Float and APT setting recommendations

[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.

[T/A_455]

And last update for the night...  Sorry for the running dialogue here - I just figure it's better to provide too much info, rather than not enough.

After observing fuel in the secondaries at idle, I took a closer look around the airhorn and realized that the gasket was saturated.  So I popped the top and dropped the float down to 13/32".

Idle is now noticeably smoother.  To the point where I think I can attribute any fluctuation to the cam.  Vacuum on the gauge is also now relatively smooth - bouncing within a half inch range, versus a couple of inches, much more violently, before.

I adjusted the idle to max vacuum (about 14 in) and set it at 750 rpm and went for a test drive.  Part throttle still has a stumble, but it seems more consistent and softer now, if that makes sense.  I suspect this may be correctable with further APT adjustments?  One other observation - I set the idle at 750 rpm with the engine warned up, went for a drive - during and afterwards it would idle much closer to 1000 rpm.  I am interested to see what it does when I start it up cold tomorrow...

[Ethan1]

 Stock float height for that carb was 17/32" from the factory.

[Marx3]

what happens if you cover the secondary air flaps with a cloth ? If this affects idle, you are propably sucking air past the secondary plates and need to center them. 

It sounds like you need to go for testdrive and fine tune the APT. If you have it at 3 turns up form seated, you can try raising it 1/2 a turn, go for cruise. Go at it 1/2 turn at the time.
I'm no expert, but your jetting sounds pretty good for your engine. 
I dont know the stock MAB size for that carb number, but as far as I know, only chevy units from that time, had the large MABS.

[Cliff Ruggles]

Have you checked fuel pressure during any of this? 

That carb should be fine at 9/32" float height.  Seeing an improvement with a much lower float setting indicates high fuel pressure.....Cliff

[T/A_455]

Thank you all for the responses.

Cliff: No, I have not checked it yet.  I will do that next.  I assumed that I was safe there from the pump specs, but I do know what happens when you assume...

If pressure proves to be too high (more than 8psi? 7.5psi?), do you think a pressure regulator would be the solution?  Or would an adjustment to the needle/seat size be workable?  (Please forgive my ignorance if this is an obvious question.)

I'm afraid I don't have the current seat size handy - I did replace it as part of the kit I received from you though.

[T/A_455]

Got a fuel pressure gauge on it tonight.  At idle, I'm seeing 4.5-5 psi at the carb inlet.

I adjusted the APT and idle mixtures a bit further - I've gotten it to be much smoother, but it's still not quite there around town.

I'm still chasing down why the idle speed changes.  It'll idle pretty much steady at 750 rpm until I go for a drive - at stop lights it'll idle consistently at 1k rpm with the same steadiness.  If I shut it down and restart right away, it'll idle back at 750 until I drive.

[Cliff Ruggles]

Those symptoms indicate fundamental issues, most likely the throttle plates aren't fully closing and sealing up well.  I'd correct that issue and I'm betting everything else will get much better......Cliff