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Discussion Starter #41 (Edited)
Dissasembled today the carb and checked the throttle plates for tightness and fit. Seem to be alright, test light passed altough at a specific light angle a sun eclipse shine is present, however it‘s not possible to change much since the whole base is secured via pressfit rod and you can‘t take it further apart. Only centering would be possible but they are pretty centric. In addition I might say that the plates are slightly smaller in diameter then the bore. The edges of the plates rest tight against the bore walls. After reassembling clamped all vacuums off and watched my gauge. It rose only 1-2 points so I guess there are no leaks. Also sprayed a lot of carb cleaner all over the place without result.
Carb still running by itself at ~1300 rpm’s with 17/18 in hg. Idle screw backed all way out 👍🏻
Playing with the timing also did not help :( Going off that 6 BTDC made it only worse, stuttering,vacuum drop.
Might the float level inside the carb have any impact or maybe a leaking powervalve?
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137915
 

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I am seeing daylight in pic #1 on the lower of the 2 throttle blades. I can also see slight daylight in pic #2 on the right edge of the throttle blade. But I don't think enough to be of concern.

The shaft is not pressed in. This screws would need to come out of the throttle plates (and the screws are typically peened so they don't back out and need to be ground on first so you can remove the throttle plate screws or they will break being brass).

The throttle blade shaft in pic #1 at the top looks dark near its end against the casting. Is it wet? If the throttle shaft is worn, it can suck in air and will act as a vacuum leak and raise RPM's. Spraying carb cleaner etc. may not show this up. I don't know if they have a kit to install new bushing and shaft as they do for QuadraJets which often wear out due to age/use.

You engine should not have run poorly as you stated if you advanced the timing, it should have increased in RPM - the same as when you connect the vacuum advance. If you retarded it, it may have run poorly and it can also cause the engine to run hotter. So one direction should have cause the engine RPM to go up and should have run smoother, then other would have decreased the RPM and probably ran poorer.

So when you closed the carb completely, throttle cable disconnected, your engine still idled at 1,300 RPM"s? I can't see how with the throttle blades completely closed.

I looked at the first picture of your engine. I see the dashpot (the round canister) on the driver's side with the vacuum hose which appears to be extended and I don't think your engine is running. The rod has a nut on the end of it to adjust this. It looks like it is up against the throttle arm which would keep the throttle blades open and keep idle speed up. Make sure that is adjusted correctly. Try screwing it in to shorten it and see if that changes your idle speed.

The choke housing uses a tube from the intake to heat the air for the bi-metal spring. Is it possible that the tube going into the manifold has rusted out/through and is the source of a vacuum leak? Have you checked this for any vacuum leaks? I cannot recall if there is an opening/port that connects the choke housing directly into the carb body, but I believe some do, just not sure what years.

Take a look at this drawing and read the text. If the idle air bleeds are plugged up, they can cause problems as if the engine was running rich and the RPM's won't drop. We had a member with a tri-power have this issue and tried quite a few things only to find out that the idle air bleeds for his application were too small with his big cam. Once he drilled them larger, his idle came down to a normal setting and that cured his fast idle issue.


You verified again your float level? Did you make sure your power valve ( that long rod with the spring on it) inside the carb body was working up and down and not stuck?

Have you tried pulling a spark plug or two to see what they look like? If by chance the carb does have an internal leak, you may find a spark plug that is wet or very black from running rich.

The 2 bbl is fairly simple and really an easy rebuild. Just seems odd you are having this problem. Make sure you are using the correct base gasket, which looks like you are. It should be rectangular and only have the 2 holes for the carb barrels. Some have contours depending on intake, but I believe your should be essentially rectangular with just the 2 holes.
 

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Discussion Starter #43 (Edited)
Thank you Jim and all other for those comprehensive ideas but I'm afraid I can't handle too much of this information since I am a greenhorn in cars mechanic, despite the fact that I am dealing the very first time with an american classic. Maybe the problem lies somewhere we did not think of, even more trivial then the ones you've stated.
Let me summerize:
1) I bought this car with an already high idle (was about 1.400 rpm if I can still remember),
2) The car had never starting problems, ignition was almost instant,
3) I gave the car to a mechanic (later turned out he wasn't quite trustworthy) because the car had to have replaced all fluids, some minor body work, rebuilding brakes, engine overheated and what was most significant, it just slammed into R and D gear (probably because of those high rpm's),
(the engine was overheating when at traffic jams, but after changing a) spark plugs for new, b) flushing myself all the radiator (there was a lot of butter) the "idiot" light showed up never again.)
4) The carb was dissassembled (it looked just gross, overgrown with thick layers of oil and mud), so was the intake manifold and top cover of engine (the one between the valve cover, under the intake manifold), all gaskets at that point has been replaced),
5) The day I was going to pick up my car I ran instantly into a problem, for one there was really strange knocking coming from the engine bay (turned out it was the loose distributor cap with the rotor bangnig against it...) and because the mechanic did sth. wrong with the carb adjustment, my exhaust muffler exploaded... I did leave the car to have this repaired at once, and picked it up on the next day. Arriving at home (driving somewhat 25 miles without issues) I ran some diagnostics by myself because I did not want to put any more trust into this sloppy work of my mechanic. So I measured the rpm's on warm engine (they were quite fine, steady at around 750, with the gear in D I could even go down to 600 by backing out slightly the idle screw), checked timing (was at 6° BTDC), checked the vacuum, reading was at around 19/20 and last but not least I looked for the dwell angle and here it happend :eek:
The meter read 14°! I took my alan wrench, opened the little service window at the distributor and rose it to 28,5°. The engine responded to this immediately with higher rpm and greater timing. Cant remember if the vacuum can was disconneted at that time (like I said, I am learning by doing :) but I surely measured at some point the BTDC with the distributor vacuum hooked up.
This was the point where I have lost completly control over the engine's rpm. Could that be just a coincidence? First I came up with the idea of setting the dwell back to 14° but that did not work anymore. Next I checked all the wiring, in fact some of the spark plugs were loose and I had to tighten them (courtesy of my mechanic) and on cylinder 5 there was a shortcut (everytime I squeezed my hand between the A/C compressor and engine block to reach the spark plug I got shocked :ROFLMAO:. I looked up my distributor, the cap was in my opinion in quite a good shape (the pic is at the beginning of this topic) like it should be for 36k miles, however the rotor showed a little bit of burn at the very end of the metal plate (it was also changed for a new one). Wires were original (dated with 4-Q-65 PACKARD RADIO TVRS LR), they were not brittle, anyways I changed them for new silicon ones from AC DELCO Professional - and it did not have any impact on the engine rpm or anything else. Sparkplugs were, possibly, also factory, AC 45S (picture below, the first two are as they came out from the engine, third one I just cleaned for commparison). The mechanic put in before new ones - AC 45 RS (dunno if that's bad or good). Old orignial are a) points mechanics, b) condenser, c) coil, d) wire coil-distributor, e) vacuum can.
I even readjusted all the levers and float to factory settings - nothing helped.

Maybe you can come up with some new thoughts :p

Meanwhile I will do as you stated Jim, check again the power valve etc.

Re: Your drawing: As stated:"
  • Set the idle speed to manufactures specifications. When the RPM is too high, the idle circuit is bypassed and adjustments will not affect anything."
The rpm is in fact way too high, and I am afraid my idle circuits are just bypassed all the time. I still believe that the engine is stock so there would be no use of drilling bigger air bleeds? In fact, I rechecked and recleaned yesterday all the carb with exemption of this curious T-shaped metal piece with a spring under it, and possibly the checkball? Did not touch that since it seems to be very fragile and I don‘t really know what it does. . However all other passages in the carb are super clean. If the power valve is leaking I can not say, I will check that again but I am not sure what else I could do besides just looking at it and trying to pressfit into the guide slide.

P.s.: Trying to answer your previous questions:
  1. As I said, the plates are centric, however they are slightly smaller in diameter then the bore (maybe because of heat extension?) and there is coming at specific angle a small amount of light through, besides that the plates are not closing perpendicular to the bore but what I think at a 10°angle as engraved on the top of em,
  2. Excuse my misinterpretation, not the shaft but the end of it, at which sits the outer lever is fixed together along with the shaft by means of a pressed nut or rivet to the casting, don't think I could disengage this without damage.
  3. The shaft on the pic is wet because of carb cleaner remnants ;) However I do recall of gasoline leaks when I was pumping "via the acc. rod" gasoline into the carb when I was checking the thigtness of all the linkage. The amount of gasoline was sufficient to wetten the gasket under the carb - this leak was strange to me.
  4. Yes, advancing the timing raised rpms but it didn't smooth it out, it was just worse, almost stuttering, either way if I advanced or backed the timing from those +6°. There was no way of smooting out with the idle mixture screws.
  5. Disengaging all the linkage, backing out fully idle screw let the carb in a ghostly manner still running at ~1300 rpms (there are highs and drops of approx. 100) ,
  6. The dashpot is a nos, the original one was stucked, after breaking it free it was leaking. However it is disconnected since it's working conditions are not given (as the manual says, the dashpot should raise the idle rpm to 1030 in order to be fully working),
  7. As I reassembled yesterday the carb, I also had to reconnect the choke warm up tube, going from the housing inside the intake manifold. As far I have even extra lubed with some heavy duty grease the threads of the upper mount/port I can't speak for the tight sit inside the manifold. All I did was spraying lots of carb cleaner onto it and it was tight,
  8. I will look after the sparkplugs again.
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Again thank's for your advice and help!

Artur
 

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1) I bought this car with an already high idle (was about 1.400 rpm if I can still remember),
2) The car had never starting problems, ignition was almost instant,
3) I gave the car to a mechanic (later turned out he wasn't quite trustworthy) because the car had to have replaced all fluids, some minor body work, rebuilding brakes, engine overheated and what was most significant, it just slammed into R and D gear (probably because of those high rpm's),
(the engine was overheating when at traffic jams, but after changing a) spark plugs for new, b) flushing myself all the radiator (there was a lot of butter) the "idiot" light showed up never again.)
There's a good chance that the overheating was at least partially related to the high idle, because even at a stop it's putting the engine under load. It's probably tending to heat up your transmission fluid as well. Of course having "butter" in your cooling system wasn't helping either.

4) The carb was dissassembled (it looked just gross, overgrown with thick layers of oil and mud), so was the intake manifold and top cover of engine (the one between the valve cover, under the intake manifold), all gaskets at that point has been replaced),
5) The day I was going to pick up my car I ran instantly into a problem, for one there was really strange knocking coming from the engine bay (turned out it was the loose distributor cap with the rotor bangnig against it...) and because the mechanic did sth. wrong with the carb adjustment, my exhaust muffler exploaded... I did leave the car to have this repaired at once, and picked it up on the next day. Arriving at home (driving somewhat 25 miles without issues) I ran some diagnostics by myself because I did not want to put any more trust into this sloppy work of my mechanic. So I measured the rpm's on warm engine (they were quite fine, steady at around 750, with the gear in D I could even go down to 600 by backing out slightly the idle screw), checked timing (was at 6° BTDC), checked the vacuum, reading was at around 19/20 and last but not least I looked for the dwell angle and here it happend :eek:
The meter read 14°! I took my alan wrench, opened the little service window at the distributor and rose it to 28,5°. The engine responded to this immediately with higher rpm and greater timing. Cant remember if the vacuum can was disconneted at that time (like I said, I am learning by doing :) but I surely measured at some point the BTDC with the distributor vacuum hooked up.
Your muffler probably exploded because the car was running very rich and was passing a significant amount of unburned fuel into the exhaust where it then accumulated in the muffler. When it got hot enough and had accumulated enough fuel vapor - BOOM!

With a factory points ignition system, changing the dwell angle will also change ignition timing. This happens because when you change the dwell, it also changes the "point" on the distributor cam where the points open, and that's what causes the spark energy to be release from the coil.

This was the point where I have lost completly control over the engine's rpm. Could that be just a coincidence? First I came up with the idea of setting the dwell back to 14° but that did not work anymore. Next I checked all the wiring, in fact some of the spark plugs were loose and I had to tighten them (courtesy of my mechanic) and on cylinder 5 there was a shortcut (everytime I squeezed my hand between the A/C compressor and engine block to reach the spark plug I got shocked :ROFLMAO:. I looked up my distributor, the cap was in my opinion in quite a good shape (the pic is at the beginning of this topic) like it should be for 36k miles, however the rotor showed a little bit of burn at the very end of the metal plate (it was also changed for a new one). Wires were original (dated with 4-Q-65 PACKARD RADIO TVRS LR), they were not brittle, anyways I changed them for new silicon ones from AC DELCO Professional - and it did not have any impact on the engine rpm or anything else. Sparkplugs were, possibly, also factory, AC 45S (picture below, the first two are as they came out from the engine, third one I just cleaned for commparison). The mechanic put in before new ones - AC 45 RS (dunno if that's bad or good). Old orignial are a) points mechanics, b) condenser, c) coil, d) wire coil-distributor, e) vacuum can.
I even readjusted all the levers and float to factory settings - nothing helped.

Maybe you can come up with some new thoughts :p
Here's a "new thought" for you, take a good look at the photos you posted of your carb, and then take a look at these images I found of the bottom of a Rochester 2GV. Notice anything different?

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137927



Yes, that's right - neither image has holes in the throttle plates like your carb does. It sure looks like that some point, "someone" has "messed with" that carb and either drilled those plates and made the holes much larger or changed the plates out completely. Some models of Rochester 2-jets have idle air bleeds in the throttle plates, but they aren't nearly that large. I'm betting that's the source of your problem.

Those holes are what is letting your carb pass too much air and are probably why it still runs and idles too high even with the idle speed screw backed all the way out. When I saw that photo of your carb, I noticed the different colored spot on the throttle shaft in one of the bores which made me suspect that area had been messed with before. Take a flat bladed screwdriver and see if you can remove the screws without having to grind on them or 'force' them past the area where they usually are staked over. If they come out without too much resistance, that's another clue that they've been removed at some point previously.

Are there such things as salvage yards / wrecking yards where you are? It would be worth trying to find a 'donor' carburetor that you could get the plates out of, or perhaps even use the entire throttle body from.

Bear
 

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Discussion Starter #45
Gonna look that up in my manual first, at least I could weld that up and or plug temporary. This carb has also one special feature I did not mention before - a cooling valve atop the wall between both bores, right below the airhorn. As I remember from the manual this was used in accordance to the factory installed a/c. It prevented the fuel from varporization, thus speaking of percolation. How both things are related I dunno but there it is. It is stated that before I am going to fine adjust the idling I should first shut this ventilation valve. However I tried many times to keep it shut but it never changed anything what makes me believe that this valve was just some gadget. I‘ll try to post some pictures of it along with the manual schemes.
 

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Here's the thing. We already know that you've got a problem with not being able to set the idle low enough even with your idle speed screw backed all the way out. The way that screw works, is to control air flow through the carburetor at idle. More air = more rpm, less air = less rpm. That's just how it works. If you can't "slow it down" using that screw, then >it has to be< that the engine is getting air flow from some other source than what passes around the edges of the throttle plates under control of that screw. There's just no other possible cause for an uncontrollably high idle rpm other than too much air flow, so that means we have to start looking for other pathways that the engine can be getting air at idle that can't be controlled by the idle speed screw. The obvious possibilities are a vacuum leak somewhere, poorly fitting/sealing throttle plates, extreme wear around the throttle shaft that's leaking air at the points where it passes through the throttle body, idle air passages that are too large, or (let's hope not) a crack in the intake or in the carb body. if the problem persists with every vacuum connection disconnected and sealed, and you haven't identified a leak around the points where the intake manifold joins to the heads, or around the ends of the throttle shaft, then the air source pretty much has to be somewhere in the carb itself.

Easy test: Pull the carb, put some sticky tape over those holes (on the top side), and try it. Not a permanent fix, but still an easy test.

Also see if you can remove or at least loosen those throttle plate screws without having to "deal with" the staking on the ends opposite the screw heads. If you can, then you know they've been removed before. Even if you can only loosen them a little, it should be enough for you to hold the throttle linkage shut with your hand and then re-tighten the screws. That will make sure they're centered in the bores and sealing as good as they can.

Bear
 

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Discussion Starter #48
Okay guys,
I have come up with sth new.
As for the drills in the throttle plates, they appear to be stock as you can see in the figure below. Probably coworking with the ominous percolation valve due to the stock a/c.
But on the next one figure (underside of the throttle cluster) there's this choke channel, which is (so I think) a direct connection between the choke housing and the lowest level of the carb- just below the throttle plates and needles, thus speaking of short cutting the idle passages. What if the choke housing has a vacuum leak? That would mean that there's a huge amount of air just bypassing the whole carb, isn't it? I always thought that the choke consisted of a spring tension located inside the choke housing, being activated by the heat dissipation of the intake manifold, thus changing the tension of the spring and so on the choke linkage and doing two things 1) opening choke throttle 2) moving the choke/fast idle cam with it's "high steps". Now there appears to be a third function with this channel running on the underside of the carburetor OR maybe it was made for adding a vacuum regulated choke instead of the springy one? However the channel is there and might leak, what do you think?
There still remains question why the leaking after I changed only the dwell 😅
P.s.: My manual unfortunately doesn't cover all basic knowledge (it's a mechanic's book) and sometimes it's hard for me to read only from figures and schemes :)
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137930

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137932
 

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Discussion Starter #49
I might want also add, that the idle screw works on cold to mid warm enigne, and than stops working when it get's hot.
 

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It's quite possible that once you corrected the dwell setting, it "unmasked" this problem that was being hidden by the dwell being so far off. I stand by my previous assertion: if your idle speed screw is ineffecitve, then you have air getting in from somewhere it's not supposed to be. The challenge is finding it.
 

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OK, here we go. I did a bunch of looking on the internet when I too saw those small holes drilled in the throttle blades. That is a "trick" in helping a carb to idle better when you have a big cam and rich carb, so thought the carb had been modified. Turned out that every carb that I looked at advertised as 1966 Pontiac had those same holes. So that is factory.

I told you about the choke housing. Was not sure which ones had the vacuum port or not. Apparently your does - so this can be a possible vacuum leak.

You mentioned that when you had gas squirting into the carb, you saw "wet" on the shaft. This means the throttle shaft is worn out. This will act the same as a vacuum leak.

That "T" thing with the spring found inside the carb is what I told you about, power piston. It sits on top of the "Power Piston" seen in your factory picture. That power piston adds more gas into the carb during wide open throttle. It has a spring inside that is pulled down to close off the piston when the engine has high vacuum. When you put the gas pedal to the floor and loose engine vacuum, the spring raised the power piston which then allows additional gasoline to be channeled into the engine. It basically will richen the air/fuel mixture when the engine needs more power. If it is not working correctly, or bad, it can become open and allow the additional gas into the intake and the engine will run rich.

You have way too many issues going on all at once and you had a bad mechanic. I suggest you begin from the start. The spark plugs look old/bad. So here is what you should do - all new parts.

*Replace all 8 spark plugs. Gap the plugs .032-.035" and no more.
*New distributor cap - brass contacts, not cheap aluminum. Brass provides a better contact.
*New rotor. - brass, not flimsy flat steel contact. These are cheap and junk.
*New wires - which you said you have and installed. Make sure they are correctly placed in the cap - Pontiac firing order goes counter clockwise. What I like to do is put the timing mark on the balancer on "0" and with the rotor installed, place the cap into position, not locked down. Then note where the #1 spark plug wire is on the cap and lift up the cap and confirm that the rotor is pointing at that wire. It might be slightly off, but not much. If the rotor is pointing to the #6 spark plug wire, rotate the engine 1 more turn to bring the balancer mark back around to "0" and then check the rotor/cap firing position again.

Skip down below before installing any distributor parts EXCEPT the rotor. With the distributor cap off, and only the rotor installed, you are going to check how much stretch/wear the timing chain has - which can effect smooth running. Look below on how to do this.

*New set of points - get New Old Stock AC Delco brand from Ebay, or AC Delco from a parts store, or Echlin which is a NAPA store item here in the US.
*New condensor - do not use store bought as many of these are made in China and junk. Get NOS AC Delco from Ebay, or Echlin brand.
You can get points & condensor as 2 individual pieces, or as a 1 piece unit.
*Look at the cam in the distributor that the points rub on. You should have a small tube of grease that gets lightly applied to the points cam to help with wear on the points rubbing block that goes up against the dist. cam. Check to see if they look terribly worn, ie rounded. Move the rotor shaft side to side to see if it has a lot of play. Should not have much. It will go up and down a little, this is OK. Do the advance weights move freely? You can check the vacuum advance canister using a vacuum pump. If you apply vacuum to the canister, it should pull on the plate that the points attach to. You will see the plate move in one direction with vacuum applied, and go the other direction when vacuum is released.
*If all looks good, install points. Gap the points with a feeler gauge, .016" to begin with. Don't worry about the Dwell right now, but it should be 30 degrees.
*Check your wire connections on the coil to make sure they are tight and now worn or frayed. A bad or weak coil can create problems. You can test a coil, but not something I do - I just replace them with new.

CHECKING TIMING CHAIN/GEARS FOR WEAR OR STRETCH.
With only the rotor installed, and the balancer timing mark lined up with the "0", turn the crank to the right by hand BUT watch the rotor for movement. The second the rotor moves, stop. This removes the slack from the timing chain. Note where the balancer mark is. Now by hand, rotate the engine in the opposite direction until you see the rotor move, stop. Look at the balancer mark. The balancer mark should have only moved a little bit, no more than about 6 degrees. You can rotate the engine back again to confirm and stop the second the rotor moves. Again, no more than about 6 degrees. If the balancer timing mark moves more than 6 degrees, the chain/gears are worn out and need replacing.

If this is good, then go back up and continue installing your new points, condensor, cap, wires, etc..

Getting all this correct, will eliminate any ignition problems. Then you can work on the carb problem and begin fresh.
 

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If you need scarce parts, assemblies or even another carb, you wold want to contact the Carburetor Shop in Missouri, I learned about them years ago when I was in AACA and they helped people with obscure parts like carb for 1941 Packard. Jon,the owner, goes by CarbKIng on the PY forum and is always very helpful. Here is the website:


Best of luck with your elusive carb problems. You have good guys helping you like Bear and Jim.
 

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Discussion Starter #53
Thank you very much,
I have recently looked up Mike’s Carb site for gaskets but I will check out yours too. I’ve noticed that getting nos parts like linkage could be quite a challenge. Speaking of replacement parts,
would you suggest any brands for the engine gaskets? I’ve got minor oil leaks at front and back axle, think those are simmerings leaking.
Also I’d like to replace the suspension bushing (getting brittle) and I have minor problems with a) speedometer- it does not move for the first several miles, following some sticky movement from 30 mph up and like after driving another 5 miles it starts working fully until the next day... it freezes again. The odometer works without issues b) alternator does have sometimes problems with sufficient charging, would it help to replace it with some higher amps?

I wan’t to underline that I am trying to stay with this car as factory stock as possible. Lately I have even redone with my best possible accuracy the writing on the aircleaner housing (on the internet there are only cheap stickers) unlike the factory printing.
Take a look if you like it:)
137972


After repainting (via tampon printing)
137973


Stay well!
Artur
 

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Discussion Starter #54
Hello mates !

It seems, that I have solved my problem with the engines rpm's. I have replaced my oem ignition coil and bought a second rpm reader, this time I took the timing pro light from Innova. As soon I hooked up the new timing light along with my old car meter it became clear that there was some technical issue on the meters themselves. When the old meter showed on hot idle around 1 100 rpms the Innova one told me 732 +/- 5 :D Believing that Innova is working properly, I must have kept detuning my engine all the time with the old meter and was way below 500 rpms back than, what caused all the engine hesitation and rough idle.
However I still have the issue with the jerking tranny. That is, as soon I put either Drive or Reverse, the whole car jerks into it. At this point I'd say that the fault lies inside the tranny and not the engine anymore. I can idle now at as low as steady 550 rpm (Innova) and still reach the 19+ inches of mercury of vacuum.
Do you have any clue what else I could do to prevent the jerking? Shifting trough the gears whilst driving is all good and soft.

Thanks,

Maczuga
 

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Hello mates !

It seems, that I have solved my problem with the engines rpm's. I have replaced my oem ignition coil and bought a second rpm reader, this time I took the timing pro light from Innova. As soon I hooked up the new timing light along with my old car meter it became clear that there was some technical issue on the meters themselves. When the old meter showed on hot idle around 1 100 rpms the Innova one told me 732 +/- 5 :D Believing that Innova is working properly, I must have kept detuning my engine all the time with the old meter and was way below 500 rpms back than, what caused all the engine hesitation and rough idle.
However I still have the issue with the jerking tranny. That is, as soon I put either Drive or Reverse, the whole car jerks into it. At this point I'd say that the fault lies inside the tranny and not the engine anymore. I can idle now at as low as steady 550 rpm (Innova) and still reach the 19+ inches of mercury of vacuum.
Do you have any clue what else I could do to prevent the jerking? Shifting trough the gears whilst driving is all good and soft.

Thanks,

Maczuga
Glad you got that solved.

The jerking could be normal, or it could be that a shift improver kit has been installed in the transmission. Make sure all your U-joints are good and check the play in the rear end - pinion gear/ring gear may have a lot of slop due to age & wear.

I don't know if a vacuum line, or the vacuum modulator could be suspect - I would check these. I believe there is a screw adjustment on the modulator and you can do a web search and see if this does anything to soften the trans.

Found this:

"One may also ask, what are the symptoms of a bad vacuum modulator? When you have a bad modulator valve, some or all of the following symptoms will begin to manifest themselves:
  • A whistling sound (from a leaking diaphragm)
  • White smoke coming out of the exhaust (from a leaking diaphragm)
  • Early or late shifting.
  • Hard shifts (usually causing the car to jerk)
  • A rough idle."


Hopefully others more qualified will provide some checks you can do.
 

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Discussion Starter #56
Hmm, as far I understand the modulator can only change the shifting points of the gears 1-3. I have a friend who’s got a 1968 Bonneville with the 400 cu in engine and I believe also TH400 tranny. His car goes into D/R almost unnoticable.
Do you think it would be safe for the engine to go further down with the idle rpm - below manufacturer specification to lower the vacuum?
I am at 650 rpms atm with tranny in Park, A/C still disconnected (no belt).
 

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I'm in the "u-joint suspect" theory until checked-off. With car off and in-park, get under it and try to rotate the driveshaft (both directions). Rotation/play should be minimal. If not your u-joints/and or differential gears need attention.
 
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