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Discussion Starter #1
Have a Quick fuel 950 race carb sitting on my 421 that has a new set of pistons, a bigger cam, and Carb rebuilt by Holley. Have just got around to getting it together and have a tuning question on the carb. Car use is Strip/Street, I hit it with nitrous at the track, like a 250 shot but I have not got that far, still tuning for the street.

AFR is about 13.5 at idle and when off the throttle while cruising.
AFR is about 12.9 WOT, little lean

The area I want to clean up is when throttling up gently from idle thru 3K. Secondaries (mechanical with a 40% progressive linkage) are not in play at this point and the AFR gets pretty fat, like between 10.9 and 11.7.

I'm using 79/89 jets and have checked the Accelerator pump adjustment, also the bowl adjustment has been done as well. Oh yeah and the power valve is a 4.5 (61600).

My initial thought is to pull 2 jet sizes from the primary and +2 the Secondary bowl but that sounds like a big spread so before I go further does anyone have other suggestions for me?

ALSO
The vacuum checks out at 5 and the idle mix screws are at 1.25 turns with the idle set at about 1000 RPM. What is weird is the idle and vacuum picks up as I turn the screws in?
 

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With the idle and vacuum picking up with turning the screws in, that means your too rich right now on the idle circuit. As for changing jets, there's no need to add secondary jets when you change primary jets. Tune each side separately. Just reduce the main jets and try it out. The secondary jets aren't doing anything until you open the secondary. To tune the idle circuit it may want slightly larger air bleeds to give it more air.
 

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The secondaries are adaptive, but only meter fuel for the airflow through the secondary ventures. Think of it more as 2 separate 2bbl carbs. You tune the primary carb first, then tune the second carb for only when its open.
 

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Well pulled 4 fired it up and got the innovate E4 error. Guess I'll be waiting a few days for a new one before I see what happens.

Anyone find a direct plug in replacement for the MTX-L sensor?

Really got to lose this Innovate and get a AEM or PLX....
If you get the number off of the sensor itself, you might actually be able to get one at the parts store or a speed shop local to you. Somehow, when my brother roasted his last AFR gauge, the local NAPA had a replacement Bosch sensor right on the damn shelf. It was pricey... but he was back to tuning in 3 hours time.

I love my PLX AFR setup, and I'll never, ever go back to Innovate. Their DM6 gauge is super handy for a number of reasons, too.
 

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There are 3 issues with a majority of the new holley type carbs built now, and one in particular with the quick fuel billet base carbs.

First, the transfer slots on all billet quick fuel baseplates are too large, causing rich running in the off idle to before the primary boosters start flowing area. You must install transfer slot restrictors of .081" size to both the primary and secondary side of the main body. Use a 5/32 drill bit to enlarge the holes, then tap with a 10-32 fluted bottoming tap only deep enough to have a 10-32 x3/16" brass set screw sit flush to 1/4 turn below the body surface. Also only drill out the hole slightly deeper than the length of the brass set screw. .081" is the size of the calculated surface area of a proper size transfer slot in a holley before everyone got the idea that bigger is better.

Now with almost every carb built, the IFR are no longer located in the bottom of the metering blocks, as holley intended with the original design. Having the jet located submerged in fuel dampens the vacuum pulses and results in much more consistent metering. Simply remove the IFR at the top of your blocks, and use a 6-32 fluted bottoming tap to tap the hole in your billet blocks in the lower position. Then install 6-32x1/8" brass set screws drilled to the same size IFR as original. Tap only 4-4.5 turns, enough so the set screw sits flush with the recessed part of the block opening.

Lastly, the overdone emulsion in newer carbs makes for rich running at low rpm and throttle openings, and lean out of the curve as the rpm rises. Change it to as Holley originally designed. On your blocks this only requires moving the blocked set screw to another location, and adding another for each emulsion package, 4 in total.
 

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Discussion Starter #8
I think I have something much more basic going on...

Again this is a Quick Fuel 950 Race carb which was rebuilt by Holley this winter. The engine is a 421 SBC which previously had no problem with the carb before it was rebuilt. Both Cruise and drag AFR was right where I wanted it, roughly 12.5. For testing I have disconnected the secondary linkage and setup the carb with basic tuning.

I just stepped the primary jets down from 78 to 66's in increments of 4. For each test the AFR continues to be in the area of 11.4 at a constant RPM between 2000 and 3000 RPM.

Now for the part I really don't get, If I screw the idle air 1/2 a turn IN (clockwise) my idle RPM INCREASES and my idle AFR cleans up a bit from 13.0 to 14.0 but of real interest is my AFR around 2000-3000 RPM actually improved to 12.1 If I continue to screw the idle air screw in the RPM continues to increase.
NOTE: I only messed with the idle air screws with the 66 primary jets.
 

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Discussion Starter #10
been a while but I'm heading the right direction.

Tried a 2.5 Power valve and liked the results.
Idle air adjustment is now around 1.25 out from bottom and I'm running a 74 jet in the primary. AFR at idle is 13.7 and between 1000-3000 RPM hangs out around 12.5.

I have a 91 jet in the secondary and at WOT AFR is right around 13.5, too lean for me so I'm heading to a 94 jet and that should get me in the 12.5 range.

The problem is I have a 40% progressive mechanical secondary linkage and just before the transition to opening the secondaries the car goes lean, like the AFR is around 15. Is there a 20-30% secondary linkage for QF, Feels like getting into the secondaries just a bit sooner would help clear this up.

I saw this from Holley but I don't get how it would work with the secondary cam or even if it would work with a QF carb.
https://www.summitracing.com/parts/...vdGcOer2Pr5o3mhRWfKM4wpgXAm-CjvhoCPLwQAvD_BwE
 
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