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 Standard or range for 750 IFR? 
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Joined: Tue Aug 23, 2016 12:23 am
Posts: 66
I have a non-standard Quick Fuel Q-750 that I have been writing on. I'm close but it still seems like there is more. Recently I went from .031 IFRs to .029 since the idle mix screws are so extremely sensitive. 1/16 of a turn at all four corners gives me almost a full point AFR change. Seems that a smaller IFR would allow more easier adjustment with the mix screws. However, the change didn't seem to do much. I am still at about 3/4 turn out on all screws to get my best idle vacuum. Seems like it should be 1 1/2 turns out. I can run each screw all the way in (one at a time) and it doesn't die. It gets real lean but doesn't die.

Front transfer slots are square, rear slots are covered. No throttle blade holes. New bowl gaskets, new base gaskets. Brake, vacuum lines all plugged, no vacuum leak.
Idle air bleeds are drilled to 0.063.
Main air bleeds are .028
Main jets are 71,
Rear main jets are 74.
PVCRs are.051.
IFRs are in the lower channel

Seems like something is still not right. It idles at 13.5 with 13 in hg. At first it was a bit lean with an occasional misfire) in the upper transition so I tried to decrease the IAB from .066 to .063. Then the flat ground cruise at 3000 was 16.5 with a little misfire so i went from 70-71 on the from main jets.

Quotations: what are the normal IFRs for a QF Q-750 carb? Should I keep going down on the IFRs until i get the mixture screws around 1.5 turns out?

Is it bad the car won't die when i turn in one screw all the way but the transfer slots are ok?

Other numbers look ok?


Sun Apr 16, 2017 11:50 pm
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Joined: Sun Oct 23, 2016 5:52 pm
Posts: 138
Location: Canada
From their (QF) web site:
https://www.quickfueltechnology.com/car ... -race.html

I didn't roam around there to try to see if I can find it's IFR sizes. They're not listed on that page, but here's what is listed:
Throttle Bore Size: 1-11/16"
Primary Jet: 22-74
Secondary Jet: 22-84 (was your 74 a typo, or is there a PV out back too?)
Idle Bleed: 26-73
Hi-Speed Bleed: 26-33
Pump Nozzle: 21-31
Power Valve: 25-45

I would hazard a guess that 0.063" IABs are too small for starters, maybe causing the mixture screws' touchiness, and maybe also the one-at-a-time-mixture-screw-wound-all-the-way-in not killing the engine. Real lean isn't lean enough if you can't make it die, which seems it's getting too much fuel from somewhere then. What power valve, is it okay, and what vacuum level was that level road 16.5:1 @ 3000rpm? (PVCRs @ 0.051" seems small for a 750 as well. Most Holley 750s I've seen have 0.063")

_________________
Mike

1986 Mustang convertible. Originally 3.8L V6 with EEC-IV CFI.
Now has a Holley List-4412-2, and averages 34 (28 US) MPG.
If you're ever bored - some narrow band O2 tuning video of this thing:
http://www.youtube.com/channel/UC9glRIa3jAAtNhaXfg_cZXQ


Mon Apr 17, 2017 10:11 am
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Joined: Tue Aug 23, 2016 12:23 am
Posts: 66
Those stock settings are what bugs me. They don't list the stock IFR size anywhere. I keep going smaller on the jets to make it leaner, but it seems I'm way out of the normal range for this carb. I saw those "stock" numbers but I'm nowhere near those. Although my numbers for IFRs and the bleeds are diameter (drilled) sizes, not flow rates.

I have a plugged rear PV. Disconnecting the secondaries and tunung just the primaries led me to my current primary jets and PVCRs. Then I reconnected the secondaries to tune the secondary jets. Normally there is about an 8 size difference, right? Mine is 3 (71 primary, 74 secondary ). New PV in the front so it's not blown.

I think the stock IFRs on this carb are 0.033. Mine are at 28. I don't see any fuel dripping out of the boosters at idle. I start seeing fuel from the boosters around 2200 rpm.

Vacuum at idle is 13.5. Vacuum at cruise is around 17.

I may just go back to the stock settings and see what that does. Way back when I started all this, I ended up having head failure - broken springs, worn valve guides, etc.


Mon Apr 17, 2017 3:02 pm
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Joined: Thu May 08, 2014 7:47 pm
Posts: 234
If your 4 corner idle screws are to sensitive, only way to change that sensitivity, is with the idle air restriction, not the idle fuel restriction.


Mon Apr 17, 2017 4:04 pm
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Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1744
Doc, your tuneup sounds normal to me. You have done a good job of tuning this (and you know it first hand) so don't get distracted, your engine is telling you want it wants.

As you (and anybody who does carb tuning with their eyes open) have learned, your engine has its own appetite. No two engines are the same, even production engines made on the same day. No two carbs are the same, even two made on the same day. The old Chinese philosophers would say "The Ten Thousand Things" about such imponderables.

The carb manufacturers need to supply a basic unit that is rich enough to avoid catastrophic damage on any engine, therefor the basic unit is calibrated on the rich side of the range for its venturi and throttle bore size. (I think that is a suitably polite way to explain it.)

Some production OE FoMoCo carbs used .026" idle jets. The original application for the 1850 was on 430 Lincolns and they (the oooold ones) have .026". The 2x4s on 427 FE have .026" with .063" IAB. An original 289 (actual) Cobra R-3259 I have in my notes has .032" IJ with .070" IAB, some 428 CJ carbs were .029", others .031".

If the engine runs reeeeely good with the IJ and IAB in use, and you just want to reduce the sensitivity of the idle mixture screws, a way to do this is drill a .040" hole into the bore through the hole #3 in the top picture. If this causes some other problem, which is unlikely, then plug it by driving some lead in the .040" hole and clean the passages with a drill bit.

Image


Mon Apr 17, 2017 4:30 pm
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Joined: Mon Apr 17, 2017 6:57 pm
Posts: 8
Tuner,
If you have the specs for the original 2x4 Ford 427 BJ/BK carbs, can you share that info?

Thanks,
Drew


Mon Apr 17, 2017 7:01 pm
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Joined: Tue Feb 23, 2016 8:51 pm
Posts: 21
i have a 750 holley on a pretty healthy mopar 360.
i dropped the IFR's down to .028 trying to get the idle mix screws turned out more and also get it lean enough in transition that it didn't run perfectly smooth until it warmed up.
the idle bleeds are around .067, yes the idle mix screws are very sensitive and they are right around .75 turn out from closed.

seems to me, if the motor will run clean with an .028 IFR then that's what it needs.

also, the IFR change didn't seem to have much effect on how far the idle mix screws were turned out, i'm thinking the idle bleed would change that.


Tue Apr 18, 2017 10:29 am
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Joined: Thu May 08, 2014 7:47 pm
Posts: 234
JETFAST wrote:
If your 4 corner idle screws are to sensitive, only way to change that sensitivity, is with the idle air restriction, not the idle fuel restriction.


krautrock wrote:
also, the IFR change didn't seem to have much effect on how far the idle mix screws were turned out, i'm thinking the idle bleed would change that.




:thumbr: :thumbl:


Wed Apr 19, 2017 1:43 pm
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Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1744
DeepRoots wrote:
Tuner,
If you have the specs for the original 2x4 Ford 427 BJ/BK carbs, can you share that info?

Thanks,
Drew


Metering block # 4033
PIAB .063
PMAB .031
PIJ .026 located in idle well under cup plug
PMJ #66
PVCR .043
PV 6.5
APD (squirter) .025
Pump cam # 290 full stroke of 30cc pump at 1/3 throttle
Angle channel and booster passage .140
Metering block is essentially the same as an early 1850 which also has .026 IJ and an emulsion tube w/ 4 x .025" holes in tube at level of upper E-hole drilled in main well and .028 bleed to tube holder in upper part of dog-leg air well. Standard old school Holley two .028" E-holes from air well to main well.
Mixture screw metering hole .062"

SIAB .028
SMAB .025
Booster leg passage .140
Metering plate #13
Main .064
Idle .031

I think original vacuum secondary servo spring is 38R-585 "yellow"


Fri Apr 21, 2017 12:32 pm
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Joined: Mon Apr 17, 2017 6:57 pm
Posts: 8
Thank you sir....
I had some info from someone else and it looked funky, I didn't believe it.
Idle Jet was listed as .043 which made me view everything else with suspicion.


Fri Apr 21, 2017 4:32 pm
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