Can't set idle low without stalling
Thread Starter
Le Mans Master
Joined: May 2000
Posts: 8,729
Likes: 1,508
From: Redmond WA
Can't set idle low without stalling
I have a ZZ502 in place of a 427/390 that I've just put the original 427 intake on along with a new Quadrajet. After setting up the choke and vacuum break and so on it starts and fires immediately.
However, even once warm, once it comes off fast idle it stumbles and dies. Basically, I can't set the main idle speed below about 1200 or it falls over.
The cam is only 218/230, which is the "stock" ZZ502 cam. It's a little larger than a stock 427 cam would have been but the displacement is larger too... and it idled OK with the Holley 850 on it once warmed up.
I checked the timing, its at 8 BTDC. It really doesn't like to run without the vacuum advance (as you have to do when setting the initial timing), as in it seems to want a lot more timing, but I set it there for now and hooked the vacuum advance back up.
What's the typical cause for this situation? I can't spot any obvious vacuum leaks, and the mixture screws, while not having much effect, are out about 4 turns each right now. It was OK before the intake and carb swap, so I presume I can limit the issues to the air-fuel setup, but not sure -where- to start hunting.
Any direction on where to start looking would be greatly appreciated!
Thanks!
Dave
However, even once warm, once it comes off fast idle it stumbles and dies. Basically, I can't set the main idle speed below about 1200 or it falls over.
The cam is only 218/230, which is the "stock" ZZ502 cam. It's a little larger than a stock 427 cam would have been but the displacement is larger too... and it idled OK with the Holley 850 on it once warmed up.
I checked the timing, its at 8 BTDC. It really doesn't like to run without the vacuum advance (as you have to do when setting the initial timing), as in it seems to want a lot more timing, but I set it there for now and hooked the vacuum advance back up.
What's the typical cause for this situation? I can't spot any obvious vacuum leaks, and the mixture screws, while not having much effect, are out about 4 turns each right now. It was OK before the intake and carb swap, so I presume I can limit the issues to the air-fuel setup, but not sure -where- to start hunting.
Any direction on where to start looking would be greatly appreciated!
Thanks!
Dave
Drifting
Joined: Mar 2011
Posts: 1,271
Likes: 1
From: PA
Try more bypass idle air....(crack the secondaries open just a little) and see if it helps. If so you may need to drill your throttle plates.
Oops I see your running a qjet. Disregard.
Oops I see your running a qjet. Disregard.
Last edited by boltnut; Jan 14, 2012 at 07:36 AM.
Drifting
Joined: Mar 2011
Posts: 1,886
Likes: 9
Sounds like your Qjet needs to have the idle circuit recalibrated. Hard to tell from here if it's rich or lean, though. You could try the cupping your hand over the carb to "choke" it a bit. If it runs better, it's lean. If that kills it, it's rich. Without some sort of exhaust gas analyzer you can't really tell for sure. I put in an air/fuel meter, myself, to try to get the mix right all through the RPM range. With a Qjet, you can adjust the idle, part throttle, main primary jets, and secondaries, one at a time, to get everything right. Don't drill the throttle plates, if you can help it. That will just cause the main circuit to run all the time. As far as the primary venturies are concerned, air going through a hole in a throttle plate is the same as partially opening the throttle plate. Get Cliff Ruggle's book. Or Doug Roe's. Or both.
Last edited by gcusmano74; Jan 14, 2012 at 07:29 PM.
Burning Brakes
Joined: May 2009
Posts: 1,232
Likes: 6
From: Tucson AZ
St. Jude Donor '12
Also, the QJet, did you buy one set up for a 427 or a 502? Depending on the answer to this, the idle issue may just be a more obvious sign of problems throughout the rpm range.
(I'm no q-jet guru tho, so this is based on general carb knowledge)
