Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

1903 Won't Respond to Tip-In Procedure

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Cliff Ruggles:
Not really.  IF the hesitation occurs "easing" into the throttle raise the APT slightly. 

I also don't remember if you are using an exhaust crossover in the intake or not?

Most later set-ups were designed for FI and the intake is cold. 

I didn't read back over the thread but what heads and intake are you using?......

bry593:
No crossover in these Vortec heads.  Using 195F thermostat for now. 
Vortec requires an external coolant bypass similar to big block Chevy.  Current restriction is 3/8, but this takes too long to warm up.  Plan to reduce the flow area by about 2/3, which is .217" or a #3 drill bit.   The new restriction will be made from an 1/8 NPT brass plug threaded into the 3/8 NPT x 5/8 HSB intake manifold fitting.

bry593:
Cliff,

What are the specs on the vac advance I bought from you?  You said you typically use only two cans, and I bought the one that starts at a higher vacuum.  Seems like you said it was 9 distributor degrees (18 crank), but I don't recall the start and all-in vacuum signals.

I ask because I need to start tuning in my cruise curve.  Looking to have around 50 total (10 initial + 22 centrifugal + 18 vac) at cruise (2450rpm @ 75mph).  At least that is my initial target.  Not sure if the vortec heads will like having the full centrifugal in by 2500 rpm.

bry593:
According to GM, my ZZ4 distributor has a centrifugal advance curve:
16 @ 2400
22 @ 4600

initial  + mechanical @2400 + Vac @ 18 in Hg = 52 max
10      +         16                   +             X          = 52 max

X = 26 crank max @ 18 in Hg => 13 distributor

Possible vac advance candidates:
VC700 starts 8-10 in Hg,  11.5 @ 19-21, $40 @ Napa
VC1755 starts 8-10 in Hg, 12.5 @ 18-20.5, $18 @ Napa

For VC700 -> 52 - 23 - 10 = 19 mechanical. 
Need to determine what rpm makes centrifugal = 19 degrees by interpolation:
16               2400
19               X
22               4600

(19 - 16)/(22-16) = (X-2400)/(4600-2400)
X = 3500 rpm which is well above any cruise rpm.  So, this will never exceed 52 total degrees.  Provides a little safety factor for non-linearity in mechanical advance.

For VC1755 -> 52 - 25 - 10 = 17 mechanical. 
Need to determine what rpm makes centrifugal = 17 degrees by interpolation:
16               2400
17               X
22               4600

(17 - 16)/(22-16) = (X-2400)/(4600-2400)
X = 2767 rpm which is slightly above cruise rpm.  So, this would be very close to 52 total degrees but might exceed 52 if the torque converter unlocks on a grade.

The VC1755 might be a little much, probably ought to use the VC700.

bry593:
My bad, the vacuum advance listed above are for 1974 and older, points distributors.

Translating the above findings into HEI results in a couple of candidates:

VC1831 / AR7 / VC217  6-8, 12@14-16
VC1828 / AR4   6-9, 10@12-14

My goal is to keep the start of curve high, so advance drops out quickly to avoid transition ping.   

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