Leaning cruise on Quick Fuel 850

[i82much]

I have a Quick Fuel SS-850, it's basically a niced-up 4781. I've attached a spec sheet from Quick Fuel.

I have a 65 GTO, 4 speed manual, 3.54 gear, 28" tire. Dual plane intake, 236 @ .050 hyd roller, and 1 7/8 headers, edelbrock heads, 505 CID, 4.35 bore, 4.25 stroke, 10:1.

Running at 60 MPH, 2700 RPM or so, I see about 12.5 to 1 AFR on the wideband. I'd like to lean that out a bit. I am happy with the WOT numbers.

Downleg boosters, 80 primary jet, 86 secondary jet, .069 PVCR on the primary side, .093 transfer slot restrictor.

Do I:

(a) Jet down the primaries and increase the PVCR,
(b) Reduce the primary transfer slot restrictor, or
(c) drink a beer, don't mess with a car that runs good, and go for a ride?

[Walter R. Malik]

Using a slightly larger primary idle air bleed is the easiest way; of course the idle screws will need to be readjusted.

[i82much]

Using a slightly larger primary idle air bleed is the easiest way; of course the idle screws will need to be readjusted.

Thanks! I will say that this carb has a choke and changing the primary air bleeds is actually kind of difficult to do.

[rgalajda]

Leave the air bleed alone for now.
Reduce the main jet size from 80 to 78 or 76 and test. One change at a time.

I doubt if the .093" TSR is doing anything.
Start with main jet.

[i82much]

Leave the air bleed alone for now.
Reduce the main jet size from 80 to 78 or 76 and test. One change at a time.

I doubt if the .093" TSR is doing anything.
Start with main jet.

Is it bad to change the front-to-rear jet stagger? Meaning is it ok if the carb starts at 80/86 to wind up at 76/86? Or does that hurt WOT front to rear fuel distribution?

[Tuner]

Connect a vacuum gauge and determine the vacuum at the cruising condition (RPM and in. Hg) you want to adjust. Record the vacuum and A/F in several different conditions of load and RPM.

What is the vacuum at steady speed, 60 MPH, 2700 RPM, on level ground?

If it is near or higher than 15" Hg it is likely the idle/T-slot circuit will have significant influence.

The Main Jet will influence intermediate load, approximately 12" and lower down to where the PV opens,

Determine the vacuum level of the power valve opening point by two means. Read the identification marks on the valve (6.5 ?), and confirm this by crowding the throttle progressively open to progressively apply intermediate load to determine actual vacuum level when the A/F enrichment occurs as the valve opens, when the WBO2 gauge shows the A/F suddenly becomes richer.

What is the A/F in the vacuum range above the valve opening point. For example if the valve is marked 6.5 and it actually opens at that value, or between 7 and 6 inches Hg, what is the A/F at 8 or 9 inches Hg?

The 31/31/29.2 emulsion bleeds are larger than (what I think) is ideal and very likely will cause the main to start to flow early and erratically. This can result in the common problem of a too-rich and erratic A/F at small throttle opening cruising conditions. I would change them all to .026" on primary and secondary.

At the beginning of flow from the main nozzle the main air bleed and emulsion bleeds act to cause enrichment by reducing viscosity of the fuel and lifting fuel up the main well, encouraging flow at the low end and into the mid-pedal. The over-all effect depends on bleed configurations, size and number, and height below float level in the well. Too much emulsion bleed air is the cause of the common complaint of level road sruge and irregular misfire that fools people to think it is too lean because the unburned oxygen fools the WBO2 to display a lean A/F number.

[HQM383]

Most aftermarket carbs are going to have a large IFR and have you running richer than lean because they don’t know where or what their universal carb will end up on. They almost always have screw in IFR for the end user to dial in the tune to their specific combination but that doesn’t happen as much as it should. The transition slot on newer carbs tend to be larger than in the past and for this reason a reduction in IFR is in order as well as reasons above.

Change to a .034” IFR to begin with to see the effect taking note of how far into the throttle the change has effect. This might get you to where you want to be or at least guide you to the next move. If the emulsion is threaded when you get the 2700rpm steady cruise right flip the sizes top to bottom and see the effect.

[BillK]

(c) drink a beer, don't mess with a car that runs good, and go for a ride?

This . . . . or put a Quadrajet on it

[HQM383]

(c) drink a beer, don't mess with a car that runs good, and go for a ride?

This . . . . or put a Quadrajet on it

Don’t forget that a couple of dollars for smaller ifr’s will likely save gas money for more of (c)

[rgalajda]

Leave the air bleed alone for now.
Reduce the main jet size from 80 to 78 or 76 and test. One change at a time.

I doubt if the .093" TSR is doing anything.
Start with main jet.

Is it bad to change the front-to-rear jet stagger? Meaning is it ok if the carb starts at 80/86 to wind up at 76/86? Or does that hurt WOT front to rear fuel distribution?

No it is not bad to change front to rear stagger. I assumed you may be a novice at this so I kept it simple .
IFR will be the next thing to look at because it affects low to mid rpm. WOT / AFR can be addressed later if needed.

[i82much]

I went down to .035 IFR's previously and it was just a little soft off-idle. Like if you were just driving normally, feathering the clutch trying to get started on an inline, it would get pretty lean on the wideband and you'd have to give it a lot more throttle than you'd expect to get going.

I was hoping just a smaller transfer slot restrictor might help without doing anything else, since WOT is good.

I do like the idea of going for a drive to see what my vacuum is under the conditions I am trying to lean out, but going from memory I am almost certain it is well over 15". Maybe 18" or so.

[HQM383]

I went down to .035 IFR's previously and it was just a little soft off-idle. Like if you were just driving normally, feathering the clutch trying to get started on an inline, it would get pretty lean on the wideband and you'd have to give it a lot more throttle than you'd expect to get going.

I was hoping just a smaller transfer slot restrictor might help without doing anything else, since WOT is good.

I do like the idea of going for a drive to see what my vacuum is under the conditions I am trying to lean out, but going from memory I am almost certain it is well over 15". Maybe 18" or so.

You said happy with WOT. What is the WOT AFR that is making you happy? There might be room for dropping a few jet sizes if the engine has the appetite for .037” IFR.

[i82much]

I went down to .035 IFR's previously and it was just a little soft off-idle. Like if you were just driving normally, feathering the clutch trying to get started on an inline, it would get pretty lean on the wideband and you'd have to give it a lot more throttle than you'd expect to get going.

I was hoping just a smaller transfer slot restrictor might help without doing anything else, since WOT is good.

I do like the idea of going for a drive to see what my vacuum is under the conditions I am trying to lean out, but going from memory I am almost certain it is well over 15". Maybe 18" or so.

You said happy with WOT. What is the WOT AFR that is making you happy? There might be room for dropping a few jet sizes if the engine has the appetite for .037” IFR.

maybe 12:1 on a warm day, 13:1 on a cold day?

[Tuner]

I went down to .035 IFR's previously and it was just a little soft off-idle. Like if you were just driving normally, feathering the clutch trying to get started on an inline, it would get pretty lean on the wideband and you'd have to give it a lot more throttle than you'd expect to get going.

I was hoping just a smaller transfer slot restrictor might help without doing anything else, since WOT is good.

I do like the idea of going for a drive to see what my vacuum is under the conditions I am trying to lean out, but going from memory I am almost certain it is well over 15". Maybe 18" or so.

I would try a smaller T-slot jet. Take a big swing at it to clearly see the range in the pedal and vacuum gauge where the T-slot jet will influence it. .067 or .070 should be lean enough to make it stupid, so from there you step back larger until the next single step makes it run OK. For level road and light load, and at intermediate load with the vacuum high enough the PV is closed, find the lean limit where it balks and stumbles and then richen back up until the stumble dissappears. To get the best mileage With PV closed you want the leanest A/F that runs smoothly, so to burn the least fuel. Obviously, at heavy load with the PV open and WOT you want the best power rich A/F.

[HQM383]

I would try a smaller T-slot jet. Take a big swing at it to clearly see the range in the pedal and vacuum gauge where the T-slot jet will influence it. .067 or .070 should be lean enough to make it stupid, so from there you step back larger until the next single step makes it run OK.

I was thinking the same. There is some wiggle room for main jets but not enough to get from afr 12's up to an acceptable light throttle cruise afr without delving into other circuits.