Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
1903 Won't Respond to Tip-In Procedure
Cliff Ruggles:
Good news.
APT and vacuum advance tuning will get you there.
Make sure the power piston hanger arms are exactly even, and make very small changes. It may take several tanks of fuel to find the ideal APT setting. You can also use the vacuum advance to fine tune for fuel economy.
Most engines with higher compression, tight quench and smaller cams will not need much, but you still need to look there for best efficiency in the "normal" driving range....
bry593:
Apparently, I can't leave well enough alone....
Although the lean idle ran well, it was insufficient to support the choke. During choke operation, there was not enough fuel being pulled to cope with freezing ambient temps. It sort of worked, but I like my choke to be: pump gas once, turn key, engine fires immediately and idles at around 1000 rpm. With this lean setup, I had to feather the throttle a little before the high idle would stabilize. Just not enough idle fuel.
I considered adding idle fuel by drilling the DCR from .047 to .052 (would this have worked?). Instead, the idle tubes were increased from .031 to .033 so they will not plug up as easily. I also drilled the horn UIAB to .052, but left the DCR .047 and LIAB .064. This morning was 32F and it fired immediately and with a stable high idle. Seems to have lost a little pep on part throttle, but I suspect I can lower the APT a half turn and regain the lost cruise power.
Out of curiosity, what is the easiest was to verify the APT arms are even?
This HT383 has a poor quench. The pistons are .040" in the hole, then add the gasket. Not a great combination to prevent detonation. Not sure why GM did it this way considering the pistons were custom made by Speed-Pro for GM (383 with a 3.800, not 3.750 stroke).
tayto:
probably rebuilder pistons that are shorter
Kenth:
I would proceed adjusting the choke pull-off to open the choke blade a little less, about .160", measured between upper end of blade and rear wall of choke housing and increase fast idle speed some to keep it running.
For off-idle respond at least .052" idle DCR´s will help.
Cliff Ruggles:
We do NOT add fuel to the idle system to improve cold weather starts or choke operation.
The pull-off unload angle if FULLY adjustable on those carburetors via a screw. Move quickly on a cold start and simply adjust the flap so the engine doesn't stall out from being too lean and doesn't "blubber" from being too rich. It's a simple and easy procedure but you have to move quickly and I'd also lower the fast idle speed (cam on the highest step) down some to about 1400-1600rpm's at the same time.
99 percent of the power piston hanger arms on carbs I get in here at bent up, down, and not even. Spend some time there to get the arms LEVEL and EVEN. Just pull the tips together underneath the center of the power piston and gently "adjust" the arms till the tips are even.
It's not difficult or fussy but REQUIRED if you want a good end result with these things.......
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