Setting Vacuum Advance with TCS (1972) & Other Issues -----

[Bill C.]

Okay ----

I have an isue with my car.

I currently have the vacuum advance hose connected to a ported vacuum source on the carb. My TCS system works as designed. I do not have any signal at idle and don't really start to get a signal until 1400-1500 RPM.
When I am driving in 4th gear around 40 -50 MPH ( 1600 - 2400 RPM) I will get a slight surging. Once I do any type of accelration it goes away. If I am driving at a faster speed where I am more keeping the gas pedal with pressure I don't have any issues.

If I disconnect the TCS selonoid the car is smooth at all RPMs and vehicle speed.

Is there a special way to set the initial advance with a TCS equiped car?


Current Configuration:
1. I currently have it set to 8 degrees advanced with no vacuum source.
2. The vacuum lines are attached to the carb as they were when I
bought it. It had a 45K miles untouched engine/compartment.
3. Engine has a high volume/pressure oil pump
4. Distributor has stock mechanical advance plate, weights and springs.
5. Vacuum advance unit on the distributor is new 436-20 (NOS).

---- car acted this way even before I rebuilt the engine and used the new oild pump and NOS vacuum advance unit. -----

any help is VERY appreciated.

[William C.]

Re: Setting Vacuum Advance with TCS (1972) & Other Issues -----

'72 454 carb is a unique critter, you may have to up the main jet a couple of sizes to work with today's gas.

[Bill C.]

Re: Setting Vacuum Advance with TCS (1972) & Other Issues -----

How do jet sizes and TCS relate to each other?

The car, w/o any vacuum advance, runs just great.

[John M.]

Re: Setting Vacuum Advance with TCS (1972) & Other Issues -----

On my 70 Chevelle SS, I disconnected the TCS and ran it with a mocked system.

I did the same with the AIR system gutting the pump to turn freely and capping off the manifolds from the inside.

You certainly don't need them for performance and you can't tell if they're bypassed asthetically.

I concede there's strong arguments to have all systems functioning as factory produced, but in the case of emissions........


Likely not the politically correct answer, just a personal opinion and preferance.

[Jim T.]

Re: Setting Vacuum Advance with TCS (1972) & Other Issues -----

If I owned your 72 454 4-speed I would insure that the vacuum advance unit would function and hold vacuum. Next I would buy vacuum hose and use full time vacuum sourced from the QJET and connect the vacuum hose direct to the vacuum advance unit, bypassing the TCS system. Use of a plastic T, full time vacuum could be sourced from the intake where the power brake and vacuum tank source their vacuum. I would clean the centrificial advance. I use a very very light application of Blue Streak conventional point grease on the movement areas of the centrifical advance on my 68 and 70 as well on the point rubbing area on the distributor shaft. I have not allowed my 70's TCS system to control the vacuum advance unit for over 38 years. My 68 has full time vacuum and the AIR system is operational. Works well for me.

[Bill C.]

Re: Setting Vacuum Advance with TCS (1972) & Other Issues -----

I am going to Kissimmee in January for PV. The TCS has to work as designed .

I will check the vacuum advance unit to see if it is holdig vacuum. The
mechanical advance unit rotates freely, with no effort - just some resistance from the springs.

Is there any chance I need to use a stiffer spring set on the mechanical advance?

There is one other thing that I noticed - if I run the engine up to, say 2500-3000 RPM, and put the car into 4th gear (activate TCS), the RPM will drop a bit and then the engine will almost seem to have a slight miss.
At lower RPMs the miss seems less noticable.

This is very confusing to me.

How should I set the timing ?
Should I set it more retarded - say 2-3 degrees ADV instead of 8?
With or without a vacuum source?

[John M.]

Re: Setting Vacuum Advance with TCS (1972) & Other Issues -----

Forgive my ignorance on judging as this is new to me, but how would they verify if the TCS system is fully operational, or simply connected?

[Dennis D.]

Re: Setting Vacuum Advance with TCS (1972) & Other Issues -----

Okay ----

I have an isue with my car.

I currently have the vacuum advance hose connected to a ported vacuum source on the carb. My TCS system works as designed. I do not have any signal at idle and don't really start to get a signal until 1400-1500 RPM.
When I am driving in 4th gear around 40 -50 MPH ( 1600 - 2400 RPM) I will get a slight surging. Once I do any type of accelration it goes away. If I am driving at a faster speed where I am more keeping the gas pedal with pressure I don't have any issues.

If I disconnect the TCS selonoid the car is smooth at all RPMs and vehicle speed.

Is there a special way to set the initial advance with a TCS equiped car?


Current Configuration:
1. I currently have it set to 8 degrees advanced with no vacuum source.
2. The vacuum lines are attached to the carb as they were when I
bought it. It had a 45K miles untouched engine/compartment.
3. Engine has a high volume/pressure oil pump
4. Distributor has stock mechanical advance plate, weights and springs.
5. Vacuum advance unit on the distributor is new 436-20 (NOS).

---- car acted this way even before I rebuilt the engine and used the new oild pump and NOS vacuum advance unit. -----

any help is VERY appreciated.

I'm not uderstanding your explaination, unless the system operates differently from 70. You should not have to raise you idle to disengage the system. If you shift to 3rd or 4th gear a hold the clutch, the idle should increase.

[John H.]

Re: Setting Vacuum Advance with TCS (1972) & Other Issues -----

Forgive my ignorance on judging as this is new to me, but how would they verify if the TCS system is fully operational, or simply connected?

By shifting it in and out of 3rd or 4th gear while the engine is running and warmed-up during the Ops check and noting if there's a change in idle speed.

[Bill C.]

Re: Setting Vacuum Advance with TCS (1972) & Other Issues -----

Dennis:
With the vacuum line connected to a ported vacuum source on the carb, there is no vacuum signal until you get around 10% throttle plate opening. The ported hole is above the throttle plates, not under and getting signal from manifold vacuum.

My TCS system functions 100% correctly. It is not the issue.
I think I have a problem with something in the distributor.



Based on the diagram you provided, it looks like the TCS vacuum line should be connected to a manifold vacuum source. I will try this and see if there is any difference.

[Patrick H.]

Re: Setting Vacuum Advance with TCS (1972) & Other Issues -----

Dennis:
With the vacuum line connected to a ported vacuum source on the carb, there is no vacuum signal until you get around 10% throttle plate opening. The ported hole is above the throttle plates, not under and getting signal from manifold vacuum.

My TCS system functions 100% correctly. It is not the issue.
I think I have a problem with something in the distributor.



Based on the diagram you provided, it looks like the TCS vacuum line should be connected to a manifold vacuum source. I will try this and see if there is any difference.

Bill,

Did you have your distributor curve checked when you restored the car? Or did you do it? If your curve is way off I suspect you'd get the exact symptoms you describe.

I've had no issues like you describe with either my 71 or 72 SB cars. But, I've had both distributors checked and verified.

Patrick

[Paul O.]

Re: Setting Vacuum Advance with TCS (1972) & Other Issues -----

Bill if I remember right you will not get vacuum till you are in 4th gear only in 72. 70 and 71 are different and there is a 25 to 45 sec. time delay through a relay before vacuum is applied hope this helps. Paul 18046

[Bill C.]

Re: Setting Vacuum Advance with TCS (1972) & Other Issues -----

Patrick,

I did not have it checked .... and I figured since I ws putting in all new parts it would be correct.

Guess that was a bad assumption!


I had to change the mainshaft - the tach gear was messed up.
I cleaned and lubricated all the moving parts, put new upper and lower bushings in it and replaced the vacuum advance unit with a NOS one.
The weights and springs are the original ones.

Where would I take it to have it checked?
Can that be done while in the car?

[Patrick H.]

Re: Setting Vacuum Advance with TCS (1972) & Other Issues -----

Patrick,

I did not have it checked .... and I figured since I was putting in all new parts it would be correct.

Guess that was a bad assumption!


I had to change the mainshaft - the tach gear was messed up.
I cleaned and lubricated all the moving parts, put new upper and lower bushings in it and replaced the vacuum advance unit with a NOS one.
The weights and springs are the original ones.

Where would I take it to have it checked?
Can that be done while in the car?

Bill,

If you honestly replaced all those parts, I can virtually guarantee you've found your culprit. The mainshafts all had different "autocams" on the top, and the one you now have differs from your original. Also, you need to verify the function of your current vacuum advance regarding if it is working, and if so at what vacuum.

You can figure this all out with it on the car (if you have the time,and patience and a dial-back timing light), but my usual method is to ship the distributor to Bill Clupper and have him run it on his distributor machine. He will then adjust and/or change parts as needed to replicate whatever curve you want. He's done at least 6 distributors for me and my friends so far (66/70/71/72/72 Corvette and 70 Cutlass) and I've been very pleased. I don't own a bunch of original weights, springs and vacuum advance cans so I let Bill find paarts in his collection that work for me.

Patrick

[Bill C.]

Re: Setting Vacuum Advance with TCS (1972) & Other Issues -----

Patrick,

Should it be connected to a manifold or ported source?

When I timed the car, it was done with no vacuum source to the advance unit. Is this correct?

I would really like to figure this stuff out without taking the distributor out of the car - PIA to get back in.
having said that - I have never set the curve on a distributor before.
Might just be better to send it out and have it done correctly.

How long would you think the job would take?