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Sorry for the long read but want to share everything I have done in hopes it helps the experts to tell me what to do.
1972 Olds 350, Stock everything. Compression checks were 153-155 across all 8 cylinders. New plugs gapped to 0.040. Cap/Rotor/Plug Wires/Points look good.
Measured dwell - was 25 and I adjusted to 30 per spec
Measured base timing - was 14 adjusted to 12.
Measuring manifold vacuum from port on manifold - adjusted A/F screws for max vacuum. Achieved 18.
All that looks good but here is what I dont think is quite right.
Choke doesnt seem to work right. I watched several youtube videos and mine doesnt seem to work the same. I tried turning the black cover back and forth feeling for resistance with the choke flap closed but never seem to feel anything significant to me. The choke seem to moved very freely vs videos showing a springness closing them? The choke assembly has a vacuum line from the top of the intake manifold into it and an exit line going to the back/top of the carb. Is black cover for the choke intended to be sealed so it isnt a vacuum leak? Do I need to take the cover off and check for a proper seal?
Now on to the more important part. To get the highest vacuum reading during AF adjustment - I started at 3 1/2 turns out and ended up at 6 turns out. Is this reasonable amount of turns out or does this mean it is dirty? At the highest vacuum reading the idle is around 1000 rpm and cant be adjusted lower because the idle screw is backed all the way out(not touching). Shouldnt I be able to set idle lower? If so why would it be 1000 rpm with idle screw backed out all the way? AF is set wrong?
When I throw car in gear rpm drops to around 700 rpm and idles decent. When I throw it in neutral or back to park it acts like it is loaded up slightly as it is slightly rough until you hit the throttle to clean it up and it idles smoother. It isnt real rough but isnt perfectly clean. There is some slight roughness to me but maybe I am too used to modern cars and it wont be as smooth as fuel injected cars of today.
Evaluate your fast idle cam setting. You have the CSM? Note the fast idle cam setting and ensure the fast idle cam is operating smoothly and adjusted correctly.
Take the choke cover off and verify there is a gasket in place. This will also allow you to see how the thermostatic coil interacts with the choke mechanism.
Evaluate your fast idle cam setting. You have the CSM? Note the fast idle cam setting and ensure the fast idle cam is operating smoothly and adjusted correctly.
Fast Idle is 1100 rpm. Are you saying this will fix everything I wrote or is this addressing the first part? Any feedback for the second part of what I wrote when its warmed up and doing what I said.
Qjets metering systems are complex & overlap. If you have to back the screws out 6 turns, that’s pretty extreme rich. This suggests you’re off base somewhere more basic. Dig around under my user name for more on qjets. I claim no expertise, but maybe something I’ve written can help you.
More typical for idle screws is 1.5-2.5 turns out from the “lightly bottomed / aka all the way in” starting point. This depends on whether you’re using aftermarket brass or factory steel Idle screws.
Are you working on an early ‘70 series or later 170 series carb? What is your primary rod/jet combination? I know big blocks better, but the general advice is start at the stock rod/jet combination (stock secondary rod/hangers too) and experiment from there.
If you’re really in learning mode, I can recommend a wideband oxygen (02) sensor to see AFR (air/fuel ratio) or Lambda (native language of 02 sensors) in realtime. Then you can see in realtime (aka in-use-on-the-road) wether your carb is lean/rich or on target in any of its stages: idle, part-throttle/cruise, or WOT.
Carb tuning is time consuming, but rewarding when you get near optimal. You can really feel the difference once you get close.
Qjets metering systems are complex & overlap. If you have to back the screws out 6 turns, that’s pretty extreme rich. This suggests you’re off base somewhere more basic. Dig around under my user name for more on qjets. I claim no expertise, but maybe something I’ve written can help you.
More typical for idle screws is 1.5-2.5 turns out from the “lightly bottomed / aka all the way in” starting point. This depends on whether you’re using aftermarket brass or factory steel Idle screws.
Are you working on an early ‘70 series or later 170 series carb? What is your primary rod/jet combination? I know big blocks better, but the general advice is start at the stock rod/jet combination (stock secondary rod/hangers too) and experiment from there.
If you’re really in learning mode, I can recommend a wideband oxygen (02) sensor to see AFR (air/fuel ratio) or Lambda (native language of 02 sensors) in realtime. Then you can see in realtime (aka in-use-on-the-road) wether your carb is lean/rich or on target in any of its stages: idle, part-throttle/cruise, or WOT.
Carb tuning is time consuming, but rewarding when you get near optimal. You can really feel the difference once you get close.
Cheers
Chris
What are some things that would cause more turns out on AF than normal? Vacuum leaks? More air needs more fuel? At idle is does smell rich to me. Weird thing is off idle it performs well. No stumble, plenty of power. All seems to be around idle issues.
I tried turning the black cover back and forth feeling for resistance with the choke flap closed but never seem to feel anything significant to me. The choke seem to moved very freely vs videos showing a springness closing them? The choke assembly has a vacuum line from the top of the intake manifold into it and an exit line going to the back/top of the carb. Is black cover for the choke intended to be sealed so it isnt a vacuum leak? Do I need to take the cover off and check for a proper seal?
The choke housing pulls air from the top of the carb through the heater tube in the crossover and through the choke housing into the carb. This warm air heats the bimetallic coil to operate the choke as the car warms up. There should be a paper gasket between the plastic choke cover and the housing. More importantly, it sounds like whoever installed that choke cover didn't properly engage the end of the coil with the tab that operates the choke linkage. Usually you have to install it with the end of the coil back from the tab, then rotate the cover to engage the tab with the coil and preload the choke linkage.
As said, there is a certain spot to engage the coil on the choke linkage. As said make sure the fast idle cam engages and disengages. Carb cleaner on the linkage then lubricant usually fixes a sticking choke mechanism. The gasket must be in place and all lines connected. I have seen rotted tubes in the intake and at least one weak hot air choke coil fail on these Olds V8's when they can into the shop I apprenticed at. If all that is in working order, check how far the choke pull off is opening, it may not be opening far enough and causing the rougher running.
Last edited by olds 307 and 403; Jun 27, 2023 at 06:49 AM.
Got to drive the car for extended time for the first time. Drove a little over an hour to a show at interstate speed. Car runs fine on the highway under throttle. Only issues I seem to be having is with idle. It is somewhat intermittent.
When pulling up to a stop sign, most times it acts like it is loading up, drops rpm and gets rough low rpm. Smells rich as well. Once and a while it doesnt act loaded up and idles smoother without rpm drop. I have a rebuild kit on the way and will give that a shot. In researching a couple things I found common problems are leaking well plugs and warped bodies. Would either of these two things be consistent with what I am experiencing?
Leaking well plugs was an issue with early 60s carbs but is not applicable to the later carbs. Ensure the initial timing, dwell, and vacuum are within factory specs before venturing off the path into custom tuning territory.
Check the throttle plate with a known-good straight edge and you’ll be able to see if it’s warped. Check the airhorn with the straightedge too. Overtightening airhorns and warping them is common.
Warping on either the throttle plate and/or the airhorn can lead to vacuum leaks of various levels of seriousness and can also lead to bad metering (probably lean from a vacuum leak).
There are various well plug fixes out there. I’ve tried epoxy, jb-weld and so on. They all last for a while, but eventually the heating/cooling cycle of the metal on the intake overcomes the elasticity of the glue and you get a seep or a leak. Might be a year or 2, might be 5 depending on how you use your car.
I try to keep sort of “fresh” carbs on my cars with the idea that by the time the well plugs leak, you may also have vacuum leaks at the drivers and/or passenger side primary throttle body shaft.
Anyway, the quick way to see if anything’s warped is a good straight-edge. Echo back if that tool is new to you.
Finally, from Cliff Ruggles or one of the other qjet suppliers, you can get airhorn gaskets that are thick to make up (and potentially solve) leakage from a warped airhorn. Worth a few bucks and a few hours to test.
Leaking well plugs was an issue with early 60s carbs but is not applicable to the later carbs. Ensure the initial timing, dwell, and vacuum are within factory specs before venturing off the path into custom tuning territory.
Did all that and it didnt help with the issue. Seems to be an intermittent rich fuel problem at idle.
Last edited by 72Vert442; Jul 4, 2023 at 07:15 PM.
Check the throttle plate with a known-good straight edge and you’ll be able to see if it’s warped. Check the airhorn with the straightedge too. Overtightening airhorns and warping them is common.
Warping on either the throttle plate and/or the airhorn can lead to vacuum leaks of various levels of seriousness and can also lead to bad metering (probably lean from a vacuum leak).
There are various well plug fixes out there. I’ve tried epoxy, jb-weld and so on. They all last for a while, but eventually the heating/cooling cycle of the metal on the intake overcomes the elasticity of the glue and you get a seep or a leak. Might be a year or 2, might be 5 depending on how you use your car.
I try to keep sort of “fresh” carbs on my cars with the idea that by the time the well plugs leak, you may also have vacuum leaks at the drivers and/or passenger side primary throttle body shaft.
Anyway, the quick way to see if anything’s warped is a good straight-edge. Echo back if that tool is new to you.
Finally, from Cliff Ruggles or one of the other qjet suppliers, you can get airhorn gaskets that are thick to make up (and potentially solve) leakage from a warped airhorn. Worth a few bucks and a few hours to test.
Cheers
Chris
Can you tell if the well plugs have been leaking when you take it apart?
Does your choke pull-off (break) work? It’s designed to hold the choke plate open after start so it doesn’t flood i.e. burn rich. It’s operated via carb vacuum port. If it isn’t working to hold the choke open at idle your choke plate may be closing enriching your fuel burn.
Does your choke pull-off (break) work? It’s designed to hold the choke plate open after start so it doesn’t flood i.e. burn rich. It’s operated via carb vacuum port. If it isn’t working to hold the choke open at idle your choke plate may be closing enriching your fuel burn.
When it sits for a week for so it has to crank for a while to get fuel to the bowls(I think thats what is happening) to start. I read this could be from leaking well plugs draining the bowls. Once it fires it acts like it is choked for a couple seconds then acts like it wants to die. To keep it running I have to pump the throttle which takes it off the choke.
Once it has ran it starts up easy. every time. Two problem areas are cold starting and idle when warmed up. Im leaning towards leaking well plugs but wanted to get some others experience with this sort of issue.
When it sits for a week for so it has to crank for a while to get fuel to the bowls(I think thats what is happening) to start. I read this could be from leaking well plugs draining the bowls.
That is totally normal and IS NOT due to leaking well plugs. It’s simply evaporation of fuel over time and then the time it takes for the fuel pump to push fuel up to the carb. Again, totally normal.
You haven’t said you validated what JoeP has said. Are sure the choke bimetallic spring is engaged correctly with the choke cover plate? You should feel a slight resistance when engaged correctly.
You haven’t said you validated what JoeP has said. Are sure the choke bimetallic spring is engaged correctly with the choke cover plate? You should feel a slight resistance when engaged correctly.
Yes validate the gasket is in the choke and the choke spring is engaged properly.
Really trying to get a better understanding on diagnostics/troubleshooting.
The vacuum measurement is spot-on @ 18"Hg "if" the measurement is rock steady. Do you notice ANY wavering of vacuum? While the illustration demonstrates the most likely cause can be improper A/F mixture, the vacuum reading should very clearly be a steady 18"Hg w/ not wavering.
Perform the 1st test identified in the upper LH corner of the illustration. Don't be timid about it - snap the throttle vigorously noting the vacuum gauge - do it several times. Can you identify any variances?
Really trying to get a better understanding on diagnostics/troubleshooting.
The vacuum measurement is spot-on @ 18"Hg "if" the measurement is rock steady. Do you notice ANY wavering of vacuum? While the illustration demonstrates the most likely cause can be improper A/F mixture, the vacuum reading should very clearly be a steady 18"Hg w/ not wavering.
Perform the 1st test identified in the upper LH corner of the illustration. Don't be timid about it - snap the throttle vigorously noting the vacuum gauge - do it several times. Can you identify any variances?
The vacuum reading was steady at 18. When snapping the throttle it would drop to around 5 and then return quickly back to 18 and was steady.
Your distributor vacuum advance canister should be connected to a manifold vacuum source NOT a ported vacuum source. Ensure you're connected to a manifold vacuum source. This means the vacuum advance canister hose should be connected to the carburetor BELOW the carburetor throttle plate NOT above the carburetor throttle plate. Above the carburetor throttle plate is ported vacuum, below the carburetor throttle plate is manifold vacuum.
The choke assembly has a vacuum line from the top of the intake manifold into it and an exit line going to the back/top of the carb.
It took me several reads trying to understand what you were talking about in the above statement until it dawned on me you're talking about the choke stove hot air tube assembly. You have your concept backwards. It is not an exit line going to the back/top of the carburetor. The back top of the carburetor is actually where the line BEGINS which feeds INTO the choke stove hot air assembly. It ENDS (EXITS) INTO the choke where there exists a fine drilled vacuum source where the choke attaches to the carburetor. Joe explained this operation in his post, I want to make sure you understand this. Filtered air is sourced (drawn INTO) from the top rear of the carburetor - it does NOT exit going to the back/top of the carburetor. The following video demonstrates operation of the choke stove hot air assembly. This video demonstrates operation/setup on a Chevy. The choke housing assembly will look different but the operation is exactly the same. Ensure you know which way the air is being sourced from. I'll mention (once again) which another member has already mentioned, as well: (1) make certain your choke pull-off (break) is actually working and it is adjusted "properly" - there should be resistance inside that choke pull-off (break) as it contains a diaphragm - suck and blow into it with your mouth to check it then check its adjustment: and, (2) ensure the fast-idle cam is adjusted properly. Both of these are clearly defined in the CSM. I do hope you own a copy of the CSM which contains the exact adjustment procedures for the fast-idle cam, the choke stove hot air assembly and the choke pull-off adjustment.
Now on to the more important part. To get the highest vacuum reading during AF adjustment - I started at 3 1/2 turns out and ended up at 6 turns out. Is this reasonable amount of turns out or does this mean it is dirty? At the highest vacuum reading the idle is around 1000 rpm and cant be adjusted lower because the idle screw is backed all the way out(not touching). Shouldnt I be able to set idle lower? If so why would it be 1000 rpm with idle screw backed out all the way? AF is set wrong
Screams vacuum leak or too much idle air. Any chance somebody put drilled primary throttle plates in this carb at some point? People do that to allow more idle air into the engine with bigger cams. Other times, mass rebuilders mix up airhorns and baseplates and you can end up with idle bypass air going through the casting, though I find that less likely. If the primary throttle shafts haven't been bushed, you may want to check those as a source of the vacuum leak as well, they can wear quite a bit.
Screams vacuum leak or too much idle air. Any chance somebody put drilled primary throttle plates in this carb at some point? People do that to allow more idle air into the engine with bigger cams. Other times, mass rebuilders mix up airhorns and baseplates and you can end up with idle bypass air going through the casting, though I find that less likely. If the primary throttle shafts haven't been bushed, you may want to check those as a source of the vacuum leak as well, they can wear quite a bit.
I have corrected that since it was written. AF needles are at 3 1/2 turns out now.
It took me several reads trying to understand what you were talking about in the above statement until it dawned on me you're talking about the choke stove hot air tube assembly. You have your concept backwards. It is not an exit line going to the back/top of the carburetor. The back top of the carburetor is actually where the line BEGINS which feeds INTO the choke stove hot air assembly. It ENDS (EXITS) INTO the choke where there exists a fine drilled vacuum source where the choke attaches to the carburetor. Joe explained this operation in his post, I want to make sure you understand this. Filtered air is sourced (drawn INTO) from the top rear of the carburetor - it does NOT exit going to the back/top of the carburetor. The following video demonstrates operation of the choke stove hot air assembly. This video demonstrates operation/setup on a Chevy. The choke housing assembly will look different but the operation is exactly the same. Ensure you know which way the air is being sourced from. I'll mention (once again) which another member has already mentioned, as well: (1) make certain your choke pull-off (break) is actually working and it is adjusted "properly" - there should be resistance inside that choke pull-off (break) as it contains a diaphragm - suck and blow into it with your mouth to check it then check its adjustment: and, (2) ensure the fast-idle cam is adjusted properly. Both of these are clearly defined in the CSM. I do hope you own a copy of the CSM which contains the exact adjustment procedures for the fast-idle cam, the choke stove hot air assembly and the choke pull-off adjustment.
What is in the video is what I have. Here are some more details with photos that helps explain better.
Here is a photo of the engine cold with choke set. Notice the blade is open slightly I think as it should be. There is resistance to open it further but can be moved closed by hand. Now on to starting cold. It cranks for a while, then pops off at fast idle but only for a second then starts to die. The only way to keep it from dying is to pump the throttle which takes it off choke. Perhaps the choke pull off is opening it too much but I cant look at it until I can keep it running by working the throttle. Next step is I will have someone else crank it when cold so I can see what the choke is doing.
Now on to the issue of where vacuum adv for distr is connected. It is connected to this part with the electrical connection at the top and 3 lines coming off from it. One goes to the tranny and one to vacuum adv. The top of it goes to ported vacuum on carb. What is this and what does it do vs direct connections?
The blade is definitely wide open once the engine is warm. I dont think the choke is causing the idle in gear issues I need to solve. I understand if the choke wasnt disengaging when warm it would make it run rich but havent seen that. According to receipts I have it got a rebuild kit in 2009. Who know what could have been done? Bigger jets? IMO there is definitely a fueling issue at idle in gear.
Last edited by 72Vert442; Jul 5, 2023 at 03:11 PM.
DO NOT use this image as a reference for anything pertaining to your DVCS on your 1972 350 as it is NOT applicable. This is an image from my 1971 CSM (again, NOT applicable to your car). It will however demonstrate what it looks like. I can't be of any further assistance in helping you determine if your hoses are routed correctly since I don't own a 1972 CSM. I also don't have an image of a 1971 non-A/C 350 DVCS routing. You, however, need to own a 1972 CSM - buy an original paperback, used 1972 Oldsmobile Chassis Service Manual - many on e-Bay. You might perform a Search on our site to see if anyone has posted an image for a 1972 non-A/C DVCS hose routing. I provided you w/ a link to the several DVCS threads.
Notice the blade is open slightly I think as it should be. There is resistance to open it further but can be moved closed by hand. Now on to starting cold. It cranks for a while, then pops off at fast idle but only for a second then starts to die. The only way to keep it from dying is to pump the throttle which takes it off choke.
That is not correct operation. It should not come off the fast idle cam until the choke coil warms up and pulls the air valve open. Pumping the throttle before starting the engine is what sets the choke and fast idle cam, so pumping it again should not disengage them.
