Hello

For fun mostly I picked up a shiny new QFT 600 with mech 2nds...a double pumper.

I have a well tuned holley street avenger 670 with vac seconds. This SA runs well but like I said I wanted to try the QFT because I like the idea of a double pumper.

The QFT has changeable air bleeds it comes with 70 primary and 73 secondary idle air bleed.

The Holley has 80 primary and 46 secondary idle air bleeds.

Both carbs have 4 corner idle and same size venturies. Why are air bleeds not matched between primary and secondary size and why does the holley have such a large offset between pri and sec idle air bleeds what is this attempting to compensate for?

Thanks


-Gun

 

Depends on metering blocks and idle feed restrictors (basically a jet for the idle circuit).

The Holley street carbs often run a completely different rear metering block/plate. So the IFR and IAB are totally different from the front. Then you add in vac secondary.....

The slight difference in the QFT may be to account for slight differences in throttle position between the front/rear blades at idle.

Keep in mind the IFR and IAB have a very large impact on fueling under cruise.

 

As its setup now the QFT comes off the line great under power, makes tons of power, and cruises fine with about a 4.4 AFR indicated.

The issue is the QFT is prone to stumble during smooth accel as the RPMs get lower its more common and more sever. For example 55 in 5th at 1700 rpm will produce a stumble if you try to accelerate nice and easy. The Holley SA is much less prone to this type of stumble yes it still does it but less sever and takes lower RPMs.

The QFT also runs slightly rich during smooth accel from a stop to say 50 mph mid 10s to upper 12s The holly does mid 12s to mid 13s

Changing IABs from 70/73s to 78/78 in an attempt to reduce this rich accel issue did little to no good but made the stumble issue worse (I think even after re-adusting idle mix)

I think I can change the IFR jets in the qft could I use that to get it to perform more like the holley so it responds better to low rpm acceleration say 55 mph in 5th. Would I go up say .004 on the IFR? would this make the rich easy accel issue worse?

So my goals are 1) lean easy accel from 0 to 50, 2) make richer during smooth accel at low rpm in higher gears....The holley does it seems I could find a combo to get the qft to do it.

Thanks

-Gun

 

You could try smaller air bleeds first. What size is the IFR in it now? It doesn't take much of a change on the internal stuff to make a huge difference.

 

It does have screw in IFRs but they aren't stamped...I'll have to track down the size.

If I were to increase the size of the IFR that would cause the idle mix to become richer correct? How is that different from simply backing out the 4 idle mix screws.

If Ive learned one things its that "idle" circuit is a misnomer...Its should be called "this is the circuit used 90% of the time when driving"

Thanks

-Gun

 

Yeah, it makes idle richer....but the IFR feeds the whole idle circuit, which feeds the transition slot as well. That affects a lot more than just idle.

The idle set screws affect ONLY the idle ports that are always uncovered (and at idle a tiny bit of the transition slot as well). So the idle screws have a major impact on idle fuel ratio, and some impact on fueling just a hair off idle. The whole idle circuit however, includes that plus the transition slot, and delivers fuel whenever there's sufficient intake vacuum for fuel to be forced through the circuit (pretty much any vacuum level starting from ~1-2" below atmospheric). So under cruise for instance with the throttle cracked open, you're pulling fuel out of the idle ports, the transition slot, and under enough load, the main boosters.

The IFR/IAB combo is the only way to control the fuel ratio that comes from the transition slot. Changing IFR's will have a significant impact on how the car performs as you come off idle and cruise down the road under light load.

 

Very helpful I did not know most of that..

Now here's something that's confusing me...

On the QFT what I think is the screw in IFR is located at the top of the idle down well.

On the Holley the IFR is located to either side of the power valve near the very bottom of the metering block.

I have seen both locations referred to as "IFRs"

on my QFT are those really the IFR jets?

Thanks a bunch

-Gun

 

Yes, the idle fuel comes from the main bowl into the idle fuel well and is pushed up over the top where it mixes with air from the IAB, then the fuel emulsion of pre-mixed air and fuel falls down into the main idle feed well and goes to the idle screws and transition slot.

You can place IFR's at the top or bottom of the idle fuel well, some do it one way and some the others.

 

after switching back to the QFT I have noticed that with the way I have it setup now it actually out performs the holley during low rpm smooth acceleration under load (example 50-55mph 5th gear easy throttle roll on. No power valve, very little to no
accelerator pump) So that's great!

The one huge issue is that the QFT is horribly rich during easy to normal acceleration. If I accelerate fairly hard (noticeably faster than normal traffic accelerates) I get good AFRs in the low 13s.

During easy to normal acceleration to 50MPH
1st: below 10 "rich" indicated by wideband AFR gauge to 10.5
2nd: 10.8 to 11
3rd: 10.9 to 11.5
4th: 10.9 to 11.5

Not smooth engine run is sometimes noticable during this I assume rich bog it feels like instantaneous stumble.

The holley performs very well during 0 to 50 mph accel

1st: 10.7 to12.8
2nd: 12.4 to 13.4
3rd: 13.5 to14.0
4th: 13.7 to 14.2

both carbs yield 14.0 to 14.7 under no load (flat ground) no acceleration...example part throttle cruise (50 mph sustained flat ground)

carbs are running 68 jets primary for holley and 69 jet primary for QFT.

Do you think I could get the QFT to lean out during easy accel and provide AFRs like the holley without causing it to run lean during easy low rpm accel or part throttle cruise by reducing the size of the IFRs?

thanks

-Gun

 

You'd prolly have to change the IAB size to reshape the curve. Basically with an IFR or jet, ir moves the whole fuel curve up or down, whereas the bleeds to to move the ends of the curve up an down without changing the middle part. So for the main air bleed, a larger bleed will tend to make the fuel ratio leaner at lower rpm/throttle and leaner at higher rpm/throttle, whereas the midrange won't change a whole lot.

What you're describing could be the high speed air bleed, if the boosters are coming in too soon, give it more air bleed to delay how much fuel they bring in and how early.