Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

Pontiac 400 - Idle bypass and Secondary rod hanger questions

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Cliff Ruggles:
A Pontiac 400 with 9.5 to 1 compression and that cam doesn't need a lot of idle bypass air.

If the throttle angle is too great, you will see two things.  Nozzle drip at idle and no control with the mixture screws. 

If you have no nozzle drip, and no control with the mixture screws, it may be getting enough fuel thru the transition slots even with the mixture screws seated.

What is the vacuum at idle, and idle quality?

With that cam it should be relatively "smooth" with a deep/heavy sound and little if any "lope".....Cliff

DieselPower:
Cliff,

At 850 rpm, I am seeing a very steady 14" vacuum with no nozzle drip.  However, in gear, it is on the edge of stalling and any input into the power steering and the engine dies.

The mixture screws do almost nothing.  It seems to run slightly better when they are full closed.  Anything more than 1/2 turn out and there is no difference.

Thanks

DieselPower:
I was just thinking, does the carb gets some "bypass air" from the PCV system?  Is it possible that some carbs have different sized PCV ports, thus the difference in required IAB diameter in similar applications?  For example, if someone else has an engine with the exact same specs as mine, but he has a factory carb and I have an Edelbrock 1910, is is possible that the 1910 has a smaller PCV port and thus less airflow for a given IAB diameter?

EDIT:  I just remembered that Pontiac engines have the PCV port in the intake manifold.  I wonder what the flow area of the elbow fitting is compared to the PCV port on my Edelbrock 1910?

Maybe the PCV valve itself is the limiting factor to airflow?

Either way, I still cannot get this thing to idle like it should.  I have the IAB at .130" now with little change.  I was able to close the throttle about 1/2 a turn.  Mixture screws still do basically nothing, but engine runs best with the screws all the way in.

Cliff Ruggles:
Sounds like too much idle fuel for the application.

I'd also check the timing, and make sure it is rock solid in and out of gear......Cliff

DieselPower:
Cliff,

Timing is steady as long as vacuum stays above 12" or so.  The vacuum advance can I installed is fully actuated above that point. 

The idle recipe I used is per emails from you and recipe 3 from your book.  If the idle circuit is too rich, how does that keep the throttle blades from being closed more so less transition slot is exposed?  Not arguing, just trying to understand.

Thanks!

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