General Category > Quadrajet Carb Talk and Tips
APT adjustment question
Greasy Harley:
I found instructions on how to set the APT using Vernier calipers. I followed that procedure and ended up with a lean condition. ( I don't think I did it wrong ?)
Using these instructions APT was set a fuzz over 2 turns up.
I pulled the top off the carb, removed the plug, verified that the rods were hanging evenly and reassembled.
I tried the tip-in procedure and the rpm's increased until the APT was 5 turns up ...but the exhaust was really stinky.
Was I doing the tip-in wrong?
* rpms up around 2k shut the choke flap 3/4 of the way, if the rpm's increase, raise the apt up 1/4 turn and repeat. do this 'til the rpms don't increase.* Right?
I decided to shoot right down the middle, if 2 turns is too lean and 5 turns is too fat...
I adjusted it to 3.5 turns up and took it for a rip. It seems happy now. so that's cool, but I would really like to know where I went wrong with the tip-in.
Closing the flap too much? rpm's not high enough? Maybe I just suck?
Anybody point me to a GOOD video on tip-in procedure?
For what it's worth, the truck is a 79 k10 4x4, with a Granny 4 speed.
1 piece rear main 350 .060 over
Flat top slugs with 4 valve reliefs
9.8:1 compression
.040 quench
TBI heads
Lunati Voodoo 10120702:
Gross Valve Lift (Int/Exh): .468/.489
Duration @ .050 (Int/Exh): 219/227
Advertised Duration (Int/Exh): 262/268
Lobe Separation: 112
RPM Range: 1400-5800
I ain't trying to win any drag races , just trying to merge with 80mph commuter traffic. :o
Thanks to everybody here for loading this forum with quality information.
-Russ
Greasy Harley:
so... actually, 3.5 turns is still a little fat judging by the puff of black smoke, but it does run pretty sweet. I'll turn the apt down 1/4 turn tomorrow and run it again.
This means my measuring procedure was closer than my tip-in procedure. Sure would like to know what I'm doing wrong.
Kenth:
I have found the APT screw does not touch the power piston pin until 2 turns up from seated.
Also, most of the carbs with this feature i have seen is set 3.5 - 4.0 turns up from factory.
I would like to see 50-100 rpms increase with slowly closed choke valve at 2200 rpms, this indicates a slightly leaned out mixture and works well.
I use the tip-in procedure after the idle/lowspeed circuit is sorted out.
Cliff Ruggles:
Not enough information.
What is the carburetor part number?
What have you done to it and exactly how did you set it up, jets, rods, idle system, etc?
The APT is simply there to allow fine tuning the height of the PP and exactly where the tapered portion of the metering rods are in the jets at light engine load (high vacuum).
Also what are "TBI" heads, and what intake manifold?....
Greasy Harley:
--- Quote from: Kenth on April 28, 2021, 12:21:29 AM ---...most of the carbs with this feature i have seen is set 3.5 - 4.0 turns up from factory.
I would like to see 50-100 rpms increase with slowly closed choke valve at 2200 rpms, this indicates a slightly leaned out mixture and works well.
I use the tip-in procedure after the idle/lowspeed circuit is sorted out.
--- End quote ---
Factory settings are out the window since I straightened the hanger. idle circuit is tuned pretty good. sounds like I might not be preforming the tip-in procedure properly
--- Quote from: Cliff Ruggles on April 28, 2021, 03:42:43 AM ---Not enough information.
What is the carburetor part number?
What have you done to it and exactly how did you set it up, jets, rods, idle system, etc?
The APT is simply there to allow fine tuning the height of the PP and exactly where the tapered portion of the metering rods are in the jets at light engine load (high vacuum).
Also what are "TBI" heads, and what intake manifold?....
--- End quote ---
I don't have the carb number handy, but I could get it after work .It's a later model carb, I believe the factory carb for that '79 truck.
Mods:
Converted to electric choke
upper Idle air bleeds ------- .695
lower idle air bleeds ------- .075
idle tubes ---------------- .036
idle restrictions ----------- .052
main air bleed (air horn) -- .0845
idle screw holes ---------- .0575
acc. pump discharge holes - .028
fuel inlet seat (no window) - .125
primary jets ------------ 74
primary Rods ----------- 50c
float set to 9/16
your kit.
I also made some small notches in the secondary air flaps
"TBI heads" swirl port heads that came factory on an '88 Suburban with a 350. mated to a cast iron Q-Jet manifold with modified center bolt holes.
Only problem I'm having now is after driving through a neighborhood (slightly above idle), there is a puff of smoke when I stab the throttle taking off from the stop sign. It clears up after that and is new since I adjusted the apt. I think this means the PP is still a little too high?
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