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Carburetor issue/tuning question, rpm lingers after foot is off the gas

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Hey I just rebuilt my 750cfm qft slayer carburetor and reinstalled it on my 1967 mustang with a 302 out of an 89 fox body that has had some work done to it. I know this carb is too big for it but I had it on there running fine before until it needed a rebuild.

i just got the floats set and my idle mixture screws are currently 1.5 turns out from fully seated. My idle speed screw is set all the way out so that there is a square made by the throttle plate and the transfer slots, the spring by my accelerator pump can that is behind the throttle arm and located on the post that my primary throttle plates sit on does not allow my primaries to close any further than the square that is showing for the transfer slots. This makes it impossible to lower idle speed via the idle speed screw. My secondaries are currently closed and I intend to set idle speed via secondary plate adjustments. The carb also has 75 primary jet and 82 secondary jet with a 65 power valve and factory air bleeds (70 31 31 70 for primary 39 31 31 39 for secondary) also my accelerator pump is set to squirt immediately upon throttle press.

at first it was hard for me to get the car to idle but now it is idling at around 900-1000 with the settings above and is relatively smooth, my vacuum gauge is reading around 20. I am now experiencing a weird issue where I press the gas and it seems a little sluggish which is to be expected with the 750 on the 302 however more sluggish then it ever was, also idling higher then it was, and the weird issue is that the rpm lingers high when quickly tap the gas and rev it. I have attached a video IMG_5100.MOV
any carb tuning tips would be much appreciated it, I am trying to get it back to around where it was.

this silver spring is preventing my throttle arm from closing the primaries all the way and make my idle speed screw obsolete in backing out any further. That causes the amount of transfer slot pictured under it to show.
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image below is how my transfer slots are set, this is with my idle speed screw all the way backed out
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Discussion Starter · #5 ·
get yourself a good old vacuum gauge , adjust you mixture screws off the vacuum gauge . lean it out to max vacuum then richen it by 1/4-1/2 turn
I've been tuning with the gauge and got the idle to 900-1000 with 19-20 hg vac but it still does what I showed in the video, however, the video was taken before I got my idle lower. any idea why it does that?
 

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Did it still do it after you fixed that spring? kip
 

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Yep, right where Bart indicated. Your transfer slots should be small squares and not the rectangles they are now. kip
 

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Discussion Starter · #9 ·
Did it still do it after you fixed that spring? kip
it’s currently rainy out here in md today so I’ll have to make the adjustment tomorrow and provide an update, however I also ordered a return spring bracket to install just in case my throttle plates still won’t close fully. The image of my transfer slots is currently the furthest I can shut them.
 

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I've found that your idle will be high until you get the transfer slots square.

Your jets seem big to me. I've got a 680 cfm QFT on my 393w, using 68 primary and 76 secondary.
 

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I'm right above you in PA getting the same. Fixing that spring help with the slots, fine tune with the screw if needed. kip
 

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Discussion Starter · #12 ·
I'm right above you in PA getting the same. Fixing that spring help with the slots, fine tune with the screw if needed. kip
Ok I got my new return springs on and placed the cam spring onto the cam hole, I believe it fixed my high idle however now I am having a lean big issue at acceleration that causes my engine to stall when I slowly apply gas to get up to speed.
my accelerator pump is adjusted to the point that as soon as my throttle arm moves slightly, gas squirts from the nozzle. It seems like a lean bog, I’m also having trouble getting my idle adjusted right, she’s just running weird when up to temp and the idle drops 200-300 rpm when I put the car into drive from park around 1100 down to 800. This makes the stumble more noticeable when accelerating under load vs accelerating in park.
Any idea or tips on adjustments to make to get rid of this stumble and tune my idle.
I started by backing out 1.5 turns on the mix and then going to highest vacuum then richening it by a quarter turn
 
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