Lean flat spot just off idle (QF 950)

[DrCharles]

On the old site there are a couple of long threads I started... one where I converted a 4780-2 to four-corner idle and replaceable brass, and then started tuning it. I realized an 800 was probably small for my combo so BradH supplied me a nice QF 950 and I worked on that for a while - then other life events intervened for 2020

It's pretty close now, but there is one really annoying lean flat spot just off idle at low rpm (1500-1800) that I haven't gotten rid of. AFR goes to 17-18 and the engine basically dies. The slightest additional throttle and it comes back on with 14-15. Not a problem above 2000 rpm or under more load (i.e. with the mains online). 14.5 cruise AFR at 60 mph. WOT around 12.5.

Combo recap: 451 ci, 10.3:1 CR with hand-ported iron heads (going to 10.6:1 aluminum 293 cfm heads this winter), 4-speed with 2.66 1st, 3.91 rear gears (60 mph = 3000 rpm). Estimated weight 3400# with me in it. Cam is slightly reground mushroom "Mini-Express" .652" gross lift, 272@.050. Edelbrock RPM dual-plane, 1" open spacer. 27 degrees idle timing, 35@4000, plus vacuum advance. Idles at 1100-1200 rpm at 8" vacuum at around 13.5 AFR.

Carb settings (pri/sec): IFR .028/.028, IAB .078/.079, TSR .074/.074, MAB .026/.026, MJ 71/86, PVCR .066, PV 10.5". T-slots square. Idle set with secondary screw. All four butterflies drilled .100". Also have Wagner tunable PCV valve.

The reason for the small TSR is to keep the oversized QF T-slot from going too rich at the top of the slot, which it did with the original .081. Should I try going slightly smaller on the IAB to get the T-slot started sooner? Enlarge the T-slot exposure instead?

Despite the big cam, it's quite driveable on the street, except for that one stumble - which of course always seems to be where my foot is at 25-30 mph cruise!

[Right hand drive]

With 1.45” venturi and 1.75” throttle bore and drilled throttle plates (and that cam and vacuum) you may need around .033” IFR (low position) and .070” iab as a starting point. With the gearing you have it wouldn’t see those revs sustained too often yet those revs can’t be avoided to get up and move to the sustained revs. Give the engine what it wants at those revs even if it is 12.5 - 13.5:1. Slight mpg penalty is a small payoff for a better driving vehicle. IFR, IAB, MAB all looks set up to run lean like a Street Avenger.

What is the emulsion stack?

[DrCharles]

Thanks!

.033 IFR, .070 IAB is where I started but as noted, that was just too rich on the t-slot. What screws things up is the vacuum increasing from idle (8") to cruise (15").

It may be a choice between my street cruising mpg (and dirtier plugs) vs. that hesitation at one small throttle angle... everything I've read says an engine should want leaner with increasing load up to a point - then enrichment, which of course is where the power valve comes in. At my current settings the 10.5" power valve may be opening a bit early. May try the 9.5 or 8.5.

I haven't learned enough about emulsion to mess with it, so it's as-delivered: .028, blank, .028, blank.

[DrCharles]

A nice warm afternoon, so I did some tinkering. I swapped out the 10.5 power valve for a 7.5. That's a little bit lean at low speeds, but the 10.5 was definitely fat below 2000 rpm (12-12.5 under light load). I have a 9.5 in the parts box, but not an 8.5 (which I think will be about right for my combo). Anyway my engine doesn't seem to mind 15-16:1 AFR at low loads. Didn't hurt the heavier acceleration AFR any, of course.

Also I decided to play "cover-up" with pump cams instead of driving myself nuts trying to get the low rpm, low load end of the fuel curve just so. I don't remember what squirter is in there now... but the original pink cam is definitely not right. I put in a black cam since I wanted the earliest possible squirt. Turned out to not be enough total shot and really balked when I winged the throttle in neutral. The other one I have is the green cam, so I put that in. (The blue cam is on my old 4780-2 and I don't want to take parts off right now).

Driveability is MUCH better with only a slight sag when just tickling the throttle to the t-slot at 1500 rpm (something I really don't do very much anyway!) Got to keep the revs up with a 4-speed and big cam

[Right hand drive]

^^^ If your satisfied with cruise speed AFR's and any changes to IFR, IAB and MAB throw that out of range of-course as discovered accelerator pump cam and squirter tuning is the way to go. I found through experimentation of a wide variety of emulsion stack and air bleed combinations on the same carburetor over 6 cfm sizes of the one brand the smaller you go with bleeds the more dependency there is on squirter to be dialed in to compensate at lower throttle angles in vehicle. When opening bleeds to modern .031" - .033" - .036" MAB and more and larger emulsion squirter can be reduced as mains start and flow earlier and faster.

[BradH]

A few comments...

The base plate isn't a Quick Fuel part; it's a Holley made by BLP to Holley's specs that came originally on an 80676 Ultra "950". I don't recall what the transition slot dimensions were/are, though.

Re pump cams, the pink cam set in the #2 bracket hole & #3 cam hole gives a large volume, but spread out over the range of the throttle travel. I like it for foot-brake cars where you still want a large amount of pump shot available after staging well above idle speed. The green cam is better for getting more pump shot before the secondaries begin to open, and the #2/#2 configuration provides more volume than #1/#1.

I still think you're making a mistake by wrapping yourself around the axle trying to get a "perfect" AFR number vs seat-of-the-pants drivability. Your cam is bleepin' big with a lot of overlap, so you're not going to get EFI-like numbers, super clean plugs, etc.

If you're (still?) running a dual-plane intake, that's not helping because the fuel distribution isn't going to be as balanced. If you try a small-runner/low-profile single-plane Holley Street Dominator, you may find the engine actually runs more smoothly, even though it gives up some torque at the bottom of the RPM range. It would be better suited to your cam and the new heads, as well.

This isn't the first time I've mentioned that chasing an AFR value is an exercise in futility, but you keep banging your head against the garage wall doing it. Unplug the wideband and tune it to run well the way you drive it, not to some arbitrary number that may not be realistic with the combination of parts you're working with.

[DrCharles]

I think you've missed my point... I am trying to achieve good driveability and a favorable AFR, since my car is almost entirely street-driven (nearest strip is 90 miles away, I don't want the trouble/noise of driving it there, and I have yet to buy a car trailer).

For the same reason, I have avoided going to a single-plane intake (preserving what low-end torque there is), and the RPM is one of the best dual-planes out there. All the comparisons I have seen say that it gives up little if any below 6000 rpm, and not too much at 6500... where my engine will spend very little time on the street! There's no chassis dyno anywhere around, either.

Once I finally put on the better heads, I could then acquire a single-plane for testing. It's easy to swap intakes on a Mopar big-block since there are no coolant passages. I actually bought a Street Dominator on ebay... but it was mis-described and when it arrived I could immediately tell it fit the RB, not my B. (Those SD's are rarer).

The AFR of 17 or 18 at tip-in only confirms that the severe stumble at one particular angle is due to excessively lean mixture; I wasn't worried about the absolute number. Similarly, I can't feel the difference between a cruise AFR of 15 or 12, but I sure notice it in fuel consumption and plugs.

Anyhow I think I have achieved my objective of streetability with a cam many would say is too big. I can highway- cruise at 9+ mpg which is acceptable, drive around town at low speeds, and to get the power, all I have to do is shift at higher RPM

Now I can tweak my advance curve, because I just can't leave well enough alone

[BradH]

If you're happy, then I'm happy.

I could squeeze out about 9 MPG during steady highway driving to the local tracks, which surprised me. The rest of the time I never bothered checking since I knew it was going to be shockingly low.

[DrCharles]

This last tank which was mostly a 55 mph cruise to Salem AR (34 miles away) and returning at 60 mph, worked out to 8.2 gallons for 83 miles = 10 mpg! And that's before I put in the lower-vacuum PV

I've seen much lower figures also during extended stop & go testing, not surprising with a double pumper