chuck_royle

Recently I picked up an 850 quickfuel double pump carb, no choke. I also installed a wideband aem wideband kit on a very knowledgeable person's advice here. Love the wideband as it does take the guess out, wish I picked up a wideband years ago!
My concern is that so far, my jetting is much smaller ( as I adjust) than I read in your combo's. Carb came with 74 pirmary and 84 secondary- no secondary p.v..
Initially, idle, cruise 11-12 and full throttle 10.5-11 was pig rich. Adjust idle mix to pretty lean at 14-15, cruise got a bit better but still 12.5
Got a holley jet kit, started by putting 70 jets in the primary and 82 secondary..road test. Cruise is now at 14-`15 which I'm happy with. I would like a lean cruise, I don't think I'm damaging anything lean at a light load and car stays at 180deg. Full throttle is still rich at 12.0. My next step is to lean out the secondary side more to #80.
These jet sizes are much smaller than I have read in other combo's, I'm not sure that I should be concerned. It seems to run great, pulls like a mother, sideways unless I'm in 3rd and it doesn't downshift on me, does not surge during cruise etc.. Have not had it back to the track since I installed this 850 qf replacing the 800 q jet I'd been playing with. That's why I got the wideband- make sure this stuff is dialed more or less before I trailer 2hrs to the track.
461
l2323's
btr hyd roller 236/245? .580" cam
proform air gap intake
1/1/4" cab spacer
speedmaster heads bowls "cleaned up" no major work
th400, 3.73, 3000 stall

The air is much cooler up here (Toronto area) than those of you that have posted combo's and locations. So, that's another reason I should be using fatter jets than you..
Thanks for your thoughts!
Chuck

Lonnies Performance

With the limited distribution accuracy of a carb style intake, I would stick to the 14.5-14.7 for lean cruise... you don't have individual injectors to be sure fuel is equally distributed to each cylinder. Your EGT's could be rather high in 1 cylinder without knowing due to things outside of your control.

Also your primaries are used for full throttle operation, so I would move the primary & secondary equally from the as delivered jetting... that would be factory 74 & 84 down to 70 & 80 as you mentioned. The primaries are mostly feeding the front cylinders & secondaries, the back ones. From there go in equal steps.

Further tuning usually is by adjusting the high speed air bleeds to lean out the top end without making the part throttle too lean. I think your carb should have screw in air bleeds.
Make a full throttle pull to see how consistent the fuel curve is. The accelerator pump will throw off the initial reading so trying to go full throttle in high gear is often best where there is more time to allow things to stabilize... unfortunately you need a place you can pull some high MPH without getting hurt or arrested.

chuck_royle

This may sound stupid but.. What about disconnecting the secondaries, jetting the primaries to 13.0 at full throttle. Then hook up the secondaries and jet them until you're at 13.0. Would this verify equal front and rear. Or does the power valve throw that off?

cutlassefi

What’s the altitude in Toronto? If you’re above 2000’ it’ll make a difference. You’ll need the smaller jets at that point.
Go up 4 sizes on the idle air bleeds, those are the ones around the outer perimeter. Readjust idle screws and go from there. Those effect low speed light throttle.
Also because you can change the power valve circuitry as well, you can go a little bit leaner on the primary yet you’ll still have the wide-open throttle fuel you need. That’s the next area you’ll need to fool with.
Great carbs, love em.

chuck_royle

NM! it would still be pulling fuel from the secondaries even though the blades are closed, dummy

chuck_royle

784 feet, so less definitely less then 2000'

Lonnies Performance

Unless I misunderstood, you do not need to lean out the part throttle. I think it is already good.

it is too rich at full throttle. Get it where you need it with main jetting (approx 12.5:1) & then see what is needed (if anything) at cruise.
You may be fine & never have to mess with air bleeds

chuck_royle

I could let it idle and check each header tube with my temp gun just for interest sake..

chuck_royle

That is what I'm saying, happy with cruise mix, still fat at full throttle. If I go smaller on the secondary side, shouldn't effect cruise. I think cutlassfi was speaking if my elevation was high

Lonnies Performance

Distribution dynamically changes with RPM & throttle position. Idle does not do you much good.
You need to check it at the conditions you are trying to adjust.

Things would be much easier if you had individual cylinder EGT's as well as a wideband in every header tube.
Without a lot of test equipment, I recommend you keep it to basics.

As for Jetting, I recommend you to do the same step changes front & rear, unless you have the ability to read each cylinder individually, then you can adjust all 4 corners separately.
4 rich cylinders & 4 lean cylinders can give you the perfect wide band reading, but a broken engine.

cutlassefi

Most square bore QF Carbs come with a 6 jet number stagger. If it was 10, you don’t necessarily need to keep it like that, although I wouldn’t go less than 6.
As mentioned the power valve circuit is adjustable as well. If all else fails you can change the orifices there up or down. That will effect your WOT Air/fuel.
And if those heads weren’t cc’d, and your deck is stock height or nearly stock height, then those 2323 pistons will only give you 9.5:1, max. Could be why you don’t need a lot of fuel.

chuck_royle

Thank you Mark(again) for the wideband suggestion and input. Also Lonnies, thanks for your advise.
Hopefully get some time after work this week to do more testing. Hopefully get back to the track this weekend.

chuck_royle

I finally made it back to the track, still runs a little rich at 12.0 is on the AEM. Unfortunately, didn't get a chance to adjust anything due to time. I brought my jet kit but I didn't get to the track until 2;50 and the place closed at 4 oclock. So, I just kept hot lapping with one break to swap out my drag radials for the welds and e/t's. It's always nice to run into strangers that are so willing to help. They saw me arrive late and frantically unloading, lots of help, good guys!
After a good chat with cutlassfi, (thanks Mark), I realize my compression is lower than I thought with these old slugs and chamber size in the speedmasters. I'm looking at some srp pistons this winter maybe. My 495 stroker dream may have to wait again.. good times

Owen Miller

Was this Q-jet modified to increase cfm when it was rebuilt?