Rich IDLE on QFT HS 680 VS Question
 

Hi everyone,
I have a QFT 680 HS Vac Secondary I got in a trade, practically brand new, everything appears stock settings 70/74 jets. Main Bleeds #28 , all as per :

https://www.holley.com/products/fuel...arts/HR-680-VS

All new rebuild kit installed, no blocked holes, no passage ways blocked.

The crazy thing is I have never had a Holley carb continually no matter what run rich at IDLE 11.0-11.5:1 ratio with idle mixture screws essentially doing nothing to get me leaner unless almost bottomed out.

yes timing , plugs and everything else is good. just rebuilding another carb.

What I've done:

√ 1/2 turn out from bottom on 4 corner idle screws
√ Rear Idle screw set so idle is roughly 850 RPM
√ Primary Idle Screw only turned CW 1/2 turn from bottom
** So front rear transfer slots should be relatively close however I'd have to flip it and check.

I actually came from the rear idle screw completely backed off and figured I may as well add more idle to see if that helped.

√ Float levels perfect front and back. Tried lowering them but did not affect AFR.
√ Fuel pressure is perfect.
√ The Main Air Bleeds are stock 28's
√ 6.5 PV tested as well, holds vacuum gasket used

I'm at a loss, I can't turn the idle screws in any more and expect that 1/4 turn out is "normal" range or practically bottomed out.

I really think this carb is out of the box settings and a bit surprised the troubles its giving.

My only guess is changing the idle circuit screw in each metering block to something smaller? (as seen in pic) ??

Aside of closing off the rear idle mixture screws and just running off the front which I'm trying to avoid.

Thanks for your help.

Some carbs had problems with the metering blocks not seating all the way because the power valve nose was touching the main body.

If you pull each metering block do you see any witness marks where the power valve might be touching?

Also note that the main jets have little effect on idle conditions in a Holley carb.

Most feedback in fb is suggesting my rear transfer slot is over exposed with symptoms I’m explaining. However the spring in diaphragm is not pressing the rod down hard enough so maybe I need to replace it a stiffer spring ? Wouldn’t want it flopping around . I dunno maybe the screw adjustment might tighten it up ?

Yes I heard that the pv or block off pv might have to thick of a gasket ? That I should try lining up metering block to body with out gasket and look for issues ? Same with main body to base plate . Also replace pv just to rule
Out . Power went out here so will
Have to try tomorrow . Thanks for suggesting the indent mark may be present .

Have you tried adding secondary idle air by adjusting the secondary throttle stop? That’s usually all that is needed to clean up the idle and restore function to the mixture screws.

Secondary transfer slot should not be exposed at idle. Starting point is to back off screw until it does not touch secondary arm. Then screw it in until the screw touches the arm, then go another 1/2 turn.

The IFRs looks large to me, but I can't see the actual restriction. Stock IFR on that carb is likely about .028 to .030".

Front transfer slot should be square at curb idle.

Starting point for 4-corner idle is to open the screws 1.25 turns. Close them in 1/8 turn increments for highest vacuum. A 4-corner usually ends up with idle mixture screws 1/2-3/4" turn open from seated.

What are LSAB sizes? 70?

High IFRs and 3 emulsions. Low speed AFR is going to be unstable. Cruise mixture is going to be rich. WOT will be about 13.1

update
 

70/28 Jet sizes on top of venturi's - stock for this 680.

The main metering block doesn't have any brass little jets along the channel(pic included - normal? ) - where the secondary metering block does. however secondary has block off PV.

I am not looking to run the carb long term.

3/26/2026 Next day: Removal and inspection:

Rear transfer slot exposed causing a ton of RICHNESS? - NO.
Nothing visible showing any slot

Front Transfer slot : CHOKE OFF – Exposure : 0.020 Small amount – no idle screw ADDED

Metering Blocks – Verified GASKETS are a match to rebuild KIT - no blocked holes
- NO WARPAGE, placed against BODY no gasket– perfectly FLUSH
- No P.V. HITTING BODY

P.V. - > Tested OK, installed another one. Which had TESTED OK too

Ensure all plates can return to their resting position.

Verify 680 MODEL # in rear of body

1) Follow advice: CHOKE OFF – SET PRIMARY throttle blades to 0.020 T-SLOT exposed
2) Follow advice: SET SECONDARY throttle blades so it’s bottomed out and closed
3) Set all idle mix SCREWS - 1.5 TURNS OUT
4) Choke FAST IDLE SCREW – ½ TURN BACK FROM RED CAM WHEN OFF – NO BINDING, VERIFY FULL ON/OFF on bench
5) Prime BOWLS with FUEL – activate shooter
6) Fire Car - Choke ON – 900 RPM
7) 11:1 AFR . All fine. Kick choke off in 2-3 minutes, won’t idle 600 rpm stall
8) Raise secondary plates slowly – bring idle up to 900
9) WARM CAR UP to 160F - IDLE goes up to 1000 on it’s own.
10) 11:1 still
11) Turn idle mixture screws ¼ leaner , no affect really. Does stall when I go right down on a couple of them so that is good.
12) Turn down secondary throttle blades to bring idle down 850 RPM
13) Mess with mixture screws leaning it out more
14) Check to see if primary throttle blade can slow down IDLE – no. I am fully relying on rear .
15) Mess again with mixture screws leaning it out – Finally see numbers in the 12:1 - 13 Range. But I only kind of see it doing it on - the last idle mixture screw. ( I make a small change on all of them at once same amount) .

16) Back and forth to the mix screws and 02

well it’s in the 12-13 range now.

This carb is just what it is. Nothing I did, this is just what it takes to run on this car. So baring changing out metering block IFR jets or some other jet (not prim/second) .

All I can hope for tomorrow is that the choke does work and it fires up to 1000 or so warms up to maybe 1200 .

When I kick it off it idles and stay at 12-13. Until tomorrow.

Thanks again everyone. I had all the same challenges with a SPEED DEMON 625.

However I can see there may be some comments about why did you expose the secondary slot, why not just raise the primary?

I guess that is the advise I got?

Fired it up today, started at 1000 rpm. Got it to 140 F. AFR was 14.5-15.5 not end of world. I'm like let's see these T-SLOTS. Pull it off.

Rear ended up if you can believe it . 0.020. Primary was about 0.060 choke off. Not super end of the world stuff. Good for me. I thought the rear was gonna be something ridiculous.

Whats the timing at idle?

I had to do a lot of modding and tuning to get my QF 750 double pumper under control. The high position IFRs really make idle mixture and low speed cruise tuning a chore. If you want a more predictable idle mixture, move those to the old school low position.

The block you show is just like the ones my carb came with. The emulsions are predrilled to .028" The 3 emulsions help drive it rich during cruise, but WOT tended to be closer to low 13s than low 12s, at least on mine.

18 Initial. Can you tell me what you mean by high position , low position? Oh in the metering block the vertical slots, take screw from top to bottom. gotcha. thanks again everyone.