Wandering AFR at idle, bad transition on throttle tip in

[Falcon67]

I don't have a leakdown tester, but I pulled the plugs and did a compression test.

#1  185
#2  190
#3  185
#4  180
#5  185
#6  190
#7  190
#8  190

Based on these numbers, I think I can rule out a leaky intake valve as the source of the oscillating vacuum, unless it is possible for the lifter to hold the valve open, but nobody has said that.

All of the plugs looked pretty much the same, black and sooty.  Some were a little cleaner, #4 seemed to have a bit more hard carbon.  It does not have a steady miss on one cylinder, I know what that sounds like, so I don't know what the painful process of pulling a plug wire on a running engine would prove.

Hmmm - I just fixed the 393C over the winter for a leaking head gasket.  Leaked enough that once over 160F the engine was pushing water out like crazy.  Like a old camp coffee pot percolation.  All my compression readings were within 15 PSI and leak down didn't show anything out of ordinary.  #5 plug looked clean, even for methanol.  So at least you have really good readings. 

Black and sooty could be from the carb having fits from the crazy vacuum readings.  Or some combination of fuel metering issues

[frnkeore]

The reason for pulling plug wires, is to see if a particular cylinder is effecting how it runs, not if it has compression loss.

It's not painful to do. You just need a plug wire puller, they are well insulated and you get no shock. Burned maybe if you touch the head or exhaust. Wear gloves

If it is a particular valve guild, it may show. You can also pull a valve cover and while running, if you have PC type valve seals, a squirt oil on the intake guilds may show a change in idle if, it's guild related.

[60sIron]

Blown power valves are talked about like an everyday thing, in 40 years I can't recall ever holding one.  Yes, I have put the back side of plenty of PV to my lips and gave 'em a "kiss", yep that one's OK.  Leaking PV gaskets - yes.  All the friggen time.  Worse with 10% pump gas.  Also, if the carb is anything "new", like last 10~15~20 years, it will have a check valve in the base plate to protect the PV from a backfire. 

If you want to check the power valve, simple test - run the engine, stop.  Carefully remove carb without tripping the pump shooters,wipe the base plate with a rag and set the carb on a coffee can or similar - quart sauce pan, etc.  Let it sit for a while and check the base plate around the vacuum hole that leads to the PV chamber.  Fuel wetness or drips into the pan = leak from something, lots of time the gasket can be a bit off center and not sealing correctly.  This does not always condemn the PV, the metering block to main body gasket can cause all kinds of trouble if not sealing well.

Also, on a normal engine if you continue to open the idle mix screws, you will see a steady drop in the vacuum reading, not a bunch of jumping around.  So a leaking PV will cause a low vacuum reading, relative to whatever is the normal level.  Same with lean until you get to some point where its too lean to idle well. 

Quick vibration of the vacuum needle screams valve seats, guide issues,  etc.  Yes, the carb can't "think" with all that going on.  If it smooths out at increased RPM, for sure it's in the valve train.  Maybe a cam lobe, that might be a stretch.  As an example, on ly dual 600 t-rammed 357C with the good size hydro roller, 1200 RPM idle, 37 degrees of timing runs right at 12 in/hg steady when everything is close to right on the idle mix, plate positions and such.   

Here's another good list of suspects I ran across on the web:
WHAT TO LOOK FOR,
(paraphrased from a rebuild manual)
1. A steady reading between 16 and 22 in.Hg. at idle...
This is normal for most vehicles.
Radically cammed engines have lower, less steady readings.

2. Normal range at idle, with sporadic drops below normal,
This could indicate a sticking valve.

3. Normal range at idle with needle vibration of about 2 in.Hg.,
This could indicate an ignition problem. Check plug gap, dwell (yes, electronic ignitions have dwell too), cap, rotor, and plug wires.

4. A steady reading slightly higher than normal.
This can be caused by a dirty air filter, or overly advanced ignition timing.

5. A steady reading 3 to 12 in.Hg. lower than normal.
This could indicate one or more of the following conditions,
Intake or carburetor vacuum leak, Late ignition or cam timing, Worn piston rings.

6. Gauge needle drifts slowly over a range of 4 to 5 in.Hg. at idle.
This could indicate an idle mixture that is too rich or too lean.

7. Gauge needle fluctuates rapidly between 10 and 21 in.Hg. at idle.
This could occur when one or more valve springs are weak or broken.

8. Gauge reads normal at idle, but drops slowly as engine speed is increased to 2,500 RPM.
This could indicate a restricted exhaust system.

9. Gauge reads below normal and fluctuates rapidly over a range of about 3 in.Hg. at idle, then the needle becomes steady as engine speed is increased.
Worn intake guides usually cause this reading.

#7 Is the closest to my condition.  I can pull the valve covers and look.  I know what a broken valve spring looks like, but how do I find a weak valve spring?

[cjshaker]

The reason for pulling plug wires, is to see if a particular cylinder is effecting how it runs, not if it has compression loss.

It's not painful to do. You just need a plug wire puller, they are well insulated and you get no shock. Burned maybe if you touch the head or exhaust. Wear gloves

Just a suggestion, but you can generally eliminate getting shocked by pulling the wire at the cap. When pulling it at the plug, all that energy at the end of the wire is looking for an escape, and the holder is that escape. Pull it at the cap and it has nowhere to go.

[Falcon67]

If this is a more standard FOMOCO with a Pertronix points eliminator, it won't bite that bad at all - if any.  Try my MSD 7AL-3 which hits the coil with 525V before firing.  That'll defib your whole being. 

[BigBlueIron]

What idle RPM are you targeting? I have ran into when trying to idle an engine very low the crazy bouncing/vibrating vacuum needle can appear at idle, even though it runs ok. 

[60sIron]

I'm not married to any particular idle RPM.  I've got a higher stall converter, so 700 works fine.  Lower would be better, but I'm not that concerned about it.

[60sIron]

Update:

I pulled the valve covers and did not find any broken springs.  These are triple coil so it is progressively harder to see if the inner coils are broken, but I'm pretty sure they are good.

I did find evidence of five of the pushrods rubbing on the intake.  I've attached a picture of the worst one.  So this is something that needs to be fixed.  These are oil through pushrods for the T&D rockers.  Are there more slender pushrods I could use, or do I need to pull the intake and grind out the offending holes?

[cammerfe]

Blown power valves are talked about like an everyday thing, in 40 years I can't recall ever holding one.  Yes, I have put the back side of plenty of PV to my lips and gave 'em a "kiss", yep that one's OK.  Leaking PV gaskets - yes.  All the friggen time.  Worse with 10% pump gas.  Also, if the carb is anything "new", like last 10~15~20 years, it will have a check valve in the base plate to protect the PV from a backfire.

Just a 'by-the-way', I suffered a blown 'rubber' in the power valve on the primary carb on my just-delivered '64 427 Custom. The car was two or three days old when I took it back to the dealer because it wasn't acting right. That's more than fifty years ago, but it DID happen.
 

KS

[1968galaxie]

Update:

I pulled the valve covers and did not find any broken springs.  These are triple coil so it is progressively harder to see if the inner coils are broken, but I'm pretty sure they are good.

I did find evidence of five of the pushrods rubbing on the intake.  I've attached a picture of the worst one.  So this is something that needs to be fixed.  These are oil through pushrods for the T&D rockers.  Are there more slender pushrods I could use, or do I need to pull the intake and grind out the offending holes?

What valve springs are you running?
I understood from your first post that you have a small hydraulic roller camshaft (236 duration and 0.556" lift)
I like the single conical springs for hydro rollers.
Perhaps the valve spring pressures are far too high?

Juts a thought I had while reading the thread.

Cheers!