converting 4 corner idle to 2 corner?

[campindog]

Is there a right/wrong way to do this?



Do the transfer slots in the secondaries of a vacuum secondary carb actually work prior to the secondary boosters kicking in?


I'm having a heck of a time getting a quickfuel hot rod series carb (680 v/s) to idle decently. I have a stockish cam in a mild 331. I've messed with both the IAB and IFRs without success. The idle circuit was waaaaay too rich out of the box. I had to keep the the idle mixture screws ~1/4 turn out. It would die with a little more or a little less. Very touchy! I opened the idle bleeds from .70-78-.83 and it's better, but still runs like crap. Then I shrunk the IFRs from .028 to .026 and brought the IAB down to .63. It idles better but tip in sputtered and fell on it's face. Not drivable. Back to .028 IFR's with .83 idle bleeds. I closed the secondary mixture screws, and put duct tape over the idle bleeds. Best yet. I was able to turn the primary mixture screws out ~1 turn and drive the car. Idle is ~14:1 AFR @ 16"Hg. Cruise is ~14-15 AFR @ 19". With the PV open <8.5" AFR goes to ~12.5. The transfer curcuit however is real lean. About 17:1 at 1500 rpm. I think next I'll try shinking the idle bleeds back to .78, leave the scondary idle circuit off and try again. I haven't done anything with the secondaries yet.

Am I on the right path?

[ehummelman]

How's your timing set up? I only ask because my QFT 735VS works great, so I wonder if there is something else going on. You'll never ever get the carb tuned if your timing isn't dead on first. And make sure you have no vacuum leaks.

[campindog]

The timing is set at 12* initial, 34* total @ 3k rpm's. No vacuum leaks. I can flip back onto my old holley S/A and it runs fine. I made the switch to QF for more performance up top and better throttle response, but the street avenger has it beat so far. Ironicaly this is a little 570 S/A that I used when my motor was a stock 289 (jetted up 8 sizes).
Thanks for the input.

[ehummelman]

Have you called QuickFuel?

[Chevelle598bb]

You would have to change the rear metering block to delete the rear idle circuit. Keep tuning the four corner. If you change to two circuit the front circuit will not be big enough and you will be in the same boat.

[campindog]

I dunno. I shrunk the IABs from 83 back to 78, closed the secondary mixture scews and blocked the secondary IABs. It ran the best yet (with this carb). It had a slight lean surge when just on the transition circuit (1200 -1800 rpm). The primary mixture screws are 1 turn out, but really sensitive. An 1/8 turn makes a huge change.

I think I want to try .070 air bleeds in this configuration before going back to tuning it as a 4 corner idle. If I go back to 4 corner, I'll need huge air bleeds. .083 bleeds seem to be as large as they go, so I'll have to drill my own. No big, but it just seems odd I would be this far out from the norm.

I did call QF (before posting) and the guy I talked just didn't seem interested. He acted like he was late to lunch or something and just wanted to get off the phone.

[campindog]

I just saw Summit has .095 in stock. I'll try that and see if I can get it lean enough with 4 corner idle.

[ehummelman]

I did call QF (before posting) and the guy I talked just didn't seem interested. He acted like he was late to lunch or something and just wanted to get off the phone.

Wow, that's disappointing. WTF ever happened to customer service?

[campindog]

I wasn't that miffed at QF, but wasn't impressed either. I don't think they have a real "tech line". I think who ever answers the phone is going to try and fix the problem.

Update:
Today I tried .095 idle air bleeds with 4 corner idle and it ran the best yet, but still a long way from the holley I want to get away from. Here's what I ended up with:
Just under 3/4 turn out on the 4 mixture screws.
Highest vacuum was 16"Hg (should be 17-18")
Idle AFR was 13:1
Tip in is very poor! Lean stumble then rich unloaded. Gotta rev the motor to get going from a stop.
Tansition circuit is still very lean. 15-16:1. It drives poorly below 2,000 rpms

If I open the mixture screws any (<1/16 turn) it goes pig rich, barely idles and the exhaust burns your eyes.

So I'm running out of ideas on how to get the carb to idle decently. Anyone have ideas or suggestions on where I could go for advice on this?
Thanks,
Derek