75 Duster carb issue
#3246092
07/21/24 08:44 AM
07/21/24 08:44 AM
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Joined: Mar 2003
Posts: 1,517 Martinsville, IN
cdwmotorsports
OP
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OP
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Martinsville, IN
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I'm currently trying to determine a carb stumble and a surge on a 75 Duster, slant 6 and 55,700 miles. Stumble is accelerating away from stop lights/signs, surging happens at cruising throttle (especially noticeable around 60-70). It has a fully intact emissions system and the carb per the owner was rebuilt. Any pointers, help would be greatly appreciated. It runs great and the owner would prefer to keep the fuel milage that it gets so a 4bbl or V8 swap isn't likely an option. The car wasn't driven much, since I started working on it I've put 300 miles on it and only running premium fuel in it hoping if it was a weird issue with fuel it would help clean it up. While it seems to have helped some the issues persist. Also did a 1975 Duster have a cat on it from the factory? Could that be part of the issue if it's bad?
Last edited by cdwmotorsports; 07/21/24 08:47 AM.
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Re: 75 Duster carb issue
[Re: cdwmotorsports]
#3246198
07/21/24 12:59 PM
07/21/24 12:59 PM
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Joined: Jan 2003
Posts: 20,356 Park Forest, IL
slantzilla
Too Many Posts
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Park Forest, IL
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Have you checked the vacuum advance to make sure it's working? Maybe bad wires on the pickup coil?
75 should have a cat.
The stock single barrel carbs are well known for issues, usually a worn out throttle shaft, and good rebuilts are almost impossible to find.
"Everybody funny, now you funny too."
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Re: 75 Duster carb issue
[Re: slantzilla]
#3246245
07/21/24 02:57 PM
07/21/24 02:57 PM
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Joined: Mar 2003
Posts: 1,517 Martinsville, IN
cdwmotorsports
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Have you checked the vacuum advance to make sure it's working? Maybe bad wires on the pickup coil?
75 should have a cat.
The stock single barrel carbs are well known for issues, usually a worn out throttle shaft, and good rebuilts are almost impossible to find. What kinds of issues because at 55,700 miles I doubt its a worn throttle shaft. I haven't checked the vacuum advance or pickup coil. It seems like a fuel issue but maybe not. Since a good rebuild is not likely an option what are options to replace it with?
Last edited by cdwmotorsports; 07/21/24 02:58 PM.
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Re: 75 Duster carb issue
[Re: cdwmotorsports]
#3246262
07/21/24 03:45 PM
07/21/24 03:45 PM
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Joined: Nov 2006
Posts: 5,328 Star Idaho
67vertman
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The stumble on acceleration could be accelerator pump, clogged fuel filter or broken fuel pump. To many variables to diagnose without being there. If you are looking for a rebuild kit, check out these guys. https://www.carburetor-parts.com/
My Monster are real!
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Re: 75 Duster carb issue
[Re: 67vertman]
#3246330
07/21/24 06:25 PM
07/21/24 06:25 PM
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Joined: Mar 2003
Posts: 1,517 Martinsville, IN
cdwmotorsports
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The stumble on acceleration could be accelerator pump, clogged fuel filter or broken fuel pump. To many variables to diagnose without being there. If you are looking for a rebuild kit, check out these guys. https://www.carburetor-parts.com/ I'm not looking for a rebuild kit the carb has been rebuilt. It has a new fuel filter and I have a pump to put on but haven't done so yet because I figured it was unlikely.
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Re: 75 Duster carb issue
[Re: 70Duster]
#3246331
07/21/24 06:26 PM
07/21/24 06:26 PM
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Joined: Mar 2003
Posts: 1,517 Martinsville, IN
cdwmotorsports
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I'm betting it's a timing issue. Try advancing the timing. Even if you don't believe me, give it a try. It's cheap and easy to try. I've been down this road before. I'll try it thanks
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Re: 75 Duster carb issue
[Re: cdwmotorsports]
#3246334
07/21/24 06:31 PM
07/21/24 06:31 PM
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Joined: Nov 2006
Posts: 5,328 Star Idaho
67vertman
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The stumble on acceleration could be accelerator pump, clogged fuel filter or broken fuel pump. To many variables to diagnose without being there. If you are looking for a rebuild kit, check out these guys. https://www.carburetor-parts.com/ I'm not looking for a rebuild kit the carb has been rebuilt. It has a new fuel filter and I have a pump to put on but haven't done so yet because I figured it was unlikely. Just throwing things out there. Chasing, can be fun. Not. Good luck with the search.
My Monster are real!
Living within your means makes life pretty easy.
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Re: 75 Duster carb issue
[Re: 67vertman]
#3246421
07/21/24 11:03 PM
07/21/24 11:03 PM
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Joined: Jan 2003
Posts: 10,905 Freeport IL USA
poorboy
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The mid 70s were the worst carb issue time that ever existed. Even new carb quality was all over the place, emissions were stepping up every year, and the gas companies were messing with the gas all the time. The chokes had electric circuits added to pull the choke off faster to meet the emission standards, and the car companies were screwing with carbs, ignition timing, and an array of EPA demanded add on stuff that also changed every year, some of which didn't work when new.
That stumble and hesitation was a common problem. To address it from a full cold motor to a full operating temp was always a struggle of compromise, you had to pick the point where the customer was the most happy. Usually a stumble or hesitation on a fully warmed up motor could be adjusted out adjusting when and how much the accelerator pump would discharge the extra fuel kick when the gas pedal was stepped into, and that adjustment was not based on how you drive, but how the customer drives. A very light footed owner is very difficult to adjust an accelerator pump for. If I was adjusting a stumble for a very light footed customer, my boss had to do the test driving while I rode along to feel and hear it.
Without knowing the operating temp of the motor when the problem is occurring, fixing it online isn't going to work out well. I need to know if the problem is when the motor is stone cold, at operating temp, or somewhere in between, or is present through the entire operation.
I was the carb adjusting guy at the largest service garage in our town from about 74 through 79. Any stumbles or hesitations came to me, even from other garages. Proper adjustment was not a one time and done process, and in that era, it was often also a fall and a spring adjustment as well. It is an act of several fine adjustments no book is going to tell you.
For the record, just because the carb was "rebuilt", that does not mean it was rebuilt correctly. Throwing new gaskets into a carb does not rebuild it.
There was a reason the car companies went to electronic fuel injection.
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Re: 75 Duster carb issue
[Re: poorboy]
#3246427
07/21/24 11:34 PM
07/21/24 11:34 PM
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Joined: Mar 2003
Posts: 1,517 Martinsville, IN
cdwmotorsports
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The mid 70s were the worst carb issue time that ever existed. Even new carb quality was all over the place, emissions were stepping up every year, and the gas companies were messing with the gas all the time. The chokes had electric circuits added to pull the choke off faster to meet the emission standards, and the car companies were screwing with carbs, ignition timing, and an array of EPA demanded add on stuff that also changed every year, some of which didn't work when new.
That stumble and hesitation was a common problem. To address it from a full cold motor to a full operating temp was always a struggle of compromise, you had to pick the point where the customer was the most happy. Usually a stumble or hesitation on a fully warmed up motor could be adjusted out adjusting when and how much the accelerator pump would discharge the extra fuel kick when the gas pedal was stepped into, and that adjustment was not based on how you drive, but how the customer drives. A very light footed owner is very difficult to adjust an accelerator pump for. If I was adjusting a stumble for a very light footed customer, my boss had to do the test driving while I rode along to feel and hear it.
Without knowing the operating temp of the motor when the problem is occurring, fixing it online isn't going to work out well. I need to know if the problem is when the motor is stone cold, at operating temp, or somewhere in between, or is present through the entire operation.
I was the carb adjusting guy at the largest service garage in our town from about 74 through 79. Any stumbles or hesitations came to me, even from other garages. Proper adjustment was not a one time and done process, and in that era, it was often also a fall and a spring adjustment as well. It is an act of several fine adjustments no book is going to tell you.
For the record, just because the carb was "rebuilt", that does not mean it was rebuilt correctly. Throwing new gaskets into a carb does not rebuild it.
There was a reason the car companies went to electronic fuel injection. Well I'm pushing for him to Holley Sniper Autolite 1100 swap it and just enjoy it. Yes I know the bases are different but its nothing an adapter can't fix.
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Re: 75 Duster carb issue
[Re: poorboy]
#3246428
07/21/24 11:37 PM
07/21/24 11:37 PM
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Joined: Jan 2003
Posts: 15,928 Omaha Ne
TJP
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With the engine at operating temp. From a dead stop, normal acceleration, If the car momentarily responds, stumbles, and picks up again, It could be the off idle transition is not responding as it should. If it stumbles immediately and recovers it usually indicates an issue with the accelerator pump circuit. Could be misadjusted, incorrect nozzle size or several other things. Your surging could be due to a lean condition caused by several things, vacuum leak or excessive timing. Going to need to find someone who understands the older vehicles or dive in yourself. As mentioned most rebuilt carbs are sketchy at best. the onlycompay I've had any luck with is Auto Line out of canada. Rock Auto sells their carbs. they will also do yours if not available keep us posted
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Re: 75 Duster carb issue
[Re: TJP]
#3246436
07/21/24 11:57 PM
07/21/24 11:57 PM
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Joined: Mar 2003
Posts: 1,517 Martinsville, IN
cdwmotorsports
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Martinsville, IN
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With the engine at operating temp. From a dead stop, normal acceleration, If the car momentarily responds, stumbles, and picks up again, It could be the off idle transition is not responding as it should. If it stumbles immediately and recovers it usually indicates an issue with the accelerator pump circuit. Could be misadjusted, incorrect nozzle size or several other things. Your surging could be due to a lean condition caused by several things, vacuum leak or excessive timing. Going to need to find someone who understands the older vehicles or dive in yourself. As mentioned most rebuilt carbs are sketchy at best. the onlycompay I've had any luck with is Auto Line out of canada. Rock Auto sells their carbs. they will also do yours if not available keep us posted I would say this is the most accurate aside from it's tendency to die... it doesn't seem to matter cold or warm it does it regardless "From a dead stop, normal acceleration, If the car momentarily responds, stumbles, and picks up again, It could be the off idle transition is not responding as it should."
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Re: 75 Duster carb issue
[Re: 70Duster]
#3247316
07/25/24 07:41 PM
07/25/24 07:41 PM
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Joined: Mar 2003
Posts: 1,517 Martinsville, IN
cdwmotorsports
OP
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I'm betting it's a timing issue. Try advancing the timing. Even if you don't believe me, give it a try. It's cheap and easy to try. I've been down this road before. So 70 Duster, are you saying that 4 degrees after TDC isn't enough? Haha. I just put a timing light on it and that's where it's at. What should I advance it to? I haven't checked anything else yet.
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Re: 75 Duster carb issue
[Re: poorboy]
#3247318
07/25/24 07:45 PM
07/25/24 07:45 PM
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Joined: Mar 2003
Posts: 1,517 Martinsville, IN
cdwmotorsports
OP
pro stock
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OP
pro stock
Joined: Mar 2003
Posts: 1,517
Martinsville, IN
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The mid 70s were the worst carb issue time that ever existed. Even new carb quality was all over the place, emissions were stepping up every year, and the gas companies were messing with the gas all the time. The chokes had electric circuits added to pull the choke off faster to meet the emission standards, and the car companies were screwing with carbs, ignition timing, and an array of EPA demanded add on stuff that also changed every year, some of which didn't work when new.
That stumble and hesitation was a common problem. To address it from a full cold motor to a full operating temp was always a struggle of compromise, you had to pick the point where the customer was the most happy. Usually a stumble or hesitation on a fully warmed up motor could be adjusted out adjusting when and how much the accelerator pump would discharge the extra fuel kick when the gas pedal was stepped into, and that adjustment was not based on how you drive, but how the customer drives. A very light footed owner is very difficult to adjust an accelerator pump for. If I was adjusting a stumble for a very light footed customer, my boss had to do the test driving while I rode along to feel and hear it.
Without knowing the operating temp of the motor when the problem is occurring, fixing it online isn't going to work out well. I need to know if the problem is when the motor is stone cold, at operating temp, or somewhere in between, or is present through the entire operation.
I was the carb adjusting guy at the largest service garage in our town from about 74 through 79. Any stumbles or hesitations came to me, even from other garages. Proper adjustment was not a one time and done process, and in that era, it was often also a fall and a spring adjustment as well. It is an act of several fine adjustments no book is going to tell you.
For the record, just because the carb was "rebuilt", that does not mean it was rebuilt correctly. Throwing new gaskets into a carb does not rebuild it.
There was a reason the car companies went to electronic fuel injection. So what would a guy like you charge to "fix" this issue if it's in fact carb and not solely timing related. We aren't far apart I'd happily bring it to you, either driven or trailered whichever makes the most sense for setting it up.
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Re: 75 Duster carb issue
[Re: cdwmotorsports]
#3247354
07/25/24 10:53 PM
07/25/24 10:53 PM
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Joined: Jan 2003
Posts: 15,928 Omaha Ne
TJP
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I'm betting it's a timing issue. Try advancing the timing. Even if you don't believe me, give it a try. It's cheap and easy to try. I've been down this road before. So 70 Duster, are you saying that 4 degrees after TDC isn't enough? Haha. I just put a timing light on it and that's where it's at. What should I advance it to? I haven't checked anything else yet. the initial timing is not what will cause the surging but rather the total or timing at 60 MPH. This is where the mechanical and vacuum come into the picture. the combination of initial, + mechanical + vacuum can create too much advance that results in the surging. DO NOT assume the timing mark is correct without verifying it indicates TDC when the #1 cylinder is at TDC. Slipping outer balancer rings, mismatched components can lead to erroneous timing. Once you verify the ignition/ timing you ca then move on to the carb
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Re: 75 Duster carb issue
[Re: TJP]
#3247362
07/25/24 11:25 PM
07/25/24 11:25 PM
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Joined: Mar 2003
Posts: 1,517 Martinsville, IN
cdwmotorsports
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Posts: 1,517
Martinsville, IN
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I'm betting it's a timing issue. Try advancing the timing. Even if you don't believe me, give it a try. It's cheap and easy to try. I've been down this road before. So 70 Duster, are you saying that 4 degrees after TDC isn't enough? Haha. I just put a timing light on it and that's where it's at. What should I advance it to? I haven't checked anything else yet. the initial timing is not what will cause the surging but rather the total or timing at 60 MPH. This is where the mechanical and vacuum come into the picture. the combination of initial, + mechanical + vacuum can create too much advance that results in the surging. DO NOT assume the timing mark is correct without verifying it indicates TDC when the #1 cylinder is at TDC. Slipping outer balancer rings, mismatched components can lead to erroneous timing. Once you verify the ignition/ timing you ca then move on to the carb Ok, I'll verify TDC tomorrow when the engine is cold and determine if the timing marks are right. I advanced it 10 degrees total to 6 BTDC (per the timing marks) and it runs vastly better. That stumble and surge are still there. The vacuum advance works (put vacuum hand pump on the advance hose and as I pumped it up it was for sure advancing the timing).
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Re: 75 Duster carb issue
[Re: cdwmotorsports]
#3247407
07/26/24 10:33 AM
07/26/24 10:33 AM
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Joined: Aug 2003
Posts: 2,481 Answering the call of the wild
ThermoQuad
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1975 emission systems had a factory work around where the egr was disabled and the vac adv hooked up to unported vauum etc. I owned such a beast and had the factory docs. Not anymore so in the meantime disconnect the egr, put the vac adv on ported
- you do not have the car set up properly with the timing so please pay attention.
Give the timing as much advance as it will tolerate without detonation
Last edited by ThermoQuad; 07/26/24 10:33 AM.
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Re: 75 Duster carb issue
[Re: ThermoQuad]
#3247409
07/26/24 10:49 AM
07/26/24 10:49 AM
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Joined: Mar 2003
Posts: 1,517 Martinsville, IN
cdwmotorsports
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1975 emission systems had a factory work around where the egr was disabled and the vac adv hooked up to unported vauum etc. I owned such a beast and had the factory docs. Not anymore so in the meantime disconnect the egr, put the vac adv on ported
- you do not have the car set up properly with the timing so please pay attention.
Give the timing as much advance as it will tolerate without detonation I literally started messing with the timing yesterday at about 7:30 PM, it's a work in progress. I am working on a car that isn't mine. I don't know who has done what to it aside from what I have done. I am reporting what I find. I chose 6 degrees BTDC based on information gathered from a thread on the slant6 forums website. Some of the posts of the users there are gone so I got limited info for a baseline. Also any idea which spot is ported vacuum or how I can figure it out?
Last edited by cdwmotorsports; 07/26/24 11:16 AM.
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