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Old 05-14-2008, 07:13 AM
DAbraham DAbraham is offline
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Question 65 GTO Vac Adv configuration

I have a 65 GTO with Carter AFB - the (65) 389 engine is from a 65 automatic car and the Transmission is a Muncie 4 sp. I am putting this car back on the road after 15 years in storage..(don't ask). It is back together and running, but when timed at book (6* BTDC) with vacuum disconnected, and then the vac line is reconnected, the timing jumps about 15* and then runs higher still at increasing revs... Can't seem to find a vac port that does not have a vacuum at idle. I have the 1965 Tempest Chassis (shop) manual - original - as well as the Pontiac Restoration guide and neither shows a clear picture of where to connect the vacuum nor comments on the high advance angles. Perhpas I do not have enough parts. I believe the auto carb (I have) is 3898S and the correct standard is 3895S. Is there some kind of device that will provide the correct vacuum profile. Any and all suggestions welcomed and very much appreciated! This sounds like a wonderful community of folks sharing ideas and information.

Thanks ... Abe - Concord, NC

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Old 05-14-2008, 08:09 AM
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Welcome to the PY Forum!!

The manual transmission GTO's used manifold vacuum, not ported vacuum for the advance mechanism. The change from 6 degrees to 15 degrees at idle is what one would expect. It's always necessary to reduce idle speed after connecting the vacuum advance. Your Carter AFB should have a large air screw in the front to adjust idle speed, not a conventional adjustable throttle stop screw.

Good luck.

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Old 05-14-2008, 01:40 PM
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Thumbs up 65 GTO Vac Advance question - resolved ..

Dick,

THANK YOU for the rapid reply! Very much appreciated. I have found this to be typical of most user communities of this nature, loaded with folks that so freely and unselfishly share their knowledge and experience with us that are far less gifted and talented in this area.

That is what I needed to hear. It had been 15 years since it last ran and after I did as much as I could and got down to a clogged pickup in the gas tank and had to move the car because we sold the house, I got a rollback and brought it to a trusted mecahnic. He had not previously seen this phenomen and was comparing the Carter to a Holley. But I called him with your information and it appears that all is ready to go.

Now to find a trusted body person in the Charlotte, NC area to take a look and see what they can clean up on the body etc.

Thanks again .... take the rest of the day off!!

best ... Abe -- Concord, NC

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Old 05-14-2008, 02:06 PM
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Glad to help. Now, get that body work done and enjoy your GTO!!
Another 15 years comes too soon and cost will be much higher then.

Dick

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Old 05-16-2008, 12:57 AM
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Sorry, Auto cars use full vacuum at idle to the distributor. Stick cars use ported (only gets vacuum as the throttle is opened). Safer to not use vac advance. Initial at 12 degees 9 or 10 in the mechanical of the distributor for a total of 32 crank degrees. Semi stock Pontiacs love it. See page 11-33 of your Tempest shop manual. Put a degreed tape on your balancer so you know what is going on. Pontiac is remiss in not doing it stock.

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Old 05-16-2008, 08:46 AM
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aceaceca,
Are you sure about the ported vacuum on stickshift cars? I am certain that on '66 GTO's with Tripower, manifold vacuum was used for both auto and stick vacuum advance.

Also, for maximum fuel economy and driveability, vacuum advance is the way to go. For drag racing and max street performance, mechanical advance only is OK.

Yes, 32-34 degrees of total advance works best on Pontiacs.

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Old 05-16-2008, 12:45 PM
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Someone else asked a bit ago about the same thing and I checked in my 65 Tempest Manual. If you have one look at page 11-33. It says specifically what I stated.

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Old 05-16-2008, 09:48 PM
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66 Stick shift cars also use manifold vacuum..... that's the only type available on a AFB.

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Old 09-24-2021, 02:39 PM
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This discussion would help me too, it seems my car was somewhat retarded on overall timing as exhaust/crossover paint is significantly burnt from new and was running hot.

I'm not sure my timing mark on the balancer is correct so I am going to confirm that soon (with degree wheel and piston stop and careful rotations).

Regardless, I'm running 110 octane right now to start on the safe side and once I know for sure how the engine (presumed stock rebuild) is configured I'll back that down.

Car runs well and cooler and doesn't ping and nice performance at 18 degrees base (if balancer mark is correct). It appears that I am getting about 10 degrees mechanical advance and about 10 degrees (from manifold) vacuum advance for a total of about 38 at 3000 just sitting still, as I held rpms around 3000 just sitting in the garage with no load and dialed back timing light on that. But when folks are saying "Pontiacs like around 32-34 degrees" I have always assumed that is only base+mechanical. So do I need to tear into my distributor to see if they have welded/limited the mechanical advance to only 10 degrees (measured at the balancer with a dial-back timing light)? Would you be expecting to see as much as 20 degrees (at the balancer) added from mechanical advance alone by 3000 rpm, to be added to a 12-14 degrees base timing to achieve the "Pontiacs like 32-34 degrees" total, assuming WOT at that point meaning zero vacuum advance?

Thanks!

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Old 09-24-2021, 04:02 PM
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Carter 3898S is from a 1965 421 A/T engine.

NO 1965 Pontiac Carter AFB has a vacuum spark port. Spark passage is open to manifold vacuum.

For 1966-67 ONLY Carter AFBīs using A.I.R. uses a vacuum spark port. ALL other Carter AFBīs for 1966 Pontiac V8 uses full manifold vacuum to the ignition vacuum advance.

If you really need a regular non-A.I.R Pontiac Carter AFB with a vacuum spark port, look for a 1964 AFB, (except 3647S) and certain earlier units.

FWIW

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Old 09-24-2021, 05:35 PM
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Normally the stock vacuum advance provides more than 10 degrees. I’ve found that 12-15 degrees initial , with about 22 degrees centrifugal advance works well. That gives you 34-37 degrees total advance.

Vacuum advance adds 15 degrees or more. Back in the day, we ran without vacuum advance, which sacriced highway gas mileage. We didn’t care when gas was 25-40 cents per gallon.


A well-respected Pontiac builder, Dan Whitmore, told me a Pontiac engine should see no more than 52 degrees total advance.

You’re welcome to disagree, or add more information.

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Old 09-24-2021, 06:59 PM
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Dick,

Thanks so much. Updated data collect from today with dial-back timing light, disconnecting manifold vacuum, manually holding engine to desired rpms at carb linkage, etc.

Vacuum:

18” at 700 rpm idle (vacuum line connected to dist),

17” at 450 rpm idle (vac line blocked to dist).

Advance:

16 degrees with no vacuum advance at 450 (low idle because no vac advance),

18 degrees with no vac advance at 600 (held throttle linkage)

38 degrees at 500 rpm idle with vacuum line connected (manifold vacuum under rear carb of tri-power)

Mechanical advance: 38 degrees at 2500 rpm holding throttle linkage (vacuum line blocked)

Total advance: 48 degrees at 2500 rpm with vacuum connected and no load, just holding center carb throttle linkage.

So, I’m pretty happy with this configuration. It previously had 12 degrees initial and was much more sluggish and running hotter. If I find I can drop to non-ethanol 93 octane gas I would likely pull the spark back to 12 and try that as I assume it will ignite much sooner. I’m just not sure yet how this engine was rebuilt and what it requires.

Thoughts?

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Old 09-25-2021, 02:55 AM
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1965 GTO uses #1111054 distributor.
Initial timing is 6° BTDC.
Centrufugal advance starts with 1-5°@900 rpmīs, 14-18°@2000 rpmīs and max 20-24°@4600 rpmīs.
Vacuum advance #1116172 starts@8-10 in/hg and advances max 20°@14.75-16.75 in/hg.
These are @ crankshaft degrees and rpmīs.
This calculates total max 50-54° while driving at 4600 rpmīs.

Also, 1965 Tempest chassis shop manual are WRONG at page 11-33 regarding source for vacuum advance as ALL 1965 Pontiac AFBīs uses full manifold vacuum for the vacuum advance.
You will find the facts in Carter carbureter manual FORM 5482 june 1964 revised september 1964

I have found using fast centrifugal advance to compensate a too lean calibrated carburetor will never make full potential from the engine, but rather destroy the engine on long term usage. This is a thing of the past when leaded high octane fuels was available.
I prefer factory ignition settings for the engine and camshaft used and THEN adjust the carburetor idle/lowspeed circuit and main and secondary jetting for proper amounts of combustable fuel/air mixture. Especially for todays leaner fuels.

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Last edited by Kenth; 09-25-2021 at 03:03 AM.
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Old 09-25-2021, 04:55 AM
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According to my Motors Manual, the vac adv on 64 to 66 389 engines added 20*.

So with 6* initial, these cars were driven off showroom floors idling at 26*.

Suddenly, 30* doesn't sound excessive for a modified engine does it??

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Old 09-25-2021, 08:47 PM
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Quote:
Originally Posted by Kenth View Post
Carter 3898S is from a 1965 421 A/T engine.

If you really need a regular non-A.I.R Pontiac Carter AFB with a vacuum spark port, look for a 1964 AFB, (except 3647S) and certain earlier units.
FWIW
So I have a 64 AFB, and tried to go port vacuum but had no port. Found a screwed-in pipe plug in the front of the carb where the Edelbrock's have a ported vacuum port. After comparing it to a newer Edelbrock with ported vacuum and seeing the same port going into the primaries, I removed the plug and screwed in a vacuum hose prong and it now has ported vacuum.

ONLY reason I want to convert to ported is to avoid the gassy smell that too much advance at idle seems to cause on this engine.

It's the ONLY Pontiac AFB I have. Not sure why it was the way it was, maybe the teat got broken off and had to be drilled and repaired. AND the driver's side fuel feed.

I'm so glad it has the adjustable air valve between the mixture screws. Edelbrock missed the boat on that one.

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Old 09-25-2021, 10:22 PM
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Quote:
Originally Posted by Kenth View Post
1965 GTO uses #1111054 distributor.
Vacuum advance #1116172 starts@8-10 in/hg and advances max 20°@14.75-16.75 in/hg.
I'll bet you are not pulling 14.75-16.75 in/hg at 4600 RPM. Probably not enough vacuum at that engine speed to give ANY vacuum advance except when you close the throttle completely

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Old 09-25-2021, 10:30 PM
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Quote:
Originally Posted by Bill Hanlon View Post
I'll bet you are not pulling 14.75-16.75 in/hg at 4600 RPM. Probably not enough vacuum at that engine speed to give ANY vacuum advance except when you close the throttle completely
So who are you challenging ?

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Old 09-26-2021, 03:50 AM
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If you are getting a 'gassy' smell with MVA, then you have an adjustment problem, not an MVA problem. These 64-66 AFBs that Pontiac used did not have an idle speed screw; the boss is there for it on the side of the carb, but is not drilled. You could drill & tap for a speed screw.
In it's factory configuration, the pri t/blades seat in the bore. The large brass screw adjusts idle air flow to adjust idle speed. Using a proper speed screw as outlined above will give you more adjustability.

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Old 09-26-2021, 03:58 AM
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Quote:
Originally Posted by Bill Hanlon View Post
I'll bet you are not pulling 14.75-16.75 in/hg at 4600 RPM. Probably not enough vacuum at that engine speed to give ANY vacuum advance except when you close the throttle completely
The ignition vacuum advance is an engine load ignition regulator/sensor.
At 4600 rpmīs you may have the pedal to the metal (WOT). High load on engine means low vacuum signal in intake and no vacuum advance needed since A/F mixture is enriched.
Or, you may drive fast at 4600 with part-throttle. Less load on engine means high vacuum signal in intake that activates the vacuum advance helps burning the leaner mixture at proper time in the combustion cycle for best efficiency

And, the load on engine (and speed of course) at 4600 "cruising" is different using 2.56:1 or 4.33:1 gears in the rearend and anything in between.

The ignition vacuum advance regulates the amounts of advance needed depending on load on engine (intake vacuum signal at all times the engine is running).

If you want to know what is going on with vacuum advance at all rpmīs and driving situation just connect a vacuum gauge to the intake at a full manifold source via a long hose so you can watch wile driving.

FWIW

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Old 09-26-2021, 08:14 AM
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Kenth, so true (load at 4600 versus gear ratios).

By my calculations, while I am running about 2800 at 70 mph with 3.23 gears and a 4-speed, you would be running 145 mph at 4600 rpm with 2.56 gears; I would say you would require WOT to achieve that!

With 4.56 gears at 4600 you’d only be at about 81 mph, so you could easily be at part throttle and would see manifold vacuum then.

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