1972 Tuning Question

[Patrick H.]

Hi All,

After ignoring Dad's car for another 2 months, I worked on some tuning issues on Tuesday. I've got a question or two to run by you.

It's a base 72 350/automatic, but no emissions controls on the engine, and distributor recurved to pre-emissions specs. The Quadrajet on the car was restored by Jerry Luck and I used it for a summer on my 71 with no problems (not that that is still any guarantee). I started out with the vacuum disconnected from the distributor and plugged, and also plugged the vacuum take off to the control systems so that there were no "hidden" vacuum leaks confounding the results. All testing was done in "Park" with the car warmed up to operating speed.

At first the car idled at 800 rpm. Timing stable at 10 degrees, and stayed as such from 600rpm-1200rpm. The vacuum gauge showed 10 inHg.
Placing my hand over the carb caused the rpms to go quickly up to 1200 and the vacuum went up to 15. Knowing this I then slowly by quarter turns, backed out the idle mixture screws 1 full turn each as I watched the gauges. At that point, the vacuum was at 12 inHg and the idle speed stayed the same. I then placed my hand over the carb, and the idle speed and vacuum went up again to 1200 rpm and 15 inHg, but at a much slower rate than the first test.

My concerns are this:
1. The idle mixture screws are now out about 5 turns each, and can easily wobble side-to-side by hand. Is there a limit to how far I back them out? I believe that I need to keep enriching the mixture (i.e. unscrewing the screws) until my vacuum is close to 15 and the idle no longer increases when I "choke" the carb with my hand. I'd hate to have the screws fall out before I reach that point.

2. All of this tuning was done with the primary throttle arm not quite closed. It seems to stick about 1/16" off the idle speed screw (possibly due to a throttle cable kink or weak throttle return springs) but every time I push the throttle arm closed, the engine quits. It's really only the difference between 800rpm and 500-600rpm of idle speed, but it still annoys me.

3. As soon as I re-connect the vacuum line to the distributor, the idle jumps up to 1500-1800. Obviously that's not palatable. My thought is that I need to find a vacuum can that doesn't come in so early, yes?

Thanks,

Patrick

[Joe L.]

Re: 1972 Tuning Question

Patrick-----

I'm not sure what the underlying problem is, but I would expect a vacuum leak could be one good possibility. I can address a few of your specific questions, though.

First, yes it IS possible to have the idle mixture screws backed too far out. I've had this happen to me in the past, but I've forgotten what condition that it was symptomatic of (or, how I cured it). When they reach the point where they "wobble", they're WAY too far out. They should never need to be anywhere near this point.

Second, the problem with the accelerator arm being "not quite closed" may be due to mis-adjusted primary throttle valves. That could also be the basic, underlying problem here.

Third, I don't think that you need a different vacuum control. I think that once you get the underlying problem identified and corrected, the one you have may be alright.

Fourth, for this particular engine, all of your vacuum readings appear too low to me. You should be getting about 18-20" Hg.

[Joe L.]

Re: 1972 Tuning Question

Patrick-----

I'm not sure what the underlying problem is, but I would expect a vacuum leak could be one good possibility. I can address a few of your specific questions, though.

First, yes it IS possible to have the idle mixture screws backed too far out. I've had this happen to me in the past, but I've forgotten what condition that it was symptomatic of (or, how I cured it). When they reach the point where they "wobble", they're WAY too far out. They should never need to be anywhere near this point.

Second, the problem with the accelerator arm being "not quite closed" may be due to mis-adjusted primary throttle valves. That could also be the basic, underlying problem here.

Third, I don't think that you need a different vacuum control. I think that once you get the underlying problem identified and corrected, the one you have may be alright.

Fourth, for this particular engine, all of your vacuum readings appear too low to me. You should be getting about 18-20" Hg.

[Duke W.]

Re: 1972 Tuning Question

Why are you messing with the idle mixture with the vacuum can disconnected?

The idle mixture should be set with the engine in normal configuration and since you have full time vacuum advance it should be set that way.

The amount of total idle advance has a big impact on manifold vacuum and since the idle fuel flow is based on manifold vacuum, the idle mixture has to be set at the normal total idle advance/vacuum.

In the engine's original emission control configuration there was no vacuum advance so the idle mixture should require leaning since increasing the total idle timing with full time vacuum advance will increase idle vacuum.

Also, on an automatic trans car, the mixture should be set in Drive. If your ignition map is set up to pre-emission specs, you should be able to achieve a smooth idle in Drive of about 450 RPM, which is the spec for the '63 327/300 with auto trans.

Duke

[Duke W.]

Re: 1972 Tuning Question

Why are you messing with the idle mixture with the vacuum can disconnected?

The idle mixture should be set with the engine in normal configuration and since you have full time vacuum advance it should be set that way.

The amount of total idle advance has a big impact on manifold vacuum and since the idle fuel flow is based on manifold vacuum, the idle mixture has to be set at the normal total idle advance/vacuum.

In the engine's original emission control configuration there was no vacuum advance so the idle mixture should require leaning since increasing the total idle timing with full time vacuum advance will increase idle vacuum.

Also, on an automatic trans car, the mixture should be set in Drive. If your ignition map is set up to pre-emission specs, you should be able to achieve a smooth idle in Drive of about 450 RPM, which is the spec for the '63 327/300 with auto trans.

Duke

[Duke W.]

PS

Since emission controlled engines typically had ported vacuum advance, the vacuum cans were different than pre-emission engines. For emission contolled engines vacuum cans typically maxed out at 20-24 degrees at 20". For full time vacuum advance should change the vacuum can to NAPA/Echlin VC680 which is 16 deg. @ 16". Set the initial timing to achieve about 36-38 based on how much centrifugal advance the dist. has. I don't have '72 specs.

Duke

[Duke W.]

PS

Since emission controlled engines typically had ported vacuum advance, the vacuum cans were different than pre-emission engines. For emission contolled engines vacuum cans typically maxed out at 20-24 degrees at 20". For full time vacuum advance should change the vacuum can to NAPA/Echlin VC680 which is 16 deg. @ 16". Set the initial timing to achieve about 36-38 based on how much centrifugal advance the dist. has. I don't have '72 specs.

Duke

[Patrick H.]

Re: 1972 Tuning Question

"Why are you messing with the idle mixture with the vacuum can disconnected?"
Probably because every (older) car I own says to disconnect the vacuum leading to the distributor and plug it when setting the idle. Seems fairly obvious then to me....?

I can rarely find an assistant with enough time to help me put the car in "drive", so I'm not sure how I'll get that one to work. In addition, it appears that I will need to work on getting the throttle plates to close before I could ever consider getting a 600rpm or so idle out of this motor whether in Drive or not if I'm supposed to leave the vacuum line to the distributor connected when setting the idle speed and mixture. At least that's my suspicion.

Patrick

[Patrick H.]

Re: 1972 Tuning Question

"Why are you messing with the idle mixture with the vacuum can disconnected?"
Probably because every (older) car I own says to disconnect the vacuum leading to the distributor and plug it when setting the idle. Seems fairly obvious then to me....?

I can rarely find an assistant with enough time to help me put the car in "drive", so I'm not sure how I'll get that one to work. In addition, it appears that I will need to work on getting the throttle plates to close before I could ever consider getting a 600rpm or so idle out of this motor whether in Drive or not if I'm supposed to leave the vacuum line to the distributor connected when setting the idle speed and mixture. At least that's my suspicion.

Patrick

[Mike M.]

Re: 1972 Tuning Question

Since the carb has been rebuilt this may be moot. But how much side to side play does the primary throttle plate shaft have in the throttle body? Bad floats and primary throttle plate shaft play cover 90% of the problems I have ever encountered with a Q-Jet. Air leakage around the shaft WILL create idle issues. On top of that, a kit that was once marketed to repair (bush) worn throttle bodies often created more issues than it fixed. The both ends of the bushing were square and it was not long enough to be pushed into the carb and properly filed so that it did not create an air leaks around the throttle plates.
Most rebuilders know of the problems this "fix" created and will look for it.
Mike

[Mike M.]

Re: 1972 Tuning Question

Since the carb has been rebuilt this may be moot. But how much side to side play does the primary throttle plate shaft have in the throttle body? Bad floats and primary throttle plate shaft play cover 90% of the problems I have ever encountered with a Q-Jet. Air leakage around the shaft WILL create idle issues. On top of that, a kit that was once marketed to repair (bush) worn throttle bodies often created more issues than it fixed. The both ends of the bushing were square and it was not long enough to be pushed into the carb and properly filed so that it did not create an air leaks around the throttle plates.
Most rebuilders know of the problems this "fix" created and will look for it.
Mike

[A L.]

Re: 1972 Tuning Question

Hi Patrick,
I went through much the same experiences as you have described on my 71. Most of my problem was vacuum related as later confirmed after 2 carb changes and vacuum advance mechanism change. This carb has ported vacuum for the distributor. The ports are just above the primary throttle plates. They normally are not exposed to engine vacuum until the plates are cracked so at idle there should be no or minimal vacuum to the distributor. The problem I dealt with was that the engine would not idle unless the plates were cracked which compounded the problem as now the dist advance was also receiving vacuum and would advance as well increasing random idling. I had also done as you have by capping off all ports on the manifold vacuum fitting except the one my vacuum gauge was connected to. It was pulling in the 13 range. Finally figuring that this was a vacuum problem, proceeded to checking for leaks around the carb and intake. Having found none, just happened to pull out the PVC valve and block the vacuum path with my thumb, viola, everything worked normal for the first time since owning the car. Tweaked the idle settings some and it did as it was supposed to. The PVC valve was discovered to be the wrong one. At idle it was supposed to cut off vacuum, mine wasn't doing that and was pulling down engine vacuum at idle and would not idle without cracking the throttle plates. It was acting like an open 1/4" vacuum port going directly to the base of the carb and full engine vacuum.
This is not to say that this will be your problem but make awareness as to the nature of the problem. I was resorting to rocket science thoughts in dealing with the problem, but when it was discovered, it turned out to be every day engine diagnosing. Probably a good mechanic would have found out in 30 minutes what it took me three years to discover.
Just a suggestion, be aware that you have the distributor vacuum hose connected to the correct vacuum port, fix the problem with the linkage cracking the throttle plates, and if problem still exists, proceed to checking possible sources for vacuum loss.
AG Logsdon

[A L.]

Re: 1972 Tuning Question

Hi Patrick,
I went through much the same experiences as you have described on my 71. Most of my problem was vacuum related as later confirmed after 2 carb changes and vacuum advance mechanism change. This carb has ported vacuum for the distributor. The ports are just above the primary throttle plates. They normally are not exposed to engine vacuum until the plates are cracked so at idle there should be no or minimal vacuum to the distributor. The problem I dealt with was that the engine would not idle unless the plates were cracked which compounded the problem as now the dist advance was also receiving vacuum and would advance as well increasing random idling. I had also done as you have by capping off all ports on the manifold vacuum fitting except the one my vacuum gauge was connected to. It was pulling in the 13 range. Finally figuring that this was a vacuum problem, proceeded to checking for leaks around the carb and intake. Having found none, just happened to pull out the PVC valve and block the vacuum path with my thumb, viola, everything worked normal for the first time since owning the car. Tweaked the idle settings some and it did as it was supposed to. The PVC valve was discovered to be the wrong one. At idle it was supposed to cut off vacuum, mine wasn't doing that and was pulling down engine vacuum at idle and would not idle without cracking the throttle plates. It was acting like an open 1/4" vacuum port going directly to the base of the carb and full engine vacuum.
This is not to say that this will be your problem but make awareness as to the nature of the problem. I was resorting to rocket science thoughts in dealing with the problem, but when it was discovered, it turned out to be every day engine diagnosing. Probably a good mechanic would have found out in 30 minutes what it took me three years to discover.
Just a suggestion, be aware that you have the distributor vacuum hose connected to the correct vacuum port, fix the problem with the linkage cracking the throttle plates, and if problem still exists, proceed to checking possible sources for vacuum loss.
AG Logsdon

[Duke W.]

Re: 1972 Tuning Question

I think you're confusing setting the initial timing with setting the idle mixture. The vacuum can should be disconnected for setting initial timing, but should remain connected when setting idle speed and mixture. (With ported vacuum advance it should not make any difference whether or not the vacuum can is connected, but with full vacuum advance it will.)

As part of a tune-up the idle speed and mixture should be adjusted after the dwell and timing and been checked and adjusted as necessary.

The specified idle speeds for the '63 250 and 300 HP engines are 500 RPM in neutral (manual tranmissions) and 475 in Drive (automatic transmission). The procedure does not say to disconnect the vacuum advacne.

When setting the idle speed/mixture on an auto trans car the parking brake should be fully engaged and the wheels chocked.

Duke

[Duke W.]

Re: 1972 Tuning Question

I think you're confusing setting the initial timing with setting the idle mixture. The vacuum can should be disconnected for setting initial timing, but should remain connected when setting idle speed and mixture. (With ported vacuum advance it should not make any difference whether or not the vacuum can is connected, but with full vacuum advance it will.)

As part of a tune-up the idle speed and mixture should be adjusted after the dwell and timing and been checked and adjusted as necessary.

The specified idle speeds for the '63 250 and 300 HP engines are 500 RPM in neutral (manual tranmissions) and 475 in Drive (automatic transmission). The procedure does not say to disconnect the vacuum advacne.

When setting the idle speed/mixture on an auto trans car the parking brake should be fully engaged and the wheels chocked.

Duke