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 Qjet to Holley jet sizing

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NOTROD Posted - 23 Dec 2017 : 12:06:33 PM
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?
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NOTROD Posted - 24 Dec 2017 : 09:01:49 AM
That's the same carb I have and the numbers I was looking for. Sounds like a good place to start. Thanks Lee!
Lee Posted - 23 Dec 2017 : 9:41:57 PM
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.
NOTROD Posted - 23 Dec 2017 : 6:17:58 PM
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.
Lee Posted - 23 Dec 2017 : 1:58:00 PM
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.

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