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 Intake / Carb and Fuel Delivery Tech
 Qjet to Holley jet sizing
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NOTROD
Two Feathers

USA
72 Posts

Posted - 23 Dec 2017 :  12:06:33 PM  Show Profile  Reply with Quote
If a Qjet has 73 primaries and 42 rods, what would be a good starting point for jet sizes when switching to a 750 Holley double pump?

Lee
Sitting Bull

171 Posts

Posted - 23 Dec 2017 :  1:58:00 PM  Show Profile  Visit Lee's Homepage  Reply with Quote
I'd install the Holley as is, then just tune/adjust as needed. Too many variables for a simple conversion formula.

Personally, I'd use a wide-band AFR meter and adjust as:
1. Set the idle mixture screws to around 14.0. If you have 4-corner mixture screws, I typically try to keep them balanced, but I've seen some people with different strategies.
2. Drive around under light/moderate throttle while observing AFR. Adjust primary jets and PV to give maybe 14.5 (see where you motor seems to run smoothest, maybe higher or lower AFR. If you get a lean-surge, then richen of course).
3. WOT test, and adjust secondary jets so that AFR is 12.8.

CURRENT: '73 T/A clone, starting to get less boring!...
67 Firebird (sold) 11.27 @ 119.6 Feb. '05 issue HPP
69 Falcon wagon, 10.51 @ 130mph Feb. '10 issue PHR (sold)
72 Cutlass Convertible, first car, owned since '82, now with a 6.0 LS, mild, putting just under 400 hp/tq to the wheels.
www.LNLPD.com
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NOTROD
Two Feathers

USA
72 Posts

Posted - 23 Dec 2017 :  6:17:58 PM  Show Profile  Reply with Quote
I should have added more details. The Holley is not new and the jets are not "out of the box", the primaries are currently 67's. I'm pretty sure that would be lean for me. I've done plenty of tuning on the Qjet to arrive at the 73/42 combo, however, I really have no idea what's in it for a PV spring. Since I have to get jets anyway, I thought I would ask if anyone could get me close. The Qjet works good, but has a hairline crack in the baseplate.
461, 9:1, 2.11/1.66, RA IV cam w/1.5 rockers, RPM intake, 1 5/8 full length headers, 2 1/2 exhaust, 4 Spd, 3.31 rear. I don't have access to an AFR meter.
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Lee
Sitting Bull

171 Posts

Posted - 23 Dec 2017 :  9:41:57 PM  Show Profile  Visit Lee's Homepage  Reply with Quote
I had a 4779 750DP on my '67 Firebird, 469", iron heads 231/239 @0.050" lift. A bit more radical than your combo, but not much. It has been a long time since I had it, but I'm pretty sure I was running around 72/80 jets with a 3.5 power valve.

CURRENT: '73 T/A clone, starting to get less boring!...
67 Firebird (sold) 11.27 @ 119.6 Feb. '05 issue HPP
69 Falcon wagon, 10.51 @ 130mph Feb. '10 issue PHR (sold)
72 Cutlass Convertible, first car, owned since '82, now with a 6.0 LS, mild, putting just under 400 hp/tq to the wheels.
www.LNLPD.com
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NOTROD
Two Feathers

USA
72 Posts

Posted - 24 Dec 2017 :  09:01:49 AM  Show Profile  Reply with Quote
That's the same carb I have and the numbers I was looking for. Sounds like a good place to start. Thanks Lee!
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