17056240 Lean(?) at Idle

[Rhett]

I’m trying to dial in the idle circuit on my 17056240 carb. It has been rebuilt with Cliff’s kit and is on a 73 Buick Stage 1. It runs great at full throttle, great throttle response, off idle response, no surging, everything is cool and has run 14.2 @ 96 so it’s mostly dialed in.

But at idle it acts like it settles into an intermittent lean condition, and the idle roughens up. I can still pull between 17-20 lbs vacuum, and I’m pretty sure I don’t have any other vacuum leaks. I can put my hand over the primary opening and it slows RPM down pretty quick.  I dialed in the APT and it helped some, and the mixture screws will change vacuum readings and idle speed.  It seems to kick in and out of the rough condition every 4-5 seconds, and sometimes just stays rough. The primary rods are staying seated, the spring is stiff but not overcoming vacuum. It feels like a lean condition, it does not blow smoke or load up.

This is the way the carb is calibrated.:

Fuel Inlet   0.135
Float Level   0.312
Accel Discharge   0.036
Idle Tube   0.039
Idle Channel Restriction   0.055
Upper Idle Air Bleed   0.070
Lower Idle Air Bleed   0.070
Main Air Bleed (upper)   0.070
Main Air Bleed (lower)   0.070
Mixture Hole   0.090
Primary Jets   0.075
Primary Rods   Tapered (cliffs)
Secondary Well restriction   0.040
Secondary POE Restriction   0.055
Secondary Tube Restriction   0.036
Secondary Rods   AY .0567 L
Secondary Valve Opening   
Secondary Hanger   I

The car has an intake and headers, with a C-118 cam, specs: Intake .478 lift 228 @ .050 duration, Exhaust .475 lift 300 duration, 118 separation, but is otherwise stock



I had a situation on my 66 carb where I had too much idle air for the car.. could that be the situation here? Do I just need a little more idle fuel?

Thanks, Rhett

[Shark Racer]

Are you running any bypass air?

How far is the throttle opened?

Any nozzle drip?

[Rhett]

Hey!

It has unmodified idle bypass, no nozzle drip and the throttle is basically closed at a 700-800 rpm idle.

Thanks!

[TommyK]

What is your initial timing?

Are you using ported or manifold vacuum?

[Rhett]

Even though the cam isnt that radiacal,  it seems to like inital timing at 20, with 15 in the disctributor in at 2500. The advance is limited to 10 degrees, and it's on manifold vacuum.

I'm looking at the carb becuse it CAN idle great, and intermittantly does. At anything above idle it runs fantastic.

[TommyK]

I have found that manifold vacuum can make for an unstable idle due to vacuum fluctuations which make the timing wander. This is especially evident with a cam that has some lope to it.

Have you tried experimenting with ported vacuum and different initial timing settings? Might be worth a try if you haven't.

If playing with the initial timing and switching the vacuum source have no effect perhaps it is time to add a little more idle fuel by opening up the tube, down channel or both.

[Rhett]

I already did open the ICR and idle fuel up some  -  what what would be the next to try? ICR to .057-.060?

Vacuum and timing arent making a big difference - the vacuum advance is so limited that it barely speeds up the idle...

[Cliff Ruggles]

Too much timing at idle will make idle tuning difficult.  Some engines just don't want or like that much timing.  I'd try a lower initial timing setting and use ported vacuum to the distributor.

My 455 idles a bit "quirky" with manifold vacuum to the advance, even though it only adds 10 degrees. It's smoother and more "stable" in and out of gear without the additional timing....Cliff

[Rhett]

Been considering that too, but if I go any lower on timing it just wont hardly run, even when I do connect the vacuum advance.

I thought 20 would be too much initial, but it still revs quick with no detonation. I keep thinking I have some kind of intermittant vacuum leak, but it'll slow right down if I start covering the intake.

One thing I will have to do is get the distributor on a machine - I know total timing and I know when it's in by, but for all I know it could already be coming in at idle. I used the stock springs, but who the hell knows?