Holley / QuickFuel Transition slots vs Manifold Vacuum relationship

[mcwho]

Trying to understand what the relationship is between Transition slot (t-slot) for primary and secondary on Holley QFT carbs. As they seem to be of similar design, not much info on QFT but lots on Holley.

I have a QF ss-650-an. It came from the factory with the primary t-slot showing about .090 and the secondary t-slot not showing at all. Was told to make the secondary t-slot show a "square" and also the primary a "square". that would make both of them about .050. Supposedly the reason is the lower manifold vacuum which causes a stumble at just off idle.

My engine vacuum is right now at 14.5 ~ 15. A normal engine (according to my vacuum guage is in the 17 ~ 22 range. Therefore I believe what is normal for a normal aspirated engine would make the factory settings work for that engine.

The theory seems to be that the lessening of the vacuum for souped up engines requires these changes to the t-slot settings. Since my Vacuum is not that far from a stock the t-slot settings need not be as radical.

Therefore if my engine vacuum is say 15 or if it is 13, would the engine with the 13 vacuum need to have twice as much t-slot derivation?

That being said, after making these adjustments, my engine would not idle below 1200 and the idle air mix screws had no effect when I turned them in and out.

After that I closed the secondary t-slot setting by turning the set screw out about 1/3 of a turn. That change now made my idle at about 900 and the ims screws to work but my front screws for best vacuum are in the 5/8 turn range and the rears are in the 1/4 ~ 1 1/8 range.

My mathematical side says that closing the secondary screw slightly more , say 1/4 to 1/3 turn should allow me to get the idle to around 800 or 850.

Is this a reasonable idle for a 331 stroker with a mild cam, edelbrock performer rpm heads and RPM air gap intake, and a cam with 224/232 duration @ .050

Before I started this I replaced the spark plugs because the previous carb, an Edelbrock always ran rich and they were black.

I marked and noted where the original settings were. I increased the set screw on the setting of the secondary
by 5/8 of a turn or.625 turn, (1/8 - .125 turn, x5 = .625)
then I backed it out by 2/6 or a turn or .333, note 1 notch on an allen wrench is 1/6 of a turn or .166
my current effective setting is (.625 - .333) or .292 of a turn
therefore that .292 of a turn lowered the idle by 300 rpm
if it is a straight line 1/3 of my last change should net me about -100 rpm or 800 which is my goal.
.292 / 3 = .097 of a turn, or just over 1/2 a hex wrench notch



The pics are:
Primary after and before
and
Secondary after and before

[CraigS]

I have read all the same stuff over the years. I use the square slot theory as a start point. Then I go from there as needed. I have never had a problem that was caused by adjusting the idle speed screw so that the T-slot is a bit off from the recommendation. At some point, to get the idle speed you want, you either adjust the idle speed screw or the timing. If my timing is where I want it, I go to the screw. At one point I found that my 351 ran a lot better in the 1000-1800 range w/ 16 degree initial rather than 12. That meant a lot of work on the distributor to reduce the mechanical advance so I could start at 16 but stay below 33 total.

[NAZ]

Bob, I know you've been doing research on carbs and you recently purchased a Quick Fuel replacement for an old problematic carb. Now you're researching tuning advice. The best advice I can give you is you're looking in the wrong place for help. No matter how knowledgable someone is that will respond to your questions there simply is not enough room in a forum to give a good explanation of what to do, how to do it, and why for each step in the process. A carb is made up of many circuits that meter air & fuel to achieve the proper ratio. Make an adjustment here and you affect something else there. I think you'll end up frustrated before you have your carb dialed in. While I've not come across any books detailing Quick Fuel carbs; there are several good books on Holley carbs that explain how they work and how to tune them. Do a search on Amazon and you should find a few that will help to demystify the tuning process. Knowing how to tune an engine is a valuable skill and can be very rewarding but is somewhat complex so I recommend you hit the books to get a good foundation to build on. Good luck.

[mcwho]

Good Comments, I agree with both, and I do have a book ordered as I do want to understand this carburetor more.

[johnnybgoode]

Your primaries look good but I'd close down the secondaries so you can just barely see the slot. Also if you haven't done this already with the carb off take the secondary throttle stop grub screw out from the bottom of the carb and reinstall it from the top so you can adjust it with the allen key from the top of the carb. Once you have the front T slot set try to use the rear adjustment to fine tune your idle. Usually a 1/4 turn or so on that rear screw will do the trick. Good Luck. Scott

[mcwho]

Scott,
I didn't think about reinstalling the set screw from the top. Right now I can get to it from the bottom and my current setting of .29 turns is a bit too much. My calc leave me to believe that a setting of .2 turns or -.09 from what I have sb close. Gonna try that today!

[mcwho]

I just adjusted the carb as mentioned and reset the IMS again. It is still idling at 900, the idle is smooth, and the off idle acceleration has no stumble. The mid range accesl is awesome. I really liking this carb, I have the secondary set screw out about .23 or .24 or a turn.

[GoDadGo]

Congratulations!

[CraigS]

Good job. It's not so difficult is it?

[mcwho]

Nope, All is good in Carburator City !