my thinking this morning.
Yeah, I don't want to start drilling holes, but it does seem like this is where guys do start drilling. I have tried to lean the idle by screwing in the idle mix screws, but the idle quality starts to degrade pretty quickly. I am only at 1/2 turn out on the idle mix screws. This may prove out eric32's contention that the overlap in the cam requires a richer mixture, and I may have to live with it.
I think maybe this morning I will try 30 IFRs and leave the 74 IABs. I would like to richen the cruise a little without richening the idle. I'm thinking that by richening the IFR, the idle mix screws will have to be screwed in a certain amount. This will get the idle mix screws to about 1/4 to 1/2 turns out. I also have experienced an off-idle stumble when the IMSs are too far in. Eric32 said earlier that when the IMS are out less than 1/2, the IFRs are probably too rich. It only takes a few minutes to change them out, so easy enough to go back. We will see.
 

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Discussion Starter · #33 ·
Well, that richened it up across the board. Idle went to 11.8 and 1800 cruise went to 14-16. I got a couple of small lean backfires when going to 1/2 - 3/4 throttle before it was completely warmed up. After it warmed up, it still has a stumble when going WOT. This surprised me as it didn't happen with the 28 IFRs and 74 IABs. ???? Anyway, I don't think this is the right direction. Seat of the pants says the 28 IFRs with the 70 IABs is the best performing combination. The idle is rich with this setup, but I might have to live with it. I guess I could go back to the 28 IFRs and try some 72 IABs.
I guess it is possible that if I got the idle leaned out, I could cause problems with the transition and be back to that off-idle stumble. I've read that when tuning for economy, the transitions are the hardest to get correct.
 

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I thought you was pretty close before. Just watch for surging from being too lean during cruise. Cool dry weather will require the most fuel so if ok now, it should be ok when it is hotter and more humid out. I wouldnt sweat the idle mix as long as it leans out off idle.

One thing... You have been installing ready-drilled jets and bleeds manufactured by Quick Fuel, correct? Switching manufacturers or drilling yourself is not the best when fine tuning. Flows can vary with same size orifice. Depends on depth and how the ends of the hole is finished.

Im really interested in hearing the fuel mileage at a 60 mph steady cruise in this light vehicle! Do you lock up the converter in OD?



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Discussion Starter · #35 ·
I thought you was pretty close before. Just watch for surging from being too lean during cruise. Cool dry weather will require the most fuel so if ok now, it should be ok when it is hotter and more humid out. I wouldnt sweat the idle mix as long as it leans out off idle.

One thing... You have been installing ready-drilled jets and bleeds manufactured by Quick Fuel, correct? Switching manufacturers or drilling yourself is not the best when fine tuning. Flows can vary with same size orifice. Depends on depth and how the ends of the hole is finished.

Im really interested in hearing the fuel mileage at a 65 mph steady cruise in this light vehicle! Do you lock up the converter in OD?



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Yes, I have been using all QFT predrilled jets. I have been thinking about drilling the 25 IFRs I bought out to 29 though. Enlarging one ought to be easier than drilling a blank.
As far as gas mileage, I am afraid that it is going to be disappointing at 65 mph. If the word aerodynamic was in the dictionary in 1926, it certainly wasn't in Henry Ford's engineer's vocabulary. Dale Earnhardt could probably "see" the air piling up in front of it.
This car really is my daily driver. It very rarely gets on the interstate, so 60 or so is its usual top speed, about 1800-2000 rpm. It will run 70-80 mph nicely, but my wife doesn't like it. Two years ago, we took a 1700 mile vacation in it and never got on the interstates.
None of my tuning has involved any cruising on the main jets, but I know that WOT is OK at 12.8-13:1. Would cruising at 3000 rpm in 3rd gear (1:1) net the same AFR as cruising at 3000 rpm in 4th (.7 OD)? I ask because 3000 rpm in 4th gear OD is 90+ miles per hour. Don't need any tickets trying to tune a carb for a range it will never see.
Thanks for your help!
Oh yeah. The converter locks up in 4th only and it is real quick.
 

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Cant be less aerodynamic than a S10 brick! 50-60 mph, 65 mph it should get pretty good mpg and still be able to get with the program at 2500#. Like i said I'm interested to see how it does. Now wishing I had gone the street rod route instead of dedicated drag race vehicle, ever since my track closes permanently last month. Once the cage is in, its pretty much difficult to go back.

Yeah, set your higher rpm cruise in a lower gear.

P.S. hand drilled jets wont flow the same as the purchased ones, so if you are trying to make steps in AFR be aware.

Sounds like you would have lots of fun with a basic data logger. Rpm, afr, mph, map sensor. My Daytona Sensors ignition box does this but its kind of inconvenient to take the data off or display it. Still I have found it useful. Also you can lock your distributor out and program the centrifugal advance curve electronically.

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I can’t say I agree on purchased jets being preferable. I’ve measured quite a number and found as much as .0025“ off the stamped size on IFR, PVR, idle and main air bleeds. Also on main jet the sizes were off a lot on a pair as original installed from Holley/Quick Fuel.

When I drill my own at least I’m getting a matched set. Flow might not be same as purchased brass but I can size up or down to achieve results desired. I use the predrilled .020” hole brass that Holley/Quick Fuel sell when space permits the flanged head. 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.
 

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When I drill my own at least I’m getting a matched set. Flow might not be same as purchased brass but I can size up or down to achieve results desired.
This is what I am warning the OP about. Mixing in hand-drilled ones with the factory ones, expecting flows to correspond to measured drill size. That's all. Either drill them all yourself using the same starting piece or buy them all from the same maker if you want to do stepwise comparison tuning. So I am agreeing with you, but not for the case of the OP who has been using factory QF jets.
 

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The only issue with that is all the purchased brass size ordered of one given size is not the same. Not always but enough to be leery of the quality. I’m not the only one to experience this as others have commented on same issue. Given a mix of “out of spec” brass may in some cases never be detectable in the carb’s performance by most.

I have a habit of pin measuring everything both purchased and home made on hand when I build. Honestly I feel the home brewed brass is preferable to me. But purchased brass out of size can alway be resized (bigger) or use as the size it pins at to use in another application.

I’ve started medicine prescription jars labeled in pin size as a stock pile to have on hand.
 

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Discussion Starter · #40 ·
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.
 
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