Transition slots QFT 680-VS
Thread Starter
Racer
Joined: Sep 2011
Posts: 414
Likes: 55
From: Humboldt county Ca
Transition slots QFT 680-VS
I'm not familiar with this carb. Always ran Edelbrocks. From the directions, the transition slots should be set "square". When I do this the car won't idle. The primary slot exposure is controlled by the curb idle screw, best I can tell. Looks like the secondary slots are controlled by the upside down hex screw on the passanger side.
Am I supposed to be "square" on the front slots and trying to idle on the secondary slots? What about the primary slots?
I'm all confused!
Stump
Am I supposed to be "square" on the front slots and trying to idle on the secondary slots? What about the primary slots?
I'm all confused!
Stump
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
You should start with the transition slots looking like squares. And you're correct, the exposure is controlled by the curb idle screw. You may need to be off "square" to get idle, but you want to be as close as possible so you don't get an off-idle stumble= that's the purpose of the transition circuit as I understand it. Back in the "old" days, the fix to stay in the transition circuit and get a fair idle with a radical cam was to drill the throttle plates. With all of the adjustability of later carbs, that old school trick is sort of past tense.
I would start by setting up the transition slot/throttle plates with the idle speed screws on both ends, then lightly seat the idle mixture screws and back out 1-1/4 to 1-1/2 turns. Go from there, keeping the mixture screws roughly equal. You might have to be off a bit on the primary slots, but as long as you don't get the off-idle stumble it will be fine.
I would start by setting up the transition slot/throttle plates with the idle speed screws on both ends, then lightly seat the idle mixture screws and back out 1-1/4 to 1-1/2 turns. Go from there, keeping the mixture screws roughly equal. You might have to be off a bit on the primary slots, but as long as you don't get the off-idle stumble it will be fine.
Melting Slicks
Joined: Mar 2008
Posts: 2,666
Likes: 289
From: Spring Park MN
Start out with small holes, .060" or so first, drilling all four blades. Try it and see how it idles. If it is still too slow, drill them a little larger. Don't go too much at a time or you can blow right past where you need to be.
Mike
Le Mans Master
Joined: Mar 2001
Posts: 5,738
Likes: 644
From: Lincoln, CA
You use a Allen wrench to adjust the secondary's. If you can't get to it you have to remove the carb and open them a little.
I could get to mine with a mirror and flash night so I could see what I was doing.
I could get to mine with a mirror and flash night so I could see what I was doing.
Le Mans Master
Joined: Oct 2010
Posts: 6,938
Likes: 848
From: melbourne florida
2023 C3 of the Year Finalist - Modified
Quickfuel makes a part that allows adjustment of the secondary transition slot without having to remove the carb or use a mirror.
I would leave your primary slot square and open the secondary slot a bit.
I would leave your primary slot square and open the secondary slot a bit.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
The whole idea is to get equal air flow through all 4 corners of the carb at idle and to avoid excessive primary transition slot exposure. To do this, you start out by opening the primaries just enough to make the transition slot appear to be "square" below the edge of the throttle plate. The secondary throttle should then be adjusted to be open exactly the same amount. On some carbs, such as Holley, the secondary transition slot is located higher in the bore than the primary slot, so you can't use the t-slot as a gauge. Other carbs, such as Demon, has the secondary t-slot in the same location, so you can set the secondary throttle opening to also produce a "square" t-slot exposure. Once you have both the primary and secondary throttles cracked open the exact same amount, all idle speed adjustment is made by tweaking both primary and secondary side the same amount - keeping them equal. In most cases, this base setting will result in a high idle, so both throttles will need to be closed just a tad. In some cases, you'll need to crack them open just a little. But the "square" t-slot setting provides a starting point baseline which will get you close, and it will give you balanced airflow if you adjust both ends of the carb evenly when setting the final idle speed.
Lars
Lars
Last edited by lars; Jun 1, 2017 at 01:26 PM.
Thread Starter
Racer
Joined: Sep 2011
Posts: 414
Likes: 55
From: Humboldt county Ca
After setting both primary and secondary slots I do have a high idle. The curb idle is all the way off the stops and the secondary is down far enough that it no longer drops idle. All vac ports plugged except vac can. Carb bolts re torqued and vacuum leaks checked around carb.
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What next?
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Melting Slicks
Joined: Mar 2008
Posts: 2,666
Likes: 289
From: Spring Park MN
The whole idea is to get equal air flow through all 4 corners of the carb at idle and to avoid excessive primary transition slot exposure. To do this, you start out by opening the primaries just enough to make the transition slot appear to be "square" below the edge of the throttle plate. The secondary throttle should then be adjusted to be open exactly the same amount. On some carbs, such as Holley, the secondary transition slot is located higher in the bore than the primary slot, so you can't use the t-slot as a gauge. Other carbs, such as Demon, has the secondary t-slot in the same location, so you can set the secondary throttle opening to also produce a "square" t-slot exposure. Once you have both the primary and secondary throttles cracked open the exact same amount, all idle speed adjustment is made by tweaking both primary and secondary side the same amount - keeping them equal. In most cases, this base setting will result in a high idle, so both throttles will need to be closed just a tad. In some cases, you'll need to crack them open just a little. But the "square" t-slot setting provides a starting point baseline which will get you close, and it will give you balanced airflow if you adjust both ends of the carb evenly when setting the final idle speed.
Lars
Lars
On engines like my own, the throttle blades had to be opened to the point where I was into the transfer slots so much that I had completely unresponsive idle mixture screws. Drilled holes in the throttle plates and then my throttle blades were where they should be and I could adjust my Idle mixture.
Mike
Melting Slicks
Joined: May 2002
Posts: 2,717
Likes: 121
From: Sulphur LA
St. Jude Donor '05-'06,'11,'13-'14,'16,'18,'19,'24, '25
If you would like to be able to adjust the secondaries with the carb on the car just pull the hex screw from the bottom and install it from the top then all you need to do is open the secondaries and adjust from the top.
Neal
Neal