When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I could move the hose for the booster, but not without changing the fitting on the intake. So here's what I'm gonna do...I'm going to check vacuum where the brake vacuum hose attaches to the carb, check vacuum where the pcv hose attaches to the carb (if necessary) , check vacuum at the hose for the vacuum canister at the firewall, check the vacuum at the hose coming up from the transmission modulator. If any of these are not necessary, let me know 😀
How about simply remove those hoses one at a time and cap the carb vacuum ports to see if there is any significant change in idle quality. Or if you have hose pliers. just pinch the hoses to block them off.
Last edited by Fun71; November 20th, 2021 at 07:56 AM.
How about simply remove those hoses one at a time and cap the carb vacuum ports to see if there is any significant change in idle quality. Or if you have hose pliers. just pinch the hoses to block them off.
I'm following your statements but maybe the manner of your written statements differ slightly than what you are thinking maybe. At any rate, you want to check the vacuum of the "sourced" vacuum ports. Example, you don't want to check vacuum at the hose coming up "from" the transmission modulator, you want to instead check the vacuum from the port which is "sourced" from the intake manifold "to" the transmission modulator since the transmission modulator is dependent on the "sourced" vacuum port and not visa versa. You're basically checking all the "sourced" vacuum ports and of course the vacuum hoses for each of those sourced ports. I think you know what I'm talking about. You can also use a "T" in the line at various places with your vacuum gauge attached to the "T", which is what I most frequently do, but then again I don't have the erratic swinging vacuum needle as you do. So, check the sourced vacuum ports and those vacuum hoses in the manner you've proposed. Well, any time you remove a vacuum hose (example vacuum advance canister) you have to cap the line from the device (vacuum advance canister) and the source (if you're not measuring that specific source). You're measuring one vacuum source at a time, all other items must be capped.
So basically I just need to check vacuum at all ports,, whether on the carb or at the intake, and then check hoses going to said ports, correct?
Correct. Once you can validate (hopefully) you have a steady needle (hopefully) pulling a healthy vacuum, you'll need to check suspected hoses which require a source of vacuum to demonstrate a particular hose doesn't have a leak and causing the vacuum issue. You got this.
How about simply remove those hoses one at a time and cap the carb vacuum ports to see if there is any significant change in idle quality. Or if you have hose pliers. just pinch the hoses to block them off.
Ok..now I'm confused...why would I need to cap the carb ports? Seems to me if I leave the hoses to the ports connected, I wouldn't need to cap them, correct?
Not being there and not really knowing anything about the motor it's hard to diagnose because alot of things can cause this condition. You did say that you had a 770 Holley. The meeting blocks have the idle restriction high on the well. Also from the video you posted it seems to run rough. This position can cause an erratic idle especially if the fuel level is to high in the bowl. Add the fact it runs rough can make a high fuel level more sensitive. I would look at you float adjustment first. You can have a bad adjustment or a bad needle and seat if it's too high. Your bouncing vacuum needle can also be from connecting it to 1 port (runner) and not the plenum and adding a rough idle makes it worse. Out of curiosity what cam is in the motor and what CR?
Engine runs smooth and all I know about the cam is that it's a W30.
Ok..now I'm confused...why would I need to cap the carb ports? Seems to me if I leave the hoses to the ports connected, I wouldn't need to cap them, correct?
Remove the hoses from the carb one at a time and cap the carb ports.
The erratic swinging of the needle is perplexing/troubling - you shouldn't have any erratic swinging of the vacuum needle at idle, it should be steady. The fact you have no misfire throughout the engine operating range does not lend itself to weak/broken valve springs, but the fact the needle swings violently at idle still remains suspicious.
Based upon results of this last test - immediately climbs to 20"Hg upon acceleration and remains there throughout the acceleration range until you go back to idle with erratic vacuum needle swings at idle, I'd like to see if you can isolate (find) a steady vacuum needle reading at idle. It's possible you have a vacuum leak at idle in at least one vacuum hose at idle. If you're able to demonstrate a steady needle @ 20"Hg upon beginning that test and throughout the RPM range, it sounds like you're pulling great vacuum. I noted you stated you have no misfiring within any RPM range, so it isn't pointing to weak valve springs.
Two things I'd look for:
(1) Check and recheck each of your vacuum hoses for any hose which is loose and any hose you suspect demonstrates cracking/wear. I think you may simply be losing vacuum at idle with no effect once you begin to accelerate; and,
(2) Try the brake booster intake manifold vacuum port as your vacuum source port. That manifold port should be rock steady on the vacuum gauge. If you can't get a steady vacuum needle on that port, I'd once again review each of your vacuum hoses for integrity of cracks and loose fittings.
Ok Norm, here are the results:
Brake booster port was 12-14 steady
Brake booster hose was 12-14 steady
PCV port was 12-13 steady
PCV hose was 12-13 steady
I did notice a cap was cracked and dry rotted, so I replaced that. Other than that, everything else (including the hoses) looked good. After I replaced the cap, I checked it at the manifold and it was the same as the first video I posted. All of the above was done with the distributor advance disconnected and capped.
Finally, I reconnected the vacuum advance and checked it at the manifold, and this is what I got...
Well, two things:
(1) You have FAR more vacuum than in your first video (based solely upon your video). You are now well w/in the normal range of vacuum as opposed to your first video where you were far below normal vacuum and FAR more erratic in your reading (based upon your videos);
(2) You don't need to remove the vacuum line from the vacuum advance canister and cap it (plus the vacuum port) - you're simply measuring vacuum.
But, since you did (which might have been the next step), I'd suggest (now that you see a normal vacuum operating range but significant erratic needle swings (based solely upon your videos) - that needle should be steady, yours is not, cap off the vacuum advance canister directly at the vacuum canister itself - not the vacuum hose attached to the vacuum advance canister. You want to remove any suspect in that line leading to the vacuum advance canister. Then, measure vacuum directly from the port you use for the vacuum advance canister. Make certain the vacuum canister is well capped off and sealed. Then, do the same procedure with the hose the vacuum canister normally uses, then with a brand new hose or a hose from some other application as a known good source of hose. You have an erratic vacuum manifestation - it may be the vacuum canister itself has gone South on you.
You still most likely have a leak somewhere. I can't say where, you're doing the tests; your video demonstrates a very erratic swing in vacuum pressure but within range; yet, you stated (above) those steady vacuum numbers (12"Hg - 13/14"Hg) are below normal range. I'd have also checked the the ATM (automatic transmission modulator) hose and the port it is attached to. You're getting there.
Review several videos regarding a steady vacuum gauge - there are a plethora of videos and all will demonstrate a good steady vacuum on the gauge. I see you making two statements: You get steady vacuum needle (12"Hg - 14"Hg) which is below normal range; and, significant vacuum needle erratic swinging with a normal range. Just have a look at this one video demonstrating a steady vacuum and the snap of the accelerator and vacuum gauge movement.
Well, two things:
(1) You have FAR more vacuum than in your first video (based solely upon your video). You are now well w/in the normal range of vacuum as opposed to your first video where you were far below normal vacuum and FAR more erratic in your reading (based upon your videos);
(2) You don't need to remove the vacuum line from the vacuum advance canister and cap it (plus the vacuum port) - you're simply measuring vacuum.
But, since you did (which might have been the next step), I'd suggest (now that you see a normal vacuum operating range but significant erratic needle swings (based solely upon your videos) - that needle should be steady, yours is not, cap off the vacuum advance canister directly at the vacuum canister itself - not the vacuum hose attached to the vacuum advance canister. You want to remove any suspect in that line leading to the vacuum advance canister. Then, measure vacuum directly from the port you use for the vacuum advance canister. Make certain the vacuum canister is well capped off and sealed. Then, do the same procedure with the hose the vacuum canister normally uses, then with a brand new hose or a hose from some other application as a known good source of hose. You have an erratic vacuum manifestation - it may be the vacuum canister itself has gone South on you.
You still most likely have a leak somewhere. I can't say where, you're doing the tests; your video demonstrates a very erratic swing in vacuum pressure but within range; yet, you stated (above) those steady vacuum numbers (12"Hg - 13/14"Hg) are below normal range. I'd have also checked the the ATM (automatic transmission modulator) hose and the port it is attached to. You're getting there.
Review several videos regarding a steady vacuum gauge - there are a plethora of videos and all will demonstrate a good steady vacuum on the gauge. I see you making two statements: You get steady vacuum needle (12"Hg - 14"Hg) which is below normal range; and, significant vacuum needle erratic swinging with a normal range. Just have a look at this one video demonstrating a steady vacuum and the snap of the accelerator and vacuum gauge movement.
Good video Norm...thanks.
So after this thread has taken so many different directions, I felt it was time to give you a quick summary:
1) The first video was directly from the intake with the distributor advance hose disconnected and capped.
2) These reading were taken at the carb port where the distributor advance connects, and also atthe other end of the hose coming from the distributor advance, respectively:
3) The last video I posted was directly at the intake with the hose from the distributor advance connected to the port on the carb.
He also said the engine has a W-30 cam, so if it is a large overlap version that 12-14" may be normal.
Completely agree - it is certainly possible. Yet, I seriously wouldn't suspect that much erratic swinging behavior but I might be wrong on this. At any rate, I don't he has a significant issue - just a troublesome/worrisome erratic vacuum at idle. Maybe too much advance? What causes that much variation of erratic swing? What about the images he posted with less than 10"Hg - I'm not sure those could even be near correct. What, did the camera capture them both at the very lowest point of Hg"?
Dave - I can't tell you specifically where your issue resides regarding your vacuum measurements. There exists some peculiarity which is not normal. The two static images you posted directly above in Post #52 are SIGNIFICANTLY below normal. I have difficulties imagining you captured those still images from your vacuum gauge based upon your two videos.
As you stated in your first post, you're not overly concerned. Based upon what you stated earlier - the engine runs great at cruising speed with no misfires but this issue only arises at idle with the engine warm, I'd suggest you don't have any significant issue which requires immediate attention. Never-the-less, something is obviously causing wild swings in vacuum gauge readings - at least from I can surmise this occurs only idle.
Earlier you stated "...vacuum went to 20, and stayed there all the way to 2500..." I have yet to witness that. Everything I've seen would demonstrate the contrary; so, I have to take your word for it.
Do you have weak valve guides, maybe? I don't know. Significantly weak valve springs? I suspect not if you have no misfires. The fact you can snap the throttle, vacuum lowers to near 5"Hg, but returns to 20"Hg and remains there at what a steady state? I can't figure any of this out, since I've seen two videos with extremely erratic vacuum needle behavior.
You still most likely have a leak somewhere. I can't say where, you're doing the tests; your video demonstrates a very erratic swing in vacuum pressure but within range; yet, you stated (above) those steady vacuum numbers (12"Hg - 13/14"Hg) are below normal range. I'd have also checked the the ATM (automatic transmission modulator) hose and the port it is attached to. You're getting there.
Norm, the hose from the modulator ties into this canister on the firewall and then tees off to the intake.
That hose is the one I disconnected to check vacuum at the intake. So if that hose is disconnected, and I'm checking the source of vacuum for that hose, then the erratic reading cannot come from the transmission modulator hose...savvy?
Well, Dave - I've never seen an ATM (automatic transmission modulator) tied into an AC vacuum canister with a tee in it. At least, not in anything I recall having been witness to.
Dave - I can't tell you specifically where your issue resides regarding your vacuum measurements. There exists some peculiarity which is not normal. The two static images you posted directly above in Post #52 are SIGNIFICANTLY below normal. I have difficulties imagining you captured those still images from your vacuum gauge based upon your two videos.
As you stated in your first post, you're not overly concerned. Based upon what you stated earlier - the engine runs great at cruising speed with no misfires but this issue only arises at idle with the engine warm, I'd suggest you don't have any significant issue which requires immediate attention. Never-the-less, something is obviously causing wild swings in vacuum gauge readings - at least from I can surmise this occurs only idle.
Earlier you stated "...vacuum went to 20, and stayed there all the way to 2500..." I have yet to witness that. Everything I've seen would demonstrate the contrary; so, I have to take your word for it.
Do you have weak valve guides, maybe? I don't know. Significantly weak valve springs? I suspect not if you have no misfires. The fact you can snap the throttle, vacuum lowers to near 5"Hg, but returns to 20"Hg and remains there at what a steady state? I can't figure any of this out, since I've seen two videos with extremely erratic vacuum needle behavior.
I really am left clueless with where to go next.
Norm, here's the second video of when I took it to 2500 rpm. You'll notice that before I start, it's indicating as in the first video, but as soon as I move the throttle, it jumps right to 20 and stays there until I back off, then it (for a split second) drops even lower to 25ish, then back to where I started.
(side note, I know there's that screeching in the background, but that's not for discussion at this point).
Dave - Here is a picture of the Vacuum Tank Assembly on the firewall as depicted in the 1972 PIM. I don't see the vacuum tank you posted as installed - it appears to be sitting on a bench. Is that the vacuum tank assembly you have in your car? If so, note the Vacuum Tank Assembly in the 1972 PIM (images below). Notice anything different?
OK Think I got it. I thought your car had A/C. Evidently it does not, as I'm familiar with my own vehicle which does have A/C and a different Vacuum Tank. My mistake.
Dave - Here is a picture of the Vacuum Tank Assembly on the firewall as depicted in the 1972 PIM. I don't see the vacuum tank you posted as installed - it appears to be sitting on a bench. Is that the vacuum tank assembly you have in your car? If so, note the Vacuum Tank Assembly in the 1972 PIM (images below). Notice anything different?
Here's my setup Norm...notice the hose coming up from underneath goes into a tee which then splits off to the AC canister and the other hose goes to the intake.
I have to look more closely at the 1972 PIM, I'm looking at the paper copy of my own 1971 PIM for a vehicle without A/C, but I'm not seeing the Tee you're referring to as being connected to the transmission modulator line. There is a Tee off the vacuum tank - one hose goes to a intake manifold port and the other goes somewhere to the left-hand side of the engine nowhere near the transmission modulate line as best I can see at this point.
OK Think I got it. I thought your car had A/C. Evidently it does not, as I'm familiar with my own vehicle which does have A/C and a different Vacuum Tank. My mistake.
It was originally an AC car, but the compressor is gone...does that make a difference? Also, I posted a video of the 2500 rpm test.
So are you saying these vacuum readings were taken from PORTED vacuum sources? If so, it is normal for there to be little/no vacuum from those sources at idle.
Originally Posted by 72455
2) These reading were taken at the carb port where the distributor advance connects, and also atthe other end of the hose coming from the distributor advance, respectively:
It was originally an AC car, but the compressor is gone...does that make a difference? Also, I posted a video of the 2500 rpm test.
Sorry, yes I did see the 2500 RPM test. Once you deliver RPMs (acceleration) the gauge goes to 20"Hg and is "rather" steady. The fact it still widely fluctuates is not correct and is not normal.
OK, the landscape changes somewhere here. You have a car which originally had A/C. You said the compressor is gone. Is that all that is gone? Look, I'm no expert on non-A/C cars, I know my own routing quite well since I rebuilt my entire A/C system from compressor through evaporator core, dryer, etc., etc. But, with that said, I am reviewing both a 1971 and a 1972 PIM, there is (I'm not certain what significance it matters) a difference in the routing and the devices contained within an A/C equipped car and a non-A/C equipped car. Currently, I'm trying to determine exactly what that difference is and how it may relate to vacuum hose routing. As an example, there are two different styles/types of firewall vacuum containers - one for non-A/C and one for A/C. The vacuum hose routing is different between the two cars - the non-A/C car traversing a transducer while the A/C car does not traverse a transducer the best I can ascertain at this point. Therefore, to answer your question - "...does it matter if a car originally equipped with AC which has the compressor disconnected makes any difference in vacuum readings (and I would suspect vacuum hose routings) as opposed to a car which never had AC. My initial answer is to suggest yes, but I do not know. You have vacuum hoses which would originally have been connected to an AC vacuum canister mounted on the firewall with two distinct ports - you have a vacuum canister mounted on the firewall with one port and a Tee which leads to a transducer. This is an area where I have no familiarity. I'm questioning why the original vacuum tank on the firewall was removed and a vacuum tank on the firewall was replaced with a non-AC vacuum tank and then determining how/if this effects idle vacuum readings. Does your car even have a transducer for routing of the non-AC type vacuum tank? As I said, this is an entire learning experience for me in attempting to determine how a once AC equipped vehicle with a non-AC equipped vacuum tank relates to vacuum hose routing and the effects of a transducer (if in fact your car has one). Not my typical Saturday afternoon.
Last edited by Vintage Chief; November 20th, 2021 at 01:01 PM.
So are you saying these vacuum readings were taken from PORTED vacuum sources? If so, it is normal for there to be little/no vacuum from those sources at idle.
If the port on the upper right hand side of the front float bowl is considered "ported", then yes, that is correct.
Sorry, yes I did see the 2500 RPM test. Once you deliver RPMs (acceleration) the gauge goes to 20"Hg and is "rather" steady. The fact it still widely fluctuates is not correct and is not normal.
OK, the landscape changes somewhere here. You have a car which originally had A/C. You said the compressor is gone. Is that all that is gone? Look, I'm no expert on non-A/C cars, I know my own routing quite well since I rebuilt my entire A/C system from compressor through evaporator core, dryer, etc., etc. But, with that said, I am reviewing both a 1971 and a 1972 PIM, there is (I'm not certain what significance it matters) a difference in the routing and the devices contained within an A/C equipped car and a non-A/C equipped car. Currently, I'm trying to determine exactly what that difference is and how it may relate to vacuum hose routing. As an example, there are two different styles/types of firewall vacuum containers - one for non-A/C and one for A/C. The vacuum hose routing is different between the two cars - the non-A/C car traversing a transducer while the A/C car does not traverse a transducer the best I can ascertain at this point. Therefore, to answer your question - "...does it matter if a car originally equipped with AC which has the compressor disconnected makes any difference in vacuum readings (and I would suspect vacuum hose routings) as opposed to a car which never had AC. My initial answer is to suggest yes, but I do not know. You have vacuum hoses which would originally have been connected to an AC vacuum canister mounted on the firewall with two distinct ports - you have a vacuum canister mounted on the firewall with one port and a Tee which leads to a transducer. This is an area where I have no familiarity. I'm questioning why the original vacuum tank on the firewall was removed and a vacuum tank on the firewall was replaced with a non-AC vacuum tank and then determining how/if this effects idle vacuum readings. Does your car even have a transducer for routing of the non-AC type vacuum tank? As I said, this is an entire learning experience for me in attempting to determine how a once AC equipped vehicle with a non-AC equipped vacuum tank relates to vacuum hose routing and the effects of a transducer (if in fact your car has one). Not my typical Saturday afternoon.
Norm, sorry my first pic didn't depict more, but here's a better shot...2 ported.
Well, this landscape is evolving then. I know for a fact my automatic transmission modulator vacuum line is connected directly to an intake manifold vacuum source on the front of my engine. There is no "Tee" located anywhere within my automatic transmission modulator vacuum hose routing. I will add my engine is the 350 cid with a THM 350 transmission. I believe your engine is a 455 cid with a THM 400 transmission. Your transmission might demonstrate employment of a transducer with a "Tee" off the firewall vacuum tank and one line leading to a manifold vacuum source and another leading to the transducer. I'll take another look at the PIM after dinner.
Well, this landscape is evolving then. I know for a fact my automatic transmission modulator vacuum line is connected directly to an intake manifold vacuum source on the front of my engine. There is no "Tee" located anywhere within my automatic transmission modulator vacuum hose routing. I will add my engine is the 350 cid with a THM 350 transmission. I believe your engine is a 455 cid with a THM 400 transmission. Your transmission might demonstrate employment of a transducer with a "Tee" off the firewall vacuum tank and one line leading to a manifold vacuum source and another leading to the transducer. I'll take another look at the PIM after dinner.
Ok Norm, and yes, I do have a TH400. While you're getting some nourishment, ponder this...1) Since I don't have an active A/C system, would I need that Tee, and 2) Would removing said tee improve, hurt, or not change my current situation?
Oh, and I sure appreciate all of the input/help/advice 😀
Dave - Somethings I recall about some folks vehicles better than I do others. I can note several aspects about your engine better only because I've read most of your threads about your engine since you became a member. I didn't know you have the AC compressor disconnected, but I will note you to do not have the DVCS installed on your car. By the time 1972 arrived, I am not certain exactly what the actual name is for your T.C.S. (as has been previously described by rocketraider; see Post #8 & Post #10). This device has received several iterations of various nomenclature but all in all the device(s) comprise what is known as the T.C.S. (Transmission Controlled Spark) system. Again, I noted you do not have this device installed on your car. That fact alone, dictates a PO has made changes to your vacuum hose routing to accommodate removal of the DVCS/DTVS device. Whether they accomplished this re-routing of the vacuum hoses appropriately has yet to be determined; but, you can bet they made changes to the vacuum hose routing to accommodate this removal. I think we're most likely headed down the path of determining if your vacuum hose routing is adequate to accommodate your engine.
Dave - Somethings I recall about some folks vehicles better than I do others. I can note several aspects about your engine better only because I've read most of your threads about your engine since you became a member. I didn't know you have the AC compressor disconnected, but I will note you to do not have the DVCS installed on your car. By the time 1972 arrived, I am not certain exactly what the actual name is for your T.C.S. (as has been previously described by rocketraider; see Post #8 & Post #10). This device has received several iterations of various nomenclature but all in all the device(s) comprise what is known as the T.C.S. (Transmission Controlled Spark) system. Again, I noted you do not have this device installed on your car. That fact alone, dictates a PO has made changes to your vacuum hose routing to accommodate removal of the DVCS/DTVS device. Whether they accomplished this re-routing of the vacuum hoses appropriately has yet to be determined; but, you can bet they made changes to the vacuum hose routing to accommodate this removal. I think we're most likely headed down the path of determining if your vacuum hose routing is adequate to accommodate your engine.
Gotcha...and it's not a complicated system. Four hoses ...one for the vacuum advance, one for the brake booster, one for the PCV, and one for the transmission. So in the event we start clearing things up, the question still remains: Why the erratic reading when connected directly to the manifold and is that the cause of the idle fluctuating?
Gotcha...and it's not a complicated system. Four hoses ...one for the vacuum advance, one for the brake booster, one for the PCV, and one for the transmission. So in the event we start clearing things up, the question still remains: Why the erratic reading when connected directly to the manifold and is that the cause of the idle fluctuating?
Dave - How about we move forward with just a basic understanding of OEM carburetor configuration for your car and it's associated vacuum routing configuration and then move onto your own carburetor configuration and it's associated vacuum routing configuration. Let's first establish some common ground we can work from. Our discussion won't even involve the purpose and design features of the T.C.S. system - no worries; you don't have one so let's not discuss it (unless provoked). But, we do need to examine what was removed (the DVCS/DTVS [T.C.S]) - lets' call this device the "XMas Tree" (since it resembles a christmas tree). I am operating from my 1971 CSM, you will be operating from your 1972 CSM. Firstly, you must establish the location of the following diagram in your 1972 CSM. The images are contained in Section 6M-0 - Carburetion. I recognize you have a Holley, don't worry the nit-picky stuff for the moment, just find the diagrams. I will comment on one in particular (below). Please NOTE: Four vacuum hoses are connected to the Xmas tree in the OEM configuration. You don't have an Xmas tree, therefore what do we do to reconfigure those vacuum hose routes when the Xmas has been removed - you can't just lay them around the top of the intake manifold. NOTE: My diagram states for ALL 455 cu. in. A-Body - that should be your vehicle and it most likely states the same in your CSM. Again, we're interested in knowing what this vacuum routing looked like before (OEM configuration) and what it looks like now (in your car) and if you car's vacuum hose routing is adequate to accommodate the Xmas removal.
NOTE: Nowhere in the depicted images is there a TEE which integrates into a firewall vacuum tank. NOTE: In my Post #61 the vacuum tank assembly obtains its vacuum from a dedicated intake manifold port (review the images and find the elbow - identified in the image "See Section 6-1 for Elbow") located on the rear of the intake manifold. That is where the vacuum tank obtains its sourced vacuum. NOTE: There is no TEE integrated into the Automatic Transmission Modulator (ATM) line. Don't worry if you have no intake manifold port available for the time being, let's just stick what is the known OEM configuration so we can establish a baseline here.
With that information alone, I'll mention one thing only at this point with regards to accommodating a Xmas tree removal. Notice the TEE in the above diagram (Post #73), that TEE provides a branch to the Xmas tree. We're going to remove that TEE and plug the ATM (MODULATOR) directly into an intake manifold source. That's it for now, that TEE is being removed, the MODULATOR will go directly to an intake manifold source thereby eliminating the TEE and the branch to the deleted Xmas tree. It should follow what we're going to do next is establish the vacuum hose routing for the remaining vacuum hoses which will not longer be connected to the Xmas tree from the distributor and the carburetor.
I tossed up (perhaps) an incorrect image (above) which states a 2bbl configuration, but I wanted to demonstrate the vacuum hose routing without a more detailed discussion of the remaining vacuum hoses. I will provide that discussion moving forward with an image I used from my own 1971 CSM with labels which will illustrate your 455 with my own handwritten statements (which BTW, follow the nomenclature in my 1971 CSM with regard to port identifications located on page 6K-9. Sorry about an earlier error on CSM Section - the pictures are in the ENGINE COOLING section pages 6K10-6K-12. Let's not worry about pages numbers, hopefully you've found the pages. Here is the image we'll work from.
(1) We need to eliminate the vacuum hose at the bottom of the Xmas tree. That's easy enough, just remove it and plug the ATM modulator into the intake manifold.
(2) We can easily eliminate the VENT PORT "V" located at the top of the Xmas tree since it's only a fresh air vent port coming from the top of the carburetor. So, we can cap that vent port on the backside of the top of the carburetor.
(3) We need to a find ported vacuum source for the distributor vacuum advance canister. Since this is a ported vacuum (as opposed to a full manifold vacuum) we can use the port which demonstrates the vacuum hose leading from the front driver side of the carburetor (just above the idle mixture screw) leading to the top of the Xmas tree. So, we can remove that piece of vacuum hose on the top of the Xmas tree and then plug the distributor vacuum advance canister into that ported vacuum source on the carburetor.
REVIEW: We've completely removed three vacuum hoses (the TEE vacuum hose off the ATM MODULATOR [full intake manifold vacuum]; the vacuum hose on the top of the Xmas tree has been removed from its carburetor ported source of vacuum; and, the VENT PORT V hose has been removed from the carburetor port on the back of the carburetor [& the carburetor port capped] to the top of the Xmas tree (no longer required). The distributor vacuum advance canister vacuum hose is now plugged into a carburetor ported vacuum source and we routed the ATM MODULATOR line into an intake manifold full vacuum source.
Last edited by Vintage Chief; November 20th, 2021 at 04:12 PM.
Since I do not know the availability of your ported vacuum sources and your full intake manifold vacuum sources, you'll have to guide the next part of the discussion with your knowledge of those ports and their availability. I suggest you review your Holley carburetor ports, but if you tell me the model number of your Holley, I can find a diagram to help with the routing, as well.
So, in essence you'll need a full vacuum source for your ATM MODULATOR. The ATM MODULATOR vacuum hose is connected to a full intake manifold vacuum source; and, the distributor vacuum advance vacuum hose is connected to a Holley carburetor ported vacuum source and the vacuum tank on the firewall should be connected to a full intake manifold vacuum source. NOTE: Full intake manifold vacuum sources can be located anywhere below the throttle plates of the Holley carburetor. The reason they are located there is because the throttle plates provide no interference\influence with full intake manifold vacuum source because the port is located below the carburetor throttle plates & directly over the intake manifold. Anything above the carburetor throttle plates is considered a ported vacuum source.
So, Dave at this point I have my suspicion the TEE you see in the vacuum tank mounted on the firewall is in appropriate for your engine vacuum hose routing. That TEE should be removed and the firewall vacuum tank should be sourced directly to an intake manifold full vacuum source with the ATM MODULATOR connected directly to a full intake manifold vacuum source. If I think about this a second, I don't know if it's even needed at this point since you have no running AC. None-the-less, if you're going to have a vacuum hose connected to the firewall mounted vacuum tank it needs to be a dedicated vacuum hose connected directly to a full vacuum source.
Gotcha...and it's not a complicated system. Four hoses ...one for the vacuum advance, one for the brake booster, one for the PCV, and one for the transmission.
You also need one for the vacuum ball that controls the HVAC. Even though your AC compressor is disconnected, you still need a manifold vacuum source to control which vents the air comes out of - dash, floor, or defrost. With no vacuum source, the air will come out of the floor vents only.
Last edited by Fun71; November 20th, 2021 at 04:38 PM.
(3) We need to a find ported vacuum source for the distributor vacuum advance canister.
I would say let's check and see which vacuum source works best. On my engine, full manifold vacuum resulted in higher idle vacuum (power brakes worked better) and a smoother idle. On ported vacuum, the idle was rough, the vacuum was bouncy, and the power brakes were so-so with the vehicle stopped.
Last edited by Fun71; November 20th, 2021 at 04:39 PM.
Gotcha...and it's not a complicated system. Four hoses ...one for the vacuum advance, one for the brake booster, one for the PCV, and one for the transmission. So in the event we start clearing things up, the question still remains: Why the erratic reading when connected directly to the manifold and is that the cause of the idle fluctuating?
Dave - If I knew the answer to that question we wouldn't be troubleshooting your scenario. Baby steps, baby steps. I have no ideas at this point in time why the ATM MODULATOR vacuum hose has a TEE into your firewall AC vacuum tank.
November 20th, 2021, 07:54 AM
