Quick Fuel Secondary air adjustment

[DanEC]

Try adjusting the secondary plates first but you don't want to go to far or you will start uncovering the transition slot in the secondary bores (assuming Quick Fuel is essentially like a Holley). I have an old Corvette with a big cam and lots of compression that only produces 8 inch vacuum at idle. I had to drill the primary throttle plates in the old Holley 3247 to tune it so unless Quick Fuel has made provisions for a big cam in it's base turning ability, that may be necessary.

Typically, a lot of tuners pull the carb (to get a better view of the actual exposed transition slot below the throttle plates) and set the primary idle setting so that a "square" exposed primary transition slot is seen below the plates. That should be the best setting for primary system idle. Then open the secondary plates from first contact of the adjustment screw with the throttle cam, about 1/2 to 3/4 turn open. Also experiment to see how many turns your can open the secondary throttle before the transition slot starts appearing below the throttle plates. Install the carb and try it and make any necessary adjustments in idle speed by opening the secondary further but stay below the max number of turns of the screw that your experiment showed would start exposing the secondary transition slot. Also adjust the idle mixture screws for max rpm or vacuum. If you are just slightly short of getting to your desired idle speed you can make a small adjustment to the curb idle setting for the primary but you don't want to uncover the transition slot very much more or it will upset the transition response and idle mixture. If you can't get to a good idle speed within this range of adjustments then you probably need to consult with Quick Fuel about drilling the plates or to see if they have any other ideas on how to adjust the carb to idle cleanly and respond smoothly from idle.

I also found that restricting the primary idle bleeds greatly boosted the vacuum signal to the primary circuit idle system and helped it run better. Old Hollys are not adjustable but I restricted it with a wire inserted in the bleed holes up at the top. I suspect Quick Fuel has replaceable air bleeds so you may want to consult with them and see if maybe you need to go a bit smaller with the bigger cam you are running.

The goal in all of this was to be able to tune the idle system to produce the highest vacuum or rpm with the least number of turns from completely closed. I finally got mine down to the 1-1/4 to 1-1/2 turns out range.

[Gaz64]

Nice description of setting up there Dan.

I would like to add, I use Holley XP Ultra or Barry Grant Demon Idle-eze to trim idle speed on my carbs.
I set the primary and secondary to just above an appropriate minimum for the engine, and trim with the bypass valve.

One carb I built for a mate of mine I fitted a set screw to the primary side, so he doesn't mess with it. I taught him to trim the idle if necessary with the secondary.
It has a lot of scope, about 500 rpm, which he never needs. 260 @ .050 Roller cam 355 idles happily (EFI quality) at 850 rpm 8 inches Hg.

Gary

[AC Ventura]

Thanks for all the replies guys and I’m sorry for my late reply, but couldn’t sign in for 2 days. Then reset my router....
Sadly, I don’t have the secondary adjustment externally as hoped. Thanks all for the info on the transfer slot exposure, which I just about get. Just not sure how I adjust the secondary throttle plate opening. But I guess that will be revealed.
But here’s the thing. Like I said, on idle, intake manifold vac is 9”. But when I rev, it rises to about 15”, which must be due to the far from standard, cam profile, right? The PV is 6.5 and the PJ’s are 72’s, with SJ’s 83’s.
As stated the problem is rich idle and bucking on take off and overrun. Some have said go to a lower number power valve, but surely that going to open even earlier. I’m thinking, maybe I need to go up to a 8.5 or 9.5 PV and also order some smaller jets, 55-70 for the primaries. What do you gurus think?