I have a QFT SS-850 (double pumper) on a 493 RB stroker. Holley Street Dominator intake, Hooker headers, Lunati 285/285 degrees cam, 10.3:1 CR. Car is a 70 Challenger for street use only.
Timing is 20° at idle (measured with vac adv. blocked), 35° all in. I use ported vacuum for vacuum advance.

The problem I have is that turning the mixture screws front or rear doesn't seem to have much of an effect on idle AFR (or idle speed).

I started with the rear butterflies left in the factory position QFT had adjusted them. Was slightly open. Front butterflies in a position that the transition slots were just square. After starting and warming up I set idle to about 800 rpm by turning the primary idle speed screw, then turned the idle mixture screws (all four of them in equal turns), quarter of a turn each time. Watched for changes in rpm, AFR and vacuum. Not much is happening from turning the idle mixture screws. Mixture screws are out about 3/4th of a turn each. Turning them in raises rpm slightly (leaner ?!).
Idle AFR is at about 14.55 and I cannot change it more than 0.1 points by mods of the mixture screws.
Idle vacuum in gear is about 11", in N is 13" with the 3/4-turns-setting.
Opening the front butterflies enrichened idle slightly. Was it approaching the transition circuit rather than idle circuit? Would that be the wrong direction? I didn't follow that way because I felt it would be wrong.

Today I went the other way: Had the carb off, openend the rear butterflies so that the transition slots were just not visible from below. Set the primaries again to squares again. Started the engine, let it warm up. Found out that I could get the idle slightly richer when I opened the rear butterflies a bit more, but idle rpm went up. I closed the front butterflies to decrease idle rpm. I had to close them completely and still idle is a bit too high for my feeling. About 50 to 100 rpms more than I am used to. So, similar behaviour as above, where I opened the primary BF and it went richer.
Idle AFR is at 14.25 with this setting (rears open, fronts closed). But still the mixture screws don't respond. Again, I only can achieve changes of about 0.1 AFR with them.

I tried the original 37 IFRs front and rear and now 33s front and rear. Behaviour is the same, AFR values the same. Mixture screws turned out nearly the same amount as before. 3/4 front, 1 rear.
What is wrong?

Are you actually idling on the idle and not the transfer or main circuits?
What happens if you screw all four screws in? Engine should die.

More than likely you have to much throttle blade opening exposing the transfer slot. Or and this is entirely likely, there is to much slot exposed. But I'm no expert..Id start by closing the throttle blades and with the mixtures screws out 1.5-2 turns.

Short version:
That's a little bit more initial timing than your engine combo seems to want or need. Set the initial timing to 18 BTDC.
That should help reduce the idle speed.

It should also let you set the initial throttle positions closer to what is needed. With four corner idle and mechanical secondaries, try factory setting for the secondaries or all four throttles about the same.

There seems to be enough manifold vacuum that it is to be expected that the idle mix screws aren't very open. However they should be responsive.
Four corner idle is something equivalent to doubling the circuits that supply fuel based on manifold pressure. If anything, make the IFRs smaller. But one change at a time!

Summary: Set timing at 18 degrees. Set throttles with primary square and secondaries the same or factory.
(Tip. Write down how far in the throttle position screw has been turned to achieve square transfer slot. Then open the primaries another 1/4 turn and another 1/4 turn. Write down how much further it opens before resetting it to square. Now you'll always know how much transfer slot is showing)
Warm up engine and see if you can achieve good idle in drive by adjusting mixture screws. If you have to adjust throttle, then just note how much.

If the 33 IFRs are in there now, leave them for now. Based on what you have written, stepping a little smaller is probably the direction to test next.

Longer version:
The lack of response is probably because most of the fuel has been coming out of the transfer slots. In the first test it was the primary transfer slots, in the second case it was when you opened the secondaries a bit more but closed the primaries.

The large displacement and strong compression helps generate half decent combustion at idle even with that cam. In other words it burns reasonably well and fast compared to many hot rodded engines with lower compression and lots of overlap in the cam.

So I'm guessing that the 11" of vac in gear probably is a little more than QF expected, so it pulls all the fuel needed with just one pair of transfer slots exposed below the blades.

Reducing the timing will generate slightly less vacuum, which should let all the butterflies be where they should. It should also time the burn so a richer mix will generate more effective pressure.

As far as IFRs go. They restrict fuel to both the idle and the transfer slot.
The transfer slot is a restriction to fuel and above the blade is a restriction to air mixing into that fuel. This is why the throttle plate's position relative to the transfer slots has to be withing a specific range.

The Idle Air Bleeds also effect both the idle port and the transfer slot.
Together the IFR and IAB will determine when the fuel from the idle system tapers off as the throttles open up.

One issue with most newer carbs is the IFRs are placed above the fuel level. This can result in less consistant behavior.
(If you find this is an issue, you can relocate the IFR.)

One other possibility is that there is too much bypass air. Bypass is when extra air is allowed past the throttle blades so they can be closed down to the correct initial position. Holley did this by drilling small holes in the throttle plates.
Barry Grant and others used an idea from Bill Jones to create an adjustable 'idle-eze'. Your carb may have that. If so it will say so in the marketing and instructions.
Another way to do achieve the same is to alter the amount of air allowed through the PCV valve at idle vacuum.

In any event, the goal in this case would appear to be allowing less air to bypass if the carb has any of these provisions to allow extra air.

Adjust the throttle blades so neither the front or rear are exposing the transfer slots
It will idle fine on both idle circuits(front and rear) only if the throttle blades are not exposing the transfer slots
BTW, there has been a lot of misinformation on this forum about adjusting the throttle blades to expose the transfer slots, DON'T DO THAT

No transfer slot exposure at all? Everything I've read (not just here) calls for about .020" (little square of the slot just showing beneath the throttle blades).

I'm no carb expert but won't you get an off-idle bog if the transfer slots are completely covered?

That is how most 4150s are designed to work. (.020 to .040 on an old school Holley) All of those relationships are pretty much right from Holley's Mike Urich, Vice President of Engineering for Holley. (see pages 121-122 of the 1987 edition, p/n 36-73).

The reason is not so much about obtaining fuel at idle speed. It is the off idle characteristics as the air bleed shrinks and the fuel port enlarges. That's explained in the side bar on p. 121
That book alone did not get me there, but it certainly provided a good start.

There are people who make carbs work in certain situations, particularly racing situations, who have other approaches. Some of them are quite successful; Braswell comes to mind as an example. I would be a fool to say they don't work. But they are not the methods for the average tuner. The average tuner needs to work within the parameters that produce the most consistant flow and response.

I have to say they worked for me. But if it was just me, I would have used a lot more qualifiers like 'maybe' and 'might' when I posted my suggestions. The fact is that Mark Whittier, Bruce Robertson and 'Tuner' (who doesn't like his real name on the web so I won't use it) amongst others have been very generous in sharing their own learning, much of it going back to the textbooks, patents and research papers on carburetor development for military aviation as well as automobiles.
Funny thing is they all seem to do very well in helping people set up their carbs.

Thanks for all the input! Interesting things to consider.

So I will to adjust for less initial and bring both throttle blades back to stock specs, writing down the needed screw positions in relation to blade opening.
I want to leave IFRs at 33 (37 is factory setting) for now as my low speed cruise is nice with them.

The QFT SS-series doesn't seem to have any further air bypass. See here:
https://www.holley.com/products/fuel_systems/carburetors/quick_fuel/street/ss-series/parts/SS-850

PCV could be a problem. I have a stock NOS one in it (for a 1972 440 engine or so), which might not work with the changes in my engine compared to stock. I will block it for a test and see if idle AFR will change significantly.

Thanks again!

The lower the manifold the vacuum (relative to stock), the more the likely the PCV will be flowing additional air at idle.

I have only found published flow numbers for AMC. Graphed here.

The stock PCV valve is probably something you could work with. If you want to experiment, my suggestion is replace the stock valve with a fixed restriction. A starting point would be Suggested restriction sizes for PCV test

What is the altitude of where you are in Germany?
If it is down south in the Alps you may need to rejet the carb. for the altitude

Altitude is about 70 meters or roughly 200ft. In the north

I spent tree weeks in Munster in 1968, way nicer people up there than in Hamburg
The English solders didn't get the same treatment as we did, U.S Army, did in town
Good luck on your tuning

Did some testing with my idle today.
Starting points were:
Pri throttles square slot (equals 1/4th of a turn in)
Sec throttles screw 1/4th of a turn open. The sec transition slots can just NOT be seen. Didn't touch it anymore.
Let the engine warm up.
Set initial timing 17 degrees BTDC, was 15 deg before (not 20 as I wrote, my error...). Drove last season with 20, though.
Mixture screws not responsive again. Just like before. Idle AFR is 14.4, mixture screws 1 turn out all four of them. Idle vacuum is 11 in D, 13.5 in N.
Idle speed screw (primary) could be screwed out slightly to lower it to 800rpm idle, is now 1/6th of a turn in, which is slightly LESS than a square on the transition slot.
So both butterflies are closed pretty much, and still the mixture screws don't react as they should.

@Cab: Munster is roughly 60mls (100km) north of me. Lots of military over there. I'd prefer Hamburg over Munster: A lot more Mopars in Hamburg

If it's not coming from the idle circuits, then the fuel has to be entering from somewhere else.

Find it and fix it... excessive fuel pressure, leaky needle/seat? Can you see anything dribbling from the boosters?

This is OK.
Now try 15 degrees initial.
The rpm in neutral should drop slightly.
Then try opening the primary mix screws.
The mixture should richen; and the rpm and vacuum drop when shifting into drive should be the same or less.

If it the rpm and vac drop going into Drive doesn't get better, then maybe this will be about as good as it gets with this carb and engine (at least without modifying the carb).

Qs Before going deeper, what do you mean by "mixture screws don't react as they should"?
If the secondary mix or primary mix screws are closed, does the engine die?
How about if they are all closed?

The primary transfer slots showing a little less than square may be OK. QF slots may be wider and/or longer than a classic Holley. This may or may not cause problems. So lets find out.
First -
Test. While the engine is idling, close off two and then all four of the Idle Air Bleeds (IAB) with your fingers. If the RPM goes up, then the engine likely wants a richer mixture at idle.

If it wants richer, then I think its worth the effort to give it what it wants. It may take a few iterations of testing. My inclination would be to try smaller primary IABs right at this point. Wires in the IABs will work as an expediant if you dont have smaller ones on hand or just want to test quick. (Run the wire under the aircleaner gasket so they can't fall into throttles.)

If it did not want a richer mix at idle, then retest at 17 to 18 deg advanced including adjusting idle mixes. Go with which ever timing setup seems to run better in Drive.

Second -
Having established the idle, the next thing to test is the off-idle. Off-idle fueling is also idle circuit except the idle ports contribute a small fraction of the fuel as more of the transfer slots under the throttle get exposed.
Test a.
Drive it. See if there is any hesitation or lazy response under normal acceleration situations on local roads.
(All tests should be without secondaries; disconnect the link if neccessary.)
If there is hesitation or lazy response, then the primary throttle position will need to be adjusted. If not, then a little less than square is probably OK.

Test b.
In neutral, very slowly open the throttle so there minimal accelerator pump shot contribution. RPM should go up with no flat spots. Some people have better luck/skill with this test than others. Urich recommends doing this with a sensitive tach. I do it be turning in the throttle screw so it is super slow.
If there is a flat spot, then the idle circuit needs more fuel. Try a slightly larger pair of Idle feed restrictions.

If it seems like its working, do the same test driving. Very slowly open the throttle from idle. If there is a flat spot, same deal, it needs more fuel in the transition. In this case, either slightly larger fuel restrictions or slightly smaller idle air bleeds.

WHAT is how they're supposed to work? Not sure which post you were replying to, but I want to know what you meant

DrCharles:
Ordered a fuel pressure gauge today. Will report.
Haven't looked for dribbling fuel yet, will keep an eye on it.
Fuel in fuel bowls is steady but wavy. Doesn't seem to run high or low. The carb is brand new, first season. Needle/seat looked great when I inspected them. Stock size 120, unchanged.

Mattax:
I had tried 15 initial before. Yes, rpms go down slightly. But I can't compensate this with mixture screws turning out. It won't respond. I'd have to open a throttle blade to up rpms. But I will give it another try.
Rpm drop between N and D isn't much. 50 rpms or so. No harsh engagement.

"mixture screws don't react as they should": I mean what I wrote above. Turning the mixture screws in or out doesn't do much to idle rpm or idle AFR (or idle quality). I can influence idle AFR by about +-0.15 by means of the mixture screws. Nearly nothing.

Will try the IAB closing trick. Interesting!
But I'd say decreasing IAB size would destroy my lower part throttle AFRs.

Yesterday I drove the car in slight rain, wet roads. Therefore I could use the throttle only very gently. Driveability was great, no bogs, lags, flat spots. Once I am off idle everything is responsive to changes (IFR, IAB, main jets etc.) and runs very well. Idle is the only thing that acts strange...

Got it. Being able to open the primary throttle blades slightly more was all that might be gained by going to 15 degrees. Based on your rainy day experience below, it doesn't seem neccessary

That's pretty good. With the 2.5 in drop in vac I thought rpm might be dropping a couple hundred rpm. Even though your meter reads 14.2 AFR, it seems there is enough power. Maybe you'd get a little more power with a richer mix, but some people are OK with near stoich at idle. Leaner saves fuel and if no misfires, less HC and CO out the exhaust. Only downsides are a little more NOx and the engine may be a little hotter than with a richer mix.



I agree with Dr Charles. If turning the mixture screws all-the-way-in does nothing of significance, then fuel is coming from somewhere else. The primary transfer slot is only the normal place it should be coming from in your current iteration. If it is, then my only suggestion is try one size smaller Primary IFRs.


IAB changes will not have as much effect on the mix as the IFR changes, but yes I agree it will normally make part throttle a little richer. (I keep saying normally because once the IAB an IFR sizes are outside a working range the fluid mix becomes inconsistant,usually too much air, and the effect is sometimes opposite from expected)
If you go smaller on the fuel restrictions, then going smaller on the idle air bleeds should help keep upper transition consistant.
Another option is to put a restriction in the passage leading to the transfer slot. My suggestion is to save that as a last resort.
If you find the idle/transition peters out too early with the smaller IFRs, the smaller IABs will help.


Well then it would seem to be rich enough through most of the idle circuit. If you left it as is, that would be an understandable decision.

If you want to try to improve it, my suggestion is try the smaller IFRs first. If you are willing to do the work, then consider placing them in the lower position.
With the smaller IFRs, you probably will then be able to use smaller IABs, at least on the primaries. Then the mix screws may start controlling a bigger proportion of the fuel at idle.

The transition slots. The fuel in them should be moving at all times.

The position of the primary throttle blades at idle should be such that 0.020" to 0.040" of the transition slots are visible below the blades.

This provides the correct balance of restriction of fuel vs. restriction of air from idle through off-idle (part throttle, blades open past the top of transition slot.)

I suspect the reason that Holley uses a slot (as opposed to a series of holes) is that they wanted the fuel flow to be continuously increasing as the throttles open.

There are other carbs with intermediate circuits, carbs with transfer fuel restrictions (some carters have two idle restrictions) and so forth. So the amount of transfer slot showing on other carbs (even Holleys) may be different.

On the secondary side, transfer slots are sometimes much higher and no amount shows below the secondary blades. I think this is most common on vac secondary carbs.