[mrmark1957]

1967 XH 400 motor with a 4413S carter AFB carb. Idles smooth but there is an annoying flat spot or hesitation as you step on the gas to accelerate. If you step on the throttle very slowly the engine almost shuts off when it hits that spot. Cleaned out the carb, set the float levels, accelerator pump is working. Timing is correct and vacuum advance is working. A small portion of the transfer slots are showing at closed throttle as per the pictures in Doug Roe's carb book. Could I have a bad carb casting requiring another carb body ? It doesn't do it with the choke on as the throttle is already past that point from the fast idle cam.

[25stevem]

Rotate the flaps out of the way on the two power piston bores on the top of the Carb and confirm that they travel very smoothly up from the fully down idle position.

it maybe that after 50 plus years the bores are worn such that the piston jams up down there just above idle, because that's where the power Pistons spend near there whole life moving, on average about a 1" long travel range.

There could also be crap like while Aluminum corrosion down there or both Rods tips could be slightly bent.

[25stevem]

Here's a question.

With the motor fully warmed up and at idle if you slowly close off the Choke does the engine die out, or is there a point where the idle rpm picks up?

[carbking]

The 4413s is just one of the Carter AFB's that use the idle air bleed idle circuit designed by GM. While this idle system worked fairly well with gasoline and a Reid Vapor Pressure from 5.5 to 7; it can be a challenge with today's fuel.

I have been suggesting for decades to defeat the idle air system by adding a conventional idle screw. One just needs a donor AFB for the screw and spring, and drill and tap the "dimple" already in the casting.

As far as the vacuum cylinders being worn, I don't like the word "never" but highly unlike. Carter early on used brass pistons, found they wore the cylinders; and obsoleted the brass pistons with aluminum pistons of an alloy softer than the base castings. The aluminum pistons are sacrificial and should be replaced at every rebuild. The pistons are included in the better rebuilding kits

The wrong A/F at idle may give a hesitation when the transition system is engaged.

Jon.

[carbking]

I had an appointment, and did not get to finish the above post.

GM used the idle air bleed idle on quite a few Rochester 2GC carbs, as well as the Carter AFB's.

Chrysler used a few of the AFB's with this system as well.

While the system alone can be challenging with modern fuel; when combined with a normal idle adjustment, the air bleed can be a wonderful tool in the world of large cams. A number of factory carbs , including the RAIV carbs, came with bleed holes located in the primary throttle plates to help the tuner maximize idle air velocity for good street manners. The adjustable air bleed allows the tuner the ability to adjust the idle air velocity, rather than the fixed holes drilled in the throttle plates.

Jon

[Ram Air IV Jack]

Originally Posted by carbking

The 4413s is just one of the Carter AFB's that use the idle air bleed idle circuit designed by GM. While this idle system worked fairly well with gasoline and a Reid Vapor Pressure from 5.5 to 7; it can be a challenge with today's fuel.

I have been suggesting for decades to defeat the idle air system by adding a conventional idle screw. One just needs a donor AFB for the screw and spring, and drill and tap the "dimple" already in the casting.

As far as the vacuum cylinders being worn, I don't like the word "never" but highly unlike. Carter early on used brass pistons, found they wore the cylinders; and obsoleted the brass pistons with aluminum pistons of an alloy softer than the base castings. The aluminum pistons are sacrificial and should be replaced at every rebuild. The pistons are included in the better rebuilding kits

The wrong A/F at idle may give a hesitation when the transition system is engaged.

Jon.

Now this brings back memories. In the early 70's while in mechanics school, we rebuilt a 65 389 GTO engine with an AFB carb. We cammed the engine with a then called 3/4 race hydraulic bump stick and it never idled well after that. I was then introduced to the air bleed screw at the front of the carb. I said to the teacher WTH is this and he said, that is how the idle is adjusted. In the end, we drilled, tapped and placed a spring and set screw like most carbs had to hold the throttle cable open at idle. Wow, still recall of this!!