Correct Vacuum Advance unit?
 

1974 T/A 455 and a TH400 with professional rebuilt Qjet. Problem is a slight off idle stumble but otherwise motor runs well. I believe the issue is with the distributor which is a Pontiac unit of unknown original application. It is a points type distributor. Carb has no stumble in park but in gear is when the stumble occurs. Rebuilt engine makes 18 steady inches of vacuum and initial timing is set to 12 degrees BTDC. The dist. weights move freely but the springs seem to have very little tension. I don't know if I should start with stiffer springs or perhaps a different/correct advance unit. Best place to start?? Thx.

If your currently running your vacuum advance on full manifold vacuum then switch it over to ported.

"professional rebuilt Qjet"

Who was the "professional" and what was done to the carb? Recipe?

A slight off idle stumble usually means lean low speed (idle) cicuit.

If you have the silver springs in the distributor from a aftermarket recal kit then that’s also a big problem because not only are they too light , but the give up what little tension they have fast.
To this day I do not know why kits come with such springs!

Here’s more details.

The 73 and 74 V8 vacuum, TCS set up had the primary function of eliminating vacuum advance at slow engine speeds.
If you have as posted above the vacuum advance hooked up to full manifold vacuum then that’s wrong if you have a 74 carb.

You have a professionally rebuilt carburetor.

Match it with a professionally rebuilt / recurved distributor that will include the correct springs and vacuum advance canister.

Worth every penny.

With what your playing with it’s time to mark your balancer so you know exactly what your doing if you have not done so or get a timing tape to put on it.
If you measure 2 1/8” around and make a mark that will be 36 degrees , if you make a mark 1/2 that then your at 18 degrees.

Sounds like carb is lean under load to me.

Just remember 90% of all carburetor problems are ignition. (from my mentor, 50 years ago). It's a joke, but some truth to it. No idea who the "professional" was that did the carburetor work, but for now, lets assume it was done correctly. I say this because we know the distributor is NOT correct. It should not have light tension, silver springs on it. You can try a couple things just to see what difference it makes. If you have an old distributor laying around or have access to one with factory springs, put them in and see what happens. Also try changing full manifold vacuum for the VA to ported and see what happens. That should tell you something. Ideally, pull the distributor and have it re-curved by one of the guys on here with a distributor machine and the necessary tuning parts.

Follow up.
 

I originally had the VA connected to ported source on carb and the stumble was much worse. When I switched the VA to intake manifold the stumble was much improved but is still present and unacceptable. The springs were on the distributor when it came with the car so I don't know if they are from an aftermarket kit or not but they appear to be like the originals I have seen in the past. I am open to a professional recurve set up on the distributor and will see if there is someone here in the Daytona Beach area that might do this. Any recommendations? The carb is a 1973 as is the motor. Thx

I also had an off-idle stumble on my '72 455 engine with the 7041262 Q-Jet and automatic transmission that I installed in my '67 LeMans. Distributor was the stock that came with the engine and initial advance was set to 9°. The carb was originally designed for TCS. I kept all the TCS components so no vacuum advance in low gears.

After tuning for best vacuum, it would stumble on mild acceleration from a stop. At first, I thought it was the acceleration pump. Previously, I rebuilt the card with Quadrajetparts.com recommended accel pump upgrade. Upon carb disassembly, I discovered that the pump cup was slighlty swollen and that the plastic retainer had gotton stck on the stem so there was pump shot on any throttle movement.

Replacing the accel pump and tuning the mixture screws for best vacuum did not solve the off idle stumble. At my wits end, I read the service manual's Idle Mixture Adjustment Procedure. I performed the Lean Drop Procedure. In a nutshell, after backing out the mixture screws six turns and setting the RPM in drive to the initial on a chart, screw in each mixture screw an equal amount till the lower RPM is reached.

This solved my off idle stumble. I attached the 1974 Idle mixture adjustment procedure in case you'd like to try it. The procedures in 1971, 72, and 73 are almost identical.

Also take into account here that since these era motors had EGR that the Carbs on the primary side where set up leaner.

If as the OP states his Carb was rebuilt to pure stock spec's and he is not using the stock hooked up EGR then that is making for a lean issue also.

Intake manifold vacuum for the VA equals stronger vacuum in intake equals richer idle mixture pushed thru the carb by atmospheric pressure.

Off-idle hesitation is a true indicator of a too lean idle circuit.

In 1972-74 Pontiac Qjets the off-idle slots sits higher, and above, the throttle blades at correct angle and does not contribute to the idle mixture volume needed with todays fuels.

Everyone of this vintage Pontiac Qjets needs larger idle tubes and down channel restrictions than the earlier vintage.
I set them up with at least .038" idle tubes and .055" (full manifold VA)-.059" (ported VA) downchannels (economizers) for best street performance.

FWIW

Update on stumble!
 

I transferred a set of springs from another distributor I had on the shelf that felt a bit stiffer than what was in my installed distributor. Just finished a 15 mile test ride and no more stumble!! Amazing how one small change makes a night and day difference in the performance of this sweet 455. Thanks again everyone. Dave

That’s great news, but for such a change to come about from just that I would say that you could still be a tad richer from just off idle .

Next time you get a chance I would open up the idle mixture screws until they just start to make the idle change.
Hopefully this will take place with no more then a added 1/2 turn, as this would also be a indication that your carb is set up pretty well besides adding some needed fuel at off idle times.

Come summer you can likely take that added 1/2 turn back to what you have it now.

Thanks.
 

I will try that Steve. Thx again.

Oh no, it is back!
 

Took the car out today for a run and the off idle stumble has returned! I then put a timing light on the timing marks and while attempting to slightly advance the timing, the groove on the dampner, suddenly disappeared from the 12 degrees BTDC position to a location of approximately 25-30 degrees ATDC. The motor continued running ok, but the dampner groove moved out of sight. Could the dampner have slipped on the hub or perhaps the vacuum advance unit in the distributor has gone bad or ?? Never observed this before!!

An old stock damper could slip as the rubber in between gets old. I’d bring up to TDC on number 1 (compression) and see where it is.

Initial timing is to be set with vacuum advance disconnected and plugged.

yes
 

Vacuum advance was disconnected and plugged.

On TDC
 

Cranked engine over to TDC on compression stroke and the grove on dampner lined up perfectly at TDC with the timing cover scale at "0" degrees. I believe this tells us that the issue must be with the Vacuum advance unit in the distributor??