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Discussion Starter #1
Hello guys,

I am new here and I apologize if this topic was already solved (tough I couldn't find my exact problem in the forum).
So to let's start with, I am owner of what looks like to be quite a survivor, a 1966 Pontiac Star Chief Executive 4-Door HT with the 389cu in engine and TH400 transmission.
The car was shipped to me (Poland) from the State of Kansas and has probably original 36k miles on it (I searched back all the previous owners, who happend to be two older women and a man). The interior looks very good (tough the dash pad is multiple times cracked), car had one repaint (recent paint is still on lead basis so I assume the job was done at least in the 80s or earlier) and only two corrsion holes, one in the trunk and right quarter panel. As far I can consider (I am no mechanic) the car seems to be all stock and had even original sparkplugs and wires. So no modifications are noticeable.

Now to get to the problem of mine, the car was a runner since the first start I have made, so there are no start up issues. But the problem occures after warming up and leaving the car idling. At the very first time the idle rpm's sat at 1600 and it was very hard to lower them to ~1350 (lowest possible before stalling). This realted to a higher vacuum and resulted in heavy transmission jerks while putting into D or R gear. That's why I gave the car to a mechanic and he did the following things:

1) disassembly, cleaning and changing gaskets of the carb (two barrel Rochester) it turned out there was a vacuum leak right at the base of the carb, at firewall's side,
2) disassembly, cleaning, repainting of intake manifold + valve covers, replacing gaskets,
3) full fluid flush of engine, transmission, powerbrakes, powersteering and cooling system + chaning filters,
4) replacing spark plugs (AC 45RS put in),
5) disassembly, cleaning and reapainting of fuel tank,
6) changing of some cracked vacuum lines around carb,
7) enigne tuning.

The day I picked up the car and drove back home (~20 miles), idling was good at 650 rpms (measured whilst put in D, dashpot unhooked and sealed). I wanted to make sure that the mechanic did everything alright, so I checked my Pontiac manual and looked up for all other things to look after and stumbled upon "engine dwell". I have the "check" window in my distributor cap so I just hooked up my car multimeter and begun to measure while having the engine idling. It clearly stated 14° and had very little play when I changed the motors rpm's (like 13,9 - 14,2°). The manual says that the dwell should be placed at around 30° +/- 2 so I took my alan screw and increased the dwell to exact 30° by steps of 5°- adjusting each time timing back to 6° BTDC . Doing this maneuver I had hooked up all the time my dwell meter and disconnected distributor vaccum as well as cabin+dashpot, all sealed. After reaching the 30° I checked back timing, and after that I looked up the rpm's - and surprisingly they were way off at 1.100 with waves of ~150 up and down.
Readjusting any of the carb screws (fast idling screw or both fuel mixture screws) did nothing. Going back with the dwell also didn't work. Installing a vacuum gauge clearly showed a vaccum fluctuation between 17,5 - 14,0 ins Hg on idle. Tranny still kicks like a horse into D/R
The engine parameters are now as follows:
Timing: 6 BTDC
Dwell: 30°
I have on my own tried to look for some vacuum leaks but found none.
Changed now also all spark wires with distributor cap and rotor.
Engine starts with set choke at once, almost instantly, doesn't even take a full turn of the starter.

Do you have any suggestions what might happened to the engine?

P.s.: I did not check yet the compression of each cylinder (36k miles).
I noticed strange knocking coming off the right side of the engine, maybe below the block, only after warming up and while stronger accelerating the car while driving. The frequency of the noise is constant and does not relay to the engine's rpm. Might that be a loose exhaust manifold heat valve? This noise was present ever since I first drove the car.
I fill the tank with Shell's V-Power Racing (100 octane) + Kleen Flo Nitro Additive
Below: Picture of the vacuum modulator of the TH400, Engine Bay and my first trip with the car:)

Thank you for any replies and greetings from Poland :)

Artur
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Your vacumn fluctuations may be moving your idle dashpot,...those were designed to not allow the throttle to slam shut on fast deceleration, like coming to a fast stop....then the car would stall. So the dashpot cushions that fast shutdown.....many guys just disconnect them.

Now just for a test you can disconnect it and plug the vacumn line. They at warm idle reset the dwell to 30 degrees but leave the distributor vac hooked up. You want the dwell to be 30 degrees when the engine is idling like it normally will. Once at 30, then disconnect and plug the vacumn to the distributor and reset your timing to 6 degrees BTDC. This is just a check and test, I think your idle dashpot is moving in and out from fluctuating vacumn. Which may be from another vacumn leak, that vacumn modulator on the trans looks rough and probably leaking. Those are about $15 from Summit, Ames, JEGS ...and are easy to install.

But you can also clamp off all other vacumn, except distributor,..which you may be doing to see if it improves your idle...

It is some give and take. Also your dashpot for the throttle may be leaking, and a weak throttle spring, that big one in the picture can make idle fluctuate...

Great looking Pontiac, super color, love the vent windows, Polish people were the greatest American allies, their bravery in every action in WW2 is legend....

This noise needs more investigation could be exhaust heat riser valve leaking.....
 

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Discussion Starter #3
Your vacumn fluctuations may be moving your idle dashpot,...those were designed to not allow the throttle to slam shut on fast deceleration, like coming to a fast stop....then the car would stall. So the dashpot cushions that fast shutdown.....many guys just disconnect them.

Now just for a test you can disconnect it and plug the vacumn line. They at warm idle reset the dwell to 30 degrees but leave the distributor vac hooked up. You want the dwell to be 30 degrees when the engine is idling like it normally will. Once at 30, then disconnect and plug the vacumn to the distributor and reset your timing to 6 degrees BTDC. This is just a check and test, I think your idle dashpot is moving in and out from fluctuating vacumn. Which may be from another vacumn leak, that vacumn modulator on the trans looks rough and probably leaking. Those are about $15 from Summit, Ames, JEGS ...and are easy to install.

But you can also clamp off all other vacumn, except distributor,..which you may be doing to see if it improves your idle...

It is some give and take. Also your dashpot for the throttle may be leaking, and a weak throttle spring, that big one in the picture can make idle fluctuate...

Great looking Pontiac, super color, love the vent windows, Polish people were the greatest American allies, their bravery in every action in WW2 is legend....

This noise needs more investigation could be exhaust heat riser valve leaking.....
Thank you for your reply,

for the dashpot - it was recently changed for a NOS (I test sucked it before installing). The engine tuning I did was with the daspot and cabin vacuum lines hooked up. I even plugged the one from the power brakes. It had no impact on the idling at all. For the tranny vacuum - I did not touch them, have to test it!
However as for the carb itself, neither the idle mixture screws really work (either the engine just runs or it stalls with a weird noisy knocking orchestra :). The acceleration screw is free and doesn't touch the acc. lever at all. I can also disconnect all other levers on the carb and the engine won't die. I'll try to add a short clip I made with my phone. There you can see the engine running with all the levers off on the carb. (Okay, had to post it on Youtube
).
The most curious thing is that all the problems above just returned (formerly the mechanic fixed most of it) after I changed the dwell from 14 to 30 deg. After that I thought maybe some electric bypass formed on the ignition system due to the old wires, so I changed them along with the distributor cap and rotor, but unfortunately it changed nothing.

Ps.: The modulator was (or still is) covered in heavy mud, so was all the engine bay before I started to clean it up. The car came from Topeka (Kansas), must have had some dusty roads back then.

Meanwhile I will try to hook up the tranny vacuum and see what happens.

Thanks,

Artur
 

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You may need a new set of points,...remove the dist cap and look at them are they pitted or burned,..in your write up I didn’t see where you changed them...but I may have missed it. Changing the dwell is basically just moving the points, so if they are original they may be bad.
 

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Discussion Starter #5
Nah, nothing burned or rusted etc (besides the centrifugal counterweights). For my layman eye the points look almost brand new. Below some pics (sry. for the quality, it's very difficult to get a good angle on that tight spots).
First - engine as it was when I received the car, secondly - one of the original spark plugs (the wires are code dated with Packard Radio TVRS LR4-Q 65, on cylinder 6 there was a current breakdown on the shoe, I could here the spark missing the plug), next view of the original distributor cap - untouched following distributor with points.
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I would make sure those screws holding the power wire and the ground wire at the points are clean and tight connections. Also the screws holding the points down make sure they are not loose, as this happened when you adjusted them with the Allen wrench.

You need to as time and money allow change that distributor cap and rotor plugs and spark plug wires, as 50 year old spark plug wires and spark plugs do deteriorate, but I don’t see any obvious signs that would be making it hunt for idle.

Now the rubbing block on the points may be worn down considerably and when you adjusted the dwell of course you move the points and it moves the rubbing block. The points have spring tension and that fades as well, so when you moved the points it may have effected how well the points can stay hard against the cam, because of wear....

I still think I would change the points, and reset not a large expense, and a good idea anyway. A normal rune up would change them and spark plugs as well.

Stay with it you will get it, keep that plug gap at 35 ....
 

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Discussion Starter #7
Oh, I must have missed that part in the description. Sorry. The rotor cap, distributor cap as well as spark plugs and spark plug wires have been already changed. I look forward to change the points and in the meanwhile I’ll hook up also the tranny vacuum line.
 

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I would make sure those screws holding the power wire and the ground wire at the points are clean and tight connections. Also the screws holding the points down make sure they are not loose, as this happened when you adjusted them with the Allen wrench.

You need to as time and money allow change that distributor cap and rotor plugs and spark plug wires, as 50 year old spark plug wires and spark plugs do deteriorate, but I don’t see any obvious signs that would be making it hunt for idle.

Now the rubbing block on the points may be worn down considerably and when you adjusted the dwell of course you move the points and it moves the rubbing block. The points have spring tension and that fades as well, so when you moved the points it may have effected how well the points can stay hard against the cam, because of wear....

I still think I would change the points, and reset not a large expense, and a good idea anyway. A normal rune up would change them and spark plugs as well.

Stay with it you will get it, keep that plug gap at 35 ....
Speaking about the distributor, I noticed some horizontal play (not excessive, just a few mm) of back and forth of the distributor cap while being mounted (screwed tight) on the distributor shaft socket. Is this normal?
 

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One step at a time. I fear your dwell meter reading is not correct. If the engine ran well at 14 degrees, then there is something wrong with your dwell meter reading. I wonder if the setting is on 4 cylinder. If you double 14 you get 28, and that is within the spec range for the dwell - 28-32. I would put it back to the 14 degree setting (measured exactly as before) and go from there.
 

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Discussion Starter #10
That’s a tought, gonna check that. Also, in theory, if the points were messed up, wouldn’t this result in a fluctuating dwell angle reading (like more than 1-2 degrees)?
 

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One step at a time. I fear your dwell meter reading is not correct. If the engine ran well at 14 degrees, then there is something wrong with your dwell meter reading. I wonder if the setting is on 4 cylinder. If you double 14 you get 28, and that is within the spec range for the dwell - 28-32. I would put it back to the 14 degree setting (measured exactly as before) and go from there.
That was my initial thought.
 

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Your rotor cap should be tight and not have side to side play, look under the side of the distributor will a little mirror once you put the cap on and make sure it is sitting inside the groves where it is supposed to be. I had a distributor recently that was on a Pontiac and someone had put it on not even near the two slots underneath. They are easy to miss, when on snug and right it has no play.
 

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Discussion Starter #15
Alright guys,

that's what I have noticed so far:

1) clamping off the transmission vacuum line and sealing off the carb's socket resulted in:
rpm's (x 10) before clamping off: 155 - 179 (amplitude of 24)
rpm's (x10) after clamping off: 131 - 163 (amplitude of 32)
- both parameters measured on hot idle,
That is about 12% difference on average idle speed between hooked/unhooked tranny line,
2) measured again the dwell, with 8-cyl setup on my multimeter and it gave me 30,1 - 30,2°
3) checked the screws distributor point's - they were already pretty tight, however there is a play between the distributor shaft and the rubber pad of the points (see link below).


P.s.: I wasn't absolutely correct while I wrote about the cap being loose, it sits absolutely tight on the centrifugal socket, but the shaft as mentioned above has some play.
 

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The distributor moves like that when the weights deploy outward, that is ok.

I would change the trans Vacumn modulator and use a new rubber line, you have a vacuum leak there that is increasing your Idle RPM..

with Dwell at .30 you are good...did you have the meter on 4 cyl as Mr. Taylor and Minesa66 noted
 

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Discussion Starter #17
The distributor moves like that when the weights deploy outward, that is ok.

I would change the trans Vacumn modulator and use a new rubber line, you have a vacuum leak there that is increasing your Idle RPM..

with Dwell at .30 you are good...did you have the meter on 4 cyl as Mr. Taylor and Minesa66 noted
You mean exchaning the factory steel tranny vacuum line with a rubber one? Also, there are many modulators on the web to buy, are there any particular I need to look for(e.g. any one that would fit the factory look)?

Thanks for all the help ;)
 

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No you can use the steel line, use what you have...AMES should be able to get you one that matches or Original Parts Group Inc (OPGI)...

if the line is steel and not leaking keep it, some have rubber going from the steel line to the modulator and they deteriorate...

You want a new modulator with no leaks
 

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Discussion Starter #19
No you can use the steel line, use what you have...AMES should be able to get you one that matches or Original Parts Group Inc (OPGI)...

if the line is steel and not leaking keep it, some have rubber going from the steel line to the modulator and they deteriorate...

You want a new modulator with no leaks
I crawled today under my fierce charriot and cleaned up that modulator. I made a suck test (it’s air tight) and replaced the little rubber connector with a new one, altough I am going to replace the modulator asap. I’ve got two questions about this area, one- what good for is the little screw atop the modulator (do I adjust with it the shifting steps)?
Secondly, the two steel lines going right into the tranny’s casing, are those the cooling lines?
Also, a question to determine the car’s history, I roughly cleaned a small part of the bottom panel on passengers sight from all the dirt and out came this quite good looking surface finish- what are the odds this could be the factory underlay protective paint? If not, what finish should I use to match factory style?
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Thanks for replies,
Artur
 

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I crawled today under my fierce charriot and cleaned up that modulator. I made a suck test (it’s air tight) and replaced the little rubber connector with a new one, altough I am going to replace the modulator asap. I’ve got two questions about this area, one- what good for is the little screw atop the modulator (do I adjust with it the shifting steps)?
Secondly, the two steel lines going right into the tranny’s casing, are those the cooling lines?
Also, a question to determine the car’s history, I roughly cleaned a small part of the bottom panel on passengers sight from all the dirt and out came this quite good looking surface finish- what are the odds this could be the factory underlay protective paint? If not, what finish should I use to match factory style?
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Thanks for replies,
Artur
The 2 steel lines are for the transmission cooler. They run to the radiator, or if so equipped, an auxiliary cooler. The screw on the modulator is for adjustment. Most modulators are sealed and not adjustable. The vacuum modulator delays the auto shifting of the trans. You can vary the shift point via the screw. If it's shifting well, I would not mess with it. As far as the originality of the floor paint, I will definitely defer to the Pontiac/body man experts.
 
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