Holley with excessively lean light throttle cruise

[Ken_Parkman]

I'm hoping to get some help with a tuning problem I'm having.

The combo is a 426 AMC street engine, 232 hyd cam, 570 hp, 5 speed OD, 3100 lbs. I have an older 1000 cfm Holley carb from a tuner house. The carb has been on a mild 401 (440 hp) for many years and worked very well, sat for a few years, now is on the current combo, and now is undriveable.

The problem is it goes excessively lean on light throttle cruising. At 1600 rpm cruise the AFR is showing 16-17, vacuum about 14, and the thing is bucking and surging so bad I will not drive it. It's the same in 4th at 2500 rpm. The tiniest bit of added throttle - just bring down the vacuum a few points - and the AFR drops to 14 and it runs fine. It runs fine at a 75 mph cruise but I did not have the O2 sensor connected then.

I've had the carb apart several times trying to find something plugged and not found anything, but it is clear I do not know enough about carb tuning and am missing something. I phoned the carb house and they said remove the power valve and square jet it.

I'm planning a more detailed effort to find the problem on the weekend and will try the power valve thing, but I'm having difficulty with understanding that concept when it's fine at a higher speed cruise.

Any suggestions on what is wrong that I'm not understanding?

[Tom68]

Different combo different needs. Is the throttle opening amount still on the idle transfer slots ?

[1980RS]

Single plane or dual plane intake?

[HQM383]

MAB definitely not blocked?

[Ken_Parkman]

Both the primary and secondary transfer slots are fully covered, the primaries more so. There is an .085" hole in each throttle plate. I have gone looking for vacuum leaks and not found any. Another thing I will look for again, but it does idle right down and it makes enough vacuum the vacuum wipers work.

The intake is a single plane torker which was also on the 401. I have not had good luck with the air gap dual planes and previous to this have had no significant drivability problems with the torker.

I have blown out the MABs and was and was confident they were not blocked, but will re-check again.

The way it drives I'm wondering if it is on the idle circuit to 60 mph?

[Walter R. Malik]

Both the primary and secondary transfer slots are fully covered, the primaries more so. There is an .085" hole in each throttle plate. I have gone looking for vacuum leaks and not found any. Another thing I will look for again, but it does idle right down and it makes enough vacuum the vacuum wipers work.

The intake is a single plane torker which was also on the 401. I have not had good luck with the air gap dual planes and previous to this have had no significant drivability problems with the torker.

I have blown out the MABs and was and was confident they were not blocked, but will re-check again.

The way it drives I'm wondering if it is on the idle circuit to 60 mph?

That carb must not be a very good fit for that application, the way it sits now..
The idle circuit does also have an affect upon part throttle driving until the opening the plate clears the entire "off idle" slot.
The easiest thing to try is making the Idle air bleed smaller. If they are fixed then before their removal make some "air bleed metering rods" from paper clips and secure them into the bleeds making them smaller in area.

Personally, I don't think it would be enough so, I would extend the bottom of the primary off idle slots to just below the bottom edge of the throttle plates on the primary side when they are set at idle speed but, that is pretty much irreversible.
You might try just soldering the holes in those plates closed and re-set the idle speed first however, I don't think it will help you enough either.

The tip of a broken coping saw blade about a .025" thick, works well for sawing the bottom of the slot however the carb base will need to be removed.

[Brktracer]

Good advice above.

What RPM does the booster start to flow? Look in the carb and start increasing the rpm until you see the booster flowing. I bet it's OVER 2,000 rpm. That said, anything you do to the main circuit is not going to do anything at rpm lower than the booster starts flowing. Keep that in mind with the direction you choose.

Step 1 is to do away with the holes in the throttle plates.
Step 2 is to decrease the IAB (but I doubt you get to this step)
Step 3 is to reduce the size of the PVCR and increase jet accordingly. Square jetting and no PV could be used as a test. This will not help at low rpm (below booster flow)

You can shape the idle fuel curve with the bleed in exactly the same way the main circuit curve is shaped with the bleed. Keep that in mind.

How far out are the idle screws now?

Make sure there is not a "jet" in the transfer slot passage in the main body. This will limit fuel and air flow through the slot at the rpm you're struggling with.

[HQM383]

The problem is it goes excessively lean on light throttle cruising. At 1600 rpm cruise the AFR is showing 16-17, vacuum about 14, and the thing is bucking and surging so bad I will not drive it. It's the same in 4th at 2500 rpm. The tiniest bit of added throttle - just bring down the vacuum a few points - and the AFR drops to 14 and it runs fine.

If this is steady state maintaining speed cruise then it would be advantageous to know the idle feed restriction size and idle air bleed size. 14” vacuum may be different than the engine it was built for at that throttle opening and so sized applicable. By 2500rpm, gearing and vehicle weight dependent, the mains are usually joining the party so main air bleed and emulsion jet sizes come into play there as well for timing of the circuit. First up though see if you can measure IFR and iab.

I phoned the carb house and they said remove the power valve and square jet it.

Solve one problem and create another wouldn’t it as 75 mph cruising would be a bit richer.

[Geoff2]

570 hp is incredibly good for a 232 @ 050 cam!


Agree with other comments. The T slot performs a see-saw action. Below the blades it delivers a A/F mix; above the blades, it acts as an air bleed. If little or any T slot is showing at idle, more slot is exposed above the blade, leaning the mixture. This will carry through to cruise mode & could explain the lean cruise condition if the T slot relationship is incorrect.

[mag2555]

I would bet that if you disabled the primary accelerator pump and with the warmed up and out of gear that if you opened up the throttle a bit on the fast side ( like with a one second count) that the motor would back fire out the carb.

When you have a carbs off idle metering right for the motors needs you should only need the accelerator pump to add in enough extra fuel to cover up for the motor being under a load.

If those intake ports and chambers have been polished then that’s adding to the fuel demand also

[ClassicComp]

Check the dia of the idle fuel bleed and get back to us please.

[treyrags]

570 hp is incredibly good for a 232 @ 050 cam!

That's what I was thinking

[BILL-C]

With that many cubes, that small of a cam, and that large of a carb I am very sure that you are running totally on the idle circuit at cruise. That combo will make 25-30 HP easily before pulling a drop of fuel through boosters. Find your idle fuel restriction holes in metering blocks and drill larger .005 and take test drive.

[treyrags]

"The problem is it goes excessively lean on light throttle cruising. At 1600 rpm cruise the AFR is showing 16-17, vacuum about 14, and the thing is bucking and surging so bad I will not drive it. It's the same in 4th at 2500 rpm."

Is your ignition timing advanced enough in the scenario above? A lean mixture burns slower requiring more timing. If you have a vacuum advance your timing may be dipping too much when tipping into the throttle in this situation if your mechanical advance is not engaged enough at that point.

[skinny z]

You might try just soldering the holes in those plates closed

This would be my first step.
It seems to me this would force a move to open the throttle a little more at cruise and bring the carb around to the way it was designed to run.