Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
Setting Up and Adjusting Idle
defiant1:
Made some adjustments:
Drilled out the idle air bypass to .086 from .081.
Installed lighter power piston spring.
Adjusted throttle plates so no more than .040 of the transition slots was showing.
Hooked everything back up. Ran motor to operating temp and kicked it off of the choke. It died. So I attempted to hold the throttle at a steady rpm and adjust the timing at the same time. Could not get it to idle. Wound up turning the throttle screw in 1 1/2 turns.
Idle mixture screws were set 5 turns out from seated.
It ran ok (still shuddered) at 850 rpm/14* BTDC and held 12" Hg for vacuum.
However, do to the fact I adjusted the throttle plates 1 1/2 turns I am sure I am no where near the .040" I originally set it at.
Oh then I leaned over the fender and looked down the carb. It looks like I have nozzle drip as well at idle.
Soooo, do I increase the Idle Air Bypass hole diameter even more? Do I back off the idle mixture screws even more? I am gaining. Vacuum is a little better, motor runs a little more consistent...rough idle but consistent.
How do I rectify the nozzle drip? I am assuming this is happening because the throttle plates are open too far at idle.
If that is the case, how can I get the motor running effectively with throttle plates nearly closed???
d1
71vette:
It seems like you are getting a lot of good help from others but I thought that I would ask a question and make a couple of observations. For an engine to jump 4" of vacuum when you removed the PCV valve, that seems like a lot. Have you checked to make sure your PCV valve is the correct one for the engine? I ask because I had this issue in the past where the PCV was not calibrated properly for the engine and it was allowing too much air. If I am correct in that it is allowing too much air, this would explain why your idle screws are not making much of a difference because it is so lean.
When you plugged the PCV hose, I am going to assume that it idled much better since the vacuum jumped to 15". Is this correct?
If you add more bypass air, I think it will only make your issue worse. The way it is now, you have had to open the throttle plates so that it would get some fuel out of the main system to keep running. Closing the throttle plates enough to stop the drip reduces the fuel but if the system is as lean as it sounds, then it won't idle.
defiant1:
I have a new PCV valve coming (just in case). I just ordered one that is for my motor Olds 350, are there different versions out there that compensate for camshafts, timing, etc.??
It did run better when I plugged the hose, rpm increased almost 500 rpm from where it was at so I guess it was not really at idle at that point.
I have not mentioned this yet, but is my APT screw (I can access it) preventing the idle circuit from working correctly? It is 4 turns out from seated, so this prevents the power piston from traveling all the way down. When I had it apart I noticed the metering rods could travel quite a bit farther when the APT was seated. Or does the jet/primary metering rods not affect the idle circuit??
Thanks for the feedback. Trying to get this figured out before summer is over. >:(
defiant1:
Drill out the IAB to .096. Idle mixture screw 9 1/2 turns out--I started out at 5 and the more I turned it out the less nozzle drip I had. Decreased my nozzle trip by 2/3. Before it was gushing out, now it is a drip. Throttle screw still turned over 1 full rotation to get to run at idle. I am still assuming the throttle blades are open too far leading to the nozzle drip.
Any other suggestions. Vacuum is around 12.5" I am slowly improving this reading with every adjustment I make.
d1
Cliff Ruggles:
Looking at the engine combination, it's going to need more idle fuel to the mixtures screws, and more idle bypass air. That's a "hefty" cam in a 350 engine at that compression ratio. Is the cam ground on a 108 LSA (Lobe Separation Angle), or is ground on a 112LSA and installed at 108 ICL (Intake Center Line)?......Cliff
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