Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

Float and APT setting recommendations

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T/A_455:
I am working on setting up the carb for my newly built Pontiac 455.  I bought a kit from Cliff, along with the book.  Everything went very smoothly (my first rebuild), but I have some questions regarding the float height and APT settings.

Specs on my build:

455, .040" over
6X-4 heads, 9.5:1 CR
RAIV cam (231/240@.050,.470') w/ Rhoads lifters
4 speed w/ 3.23 rear
'77 T/A

Carb #17057263
Primary Jets: 73
Primary Rods: 44
Secondary Rods: DA
Idle Tubes: .038"
Down Channels: .059"
Idle Bypass: .110"
Idle Mixture Holes: .096"

Primary shaft was bushed and blades were centered on both shafts w/ 90 deg stops.  Fuel is supplied by a 550hp RobbMC pump w/ vapor return.

Initial timing is currently at 12 deg.


The issue I'm having is in getting the idle under control.  It currently fluctuates and I don't seem to have much, if any, control with the mixture screws.  Exhaust is eye-stinging.  Fuel consumption also seems excessive (beyond what that 455 should eat - gauge dropped a quarter tank in about 25 miles of driving) in the limited test driving I've done so far.

Low speed part throttle and decel are also quite rough - it stutters and bucks, generally not wanting to putter around the neighborhood.  Acceleration and highway driving is smooth.

I suspect that its flooding and has a bit of a drip due to setting my float height too high.  It is currently at 9/32".  With that fuel pump, I'm thinking I may need to drop it down to a more stock like 15/32"?  Does that sound correct to anyone?

I'm also looking for advice on the APT setting.  When I tore down the carb, I found that the APT screw was missing.  I added a screw and spring back in, but I do not know what a correct setting would be.  As a baseline, I set the screw so that the inner brass sleeve of the power piston sat ~.020" above the plastic retaining collar (5 turns), but I don't know if that's right.  I've been reluctant to try the tip-in procedure in the book until after I get the idle issues squared away, but I thought I would throw it out here just for info.  I did pop out the aluminum plug in the airhorn and added a set screw during the rebuild, so I can easily adjust this at any point.

Marx3:
A kind of universal float height is 1/4", measured from the shoulder of the back of the float and up to the top of the carb main body. ( with the needle seated off course )

A good starting point for the Apt is 3 turns up from bottom position.

The problem you describe does sound like the carb is flooding. Could be the float height OR heavy nozzle drop I guess.
Take a look down the primary barrels, while the engine is idling, just to see if fuel us all over the place or if the barrels are nice and dry.

T/A_455:
Thank you for the info - I will take a look at the state of the primaries at idle to verify any drip before digging into things.

When I set the float height, I'll admit that I measured out at the toe of the float, not at the shoulder.  So my stated 9/32" is probably significantly higher when measured correctly.

And it sounds like my APT is far too high as well.

I will report back once I take a look.  Thanks again!

T/A_455:
Been tweaking things a bit this evening.  Cold start and warmed up there is no apparent drip from the primary nozzles that I can see.

Idle fluctuates about 50-100 rpm.  I'm showing about 14in of vacuum, max, with my mixture screws about four turns out.

I adjusted the APT down to 3 turns and it seemed to help a bit with part throttle, but it still seems a bit rough at low speed and decel.  Oddly, it seems like adjusting the APT is what gave me more control with the mixture screws, but perhaps I'm just imagining that?

I have not yet adjusted the float, but that is up next.

T/A_455:
I should also note that timing appears to fluctuate with the idle.  Vacuum advance disconnected, it bounces from 12 to about 16 degrees at idle.

Could this be a large vacuum leak?


EDIT:  I just looked down the secondary barrels and the walls appear wet with fuel.  If I manually open the air flaps while idling, it seems to smooth out a bit and the fuel begins to dry.  It appears it's either weeping from the airhorn gasket itself or from the secondary accelerating system holes and being carried around the seam by airflow.

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