Quick Fuel Secondary air adjustment

[AC Ventura]

Hi guys from London, UK. I hope you are all okay, although it seems we are all in the same boat. Not so good here. Most businesses closed, most people off work and we are all pretty much forced to stay at home and strictly no socialising. All Motor and other sport including Formula One, Wimbledon etc have been cancelled. Plus our Prime Minister in intensive care. The most challenging times in my lifetime that’s for sure.
So, I have some time to try and get my new QFT fuelled 427FE running right. Set the corner idles with a vac gauge, and now getting 9.5” at about 850-900 idle, where it was 8”. I’m running a fairly big cam and the plugs are still black, after idling. I’ve read about drilling holes in the butterfly’s to introduce some air, but then found this note in the instructions.

NOTE: Secondary idle air can be adjusted externally. While this is a convenient tuning aid in cases where the primary throttle plates must be opened considerably to compensate for a radical camshaft profile, it should not be opened more the 1 1⁄2 turns from base as it could cause a hesitation at wide-open throttle. Consult a carburetor tuning book to ensure proper primary throttle plate alignment.

Does anybody know what form this externally idle adjustment is and where is it located? Surely, they don’t just mean open the throttle to higher idle?

[MKS427]

I had big cam issues with my QF 780 vs. I called them a few times, 4 years ago, at no time did they mention a secondary adjustment. I can't remember, if it was QF or someone else, that told me to drill the throttle plates. SLightly better, but nowhere near enough.
I was advised to go EFI and it worked. However, the tuning instructions were non existent and the first go around, I wasn't instructed about secondary adjustment for IAC steps.

[hauss]

First I am no expert but, I was able to get my quick fuel dialed in after, installing the correct power valve size . Came with a 6.5 changed ,to a 3.5 (big cam). also changed out jets and fine tuned with air bleeds. I must also say, I had a lot of help from gazz64 . I say spend the time. Make sure all seals are in good order, keep it clean and you will be greatly rewarded!

[hauss]

Blykins also helped me out a lot sorry no disrespect mr Blykins !

[PeteF]

The adjustment for the secondary throttle plate idle is on the right side, accessible from underneath the base. If you look closely, you can see the threads protruding up through the base. Depending on your intake manifold, you may be able to insert a small Allen wrench with the carb installed. I can do this on my 351W. It is much easier with the carb off, where you can easily adjust it . The factory setting is normally 1/4 turn from full closed. You could try increments of an additional 1/4 turn to see if you get better results. With more secondary opening, you will have to reduce your primary screw to keep the same idle speed. Good luck.

[Johnnybgoode]

You might want to try one of these. Scott

https://www.holley.com/products/fuel.../parts/63-2QFT

[RET_COP]

AC Ventura you stated, "I’ve read about drilling holes in the butterfly’s to introduce some air," NO, not yet at least and is last resort. There is a lot more to consider. You may be OK right where you are. The plugs won't read properly now that you have them carboned up due to a previously bad idle mixture that you may have fixed.
Stop where you are, drain the bowls take the carb off and flip it upside down. Look at the throttle blades and the relationship to the transfer slots. Particular attention to the primary as you have stated you have a good idle. the edge of the plate should be in the length of the transfer slot maybe exposing about .020" [ it would look like a even square]or so. The secondaries will maybe only expose none to about .020 of this slot : this would be a good balance. If your idle adjustment screws had an effect on the idle chances are they are in the functionable window. I usually go for a balance with a little less exposure on the secondary side if I can. Drilling the primary plates come into play if by getting the engine to idle at desired rpm the throttle blades are open too far and out of the range of the transfer slots. Holes will cause a higher idle forcing you to close the blades and bringing them into the functionable window of the transfer slot. I hope I explained this well enough. I'm not too good at getting my thoughts in my brain onto paper sometimes.

[kyleb]

What if you’ve done all that and the corner mixture screws have an effect but it still smells rich at idle.
I turn the corner idle screws in on my 347 stroker (quickfuel 650dp) and it stumbles at about 1/2 turn from bottoming on all four screws but still smells extremely rich.
I modified the secondary so I can adjust the idle without removing the carb.
I set the primary with the transfer slot square then adjust idle with secondary adjustment.
Sorry to hijack but answers may help your situation as well.
Cheers

[jhv48]

My quickfuel dp has an externally adjustable secondary idle air screw located on the right side toward the rear. It’s a vertical screw about 3/4 inch long with a spring around it. Screwing it in (clockwise) opens the secondary blades. No need to remove the carb. Easily adjusted externally.

[DanEC]

Try adjusting the secondary plates first but you don't want to go to far or you will start uncovering the transition slot in the secondary bores (assuming Quick Fuel is essentially like a Holley). I have an old Corvette with a big cam and lots of compression that only produces 8 inch vacuum at idle. I had to drill the primary throttle plates in the old Holley 3247 to tune it so unless Quick Fuel has made provisions for a big cam in it's base turning ability, that may be necessary.

Typically, a lot of tuners pull the carb (to get a better view of the actual exposed transition slot below the throttle plates) and set the primary idle setting so that a "square" exposed primary transition slot is seen below the plates. That should be the best setting for primary system idle. Then open the secondary plates from first contact of the adjustment screw with the throttle cam, about 1/2 to 3/4 turn open. Also experiment to see how many turns your can open the secondary throttle before the transition slot starts appearing below the throttle plates. Install the carb and try it and make any necessary adjustments in idle speed by opening the secondary further but stay below the max number of turns of the screw that your experiment showed would start exposing the secondary transition slot. Also adjust the idle mixture screws for max rpm or vacuum. If you are just slightly short of getting to your desired idle speed you can make a small adjustment to the curb idle setting for the primary but you don't want to uncover the transition slot very much more or it will upset the transition response and idle mixture. If you can't get to a good idle speed within this range of adjustments then you probably need to consult with Quick Fuel about drilling the plates or to see if they have any other ideas on how to adjust the carb to idle cleanly and respond smoothly from idle.

I also found that restricting the primary idle bleeds greatly boosted the vacuum signal to the primary circuit idle system and helped it run better. Old Hollys are not adjustable but I restricted it with a wire inserted in the bleed holes up at the top. I suspect Quick Fuel has replaceable air bleeds so you may want to consult with them and see if maybe you need to go a bit smaller with the bigger cam you are running.

The goal in all of this was to be able to tune the idle system to produce the highest vacuum or rpm with the least number of turns from completely closed. I finally got mine down to the 1-1/4 to 1-1/2 turns out range.

[Gaz64]

Nice description of setting up there Dan.

I would like to add, I use Holley XP Ultra or Barry Grant Demon Idle-eze to trim idle speed on my carbs.
I set the primary and secondary to just above an appropriate minimum for the engine, and trim with the bypass valve.

One carb I built for a mate of mine I fitted a set screw to the primary side, so he doesn't mess with it. I taught him to trim the idle if necessary with the secondary.
It has a lot of scope, about 500 rpm, which he never needs. 260 @ .050 Roller cam 355 idles happily (EFI quality) at 850 rpm 8 inches Hg.

Gary

[AC Ventura]

Thanks for all the replies guys and I’m sorry for my late reply, but couldn’t sign in for 2 days. Then reset my router....
Sadly, I don’t have the secondary adjustment externally as hoped. Thanks all for the info on the transfer slot exposure, which I just about get. Just not sure how I adjust the secondary throttle plate opening. But I guess that will be revealed.
But here’s the thing. Like I said, on idle, intake manifold vac is 9”. But when I rev, it rises to about 15”, which must be due to the far from standard, cam profile, right? The PV is 6.5 and the PJ’s are 72’s, with SJ’s 83’s.
As stated the problem is rich idle and bucking on take off and overrun. Some have said go to a lower number power valve, but surely that going to open even earlier. I’m thinking, maybe I need to go up to a 8.5 or 9.5 PV and also order some smaller jets, 55-70 for the primaries. What do you gurus think?

[saki302]

If you look where the secondary throttle shaft is, there may be a set screw (flat head) which can adjust the opening. On mine it is accessed from underneath, now that I know where it is, I can get to it with a flat head bit and a low profile screwdriver ratchet. There isn't much room under there to work.

Some people replace the whole thing with an allen set screw to make adjustments easier.

[Gaz64]

What model Quick fuel carb do you have here?
What size IFRs?
A lower number PV opens later, not earlier. Your 6.5 is fine while the engine idles at 9.
The engine idles at 9 because of your cam profile, but vacuum increases with rpm increase at light throttle. Completely normal for larger than stock profiles.
Gary

[AC Ventura]

Hi Gary. I’m fairly sure I have the HR 780 QFT. Sorry don’t know size of the idle feeds. Sorry but can I get you to explain, how a 3.5 PV opens later than a 6.5, on an engine where the vacuum rises as the rpm increases? I thought the number relates to the vac that the PV opens and let’s fuel in.

[Gaz64]

A power valve is a load sensing "switch" that adds fuel in parallel with the main jet to the main well.

So while driving, we could have 15 inches at cruising speed, 10 inches at light load, then squeezing the throttle more, the vacuum drops lower.

You might want 6.5 as the opening point, or even lower vacuum at 3.5.

Drive you car with a vacuum gauge fitted to understand what is happening UNDER LOAD.

You are free revving your engine, and that is not how to understand PV operation.

This is some info about your carb:

HR-780-VS 1.375 1.375 1.687 (1-11/16) Down Leg 160 72 83 70 31 Vacuum Secondary / Gas

If you say the engine is rich, how far open are your mixture screws?

All carb engines with cams will exhibit some richness, especially the smell at idle/low speed.

Gary

[AC Ventura2]

Hi Gary. Had to re register as AC Ventura2 to enable sign in.

Thanks for the carb info. I will check under load vacuum, by taping the gauge to the screen, but can’t drive the car as yet due to severe lockdown here. . In the meantime I’ve ordered a 4.5 and 8.5PV and a selection of primary jets no’s between 70 and 60. As they at low cost, I’m prepared to take a chance on this.
If by mixture screws you mean the four corner idles, they are all about 5/8 of a turn out from bottom. This is where I found the max vac. Any further in and it drops again.

If you mean the

[Gaz64]

Originally Posted by AC Ventura2

Hi Gary. Had to re register as AC Ventura2 to enable sign in.

Thanks for the carb info. I will check under load vacuum, by taping the gauge to the screen, but can’t drive the car as yet due to severe lockdown here. . In the meantime I’ve ordered a 4.5 and 8.5PV and a selection of primary jets no’s between 70 and 60. As they at low cost, I’m prepared to take a chance on this.
If by mixture screws you mean the four corner idles, they are all about 5/8 of a turn out from bottom. This is where I found the max vac. Any further in and it drops again.

If you mean the

I think you need some help from some exhaust gas analysis.
A wideband O2 gauge is helpful. I use a datalogger, which logs tps, map, O2, rpm.
Your mixture screws at 5/8 are close, but for me I would like to see them at 3/4 to 1 turn.
If they need to be at 5/8, the IFRs are a fraction big for your engine.

And now with lockdown happening, tuning a cobra on a roadtest is just not going to happen.

And if your "richness" is idle/low speed related, replacing main jets progressively leaner from 72 to down into possibly early 60s will not solve your issue.

Gary

[AC Ventura2]

Thanks Gary. In the meantime I changed the underfloor exhaust for something more like a glass pack/cherry bomb and much less obviously restrictive than before. All I’m looking for is the power I’m supposed to have and a smooth take off and over run release. As the carbs/plugs so black, that’s that the easiest place to look. I’m also going to do a comp test, to make sure I’m not chasing my tail.

[AC Ventura]

Hi guys, sorry for the delay replying, but had a lot of log in issues. Okay, haven't changedjets or power valve yet, but have removed carb and primary bore transfer slots are .050" exposed. Obviously I can close them down to obtain the 'square' with the stop screw, then presumably open up the vac secondaries to reinstate a good idle, via the awkward secondary spindle stop screw under the carb face. However, the primary spindle lever, the one connected to the accelerator pedal, has a link bar (arrowed) that won't allow the secondary spindle rotate, unless the primary is already open. I see there are 3 positions (holes) for the link bar. Am I to move the link bar end to one of the other positions, to enable the secondaries to crack open, but maintain the primaries in the desired 'square exposed' position?