idle and choke adjustment for 1962 Buick Lesabre

[Anthony12]

My Dad and I got this lesabre about a week ago. The guy said he tried doing some adjustments on the timing and idle screws. The dwell angle was off about 6° and the idle mixture screws where all the way rich. The dwell angle is now at a perfect 30°, timing might still be off by a degree. I am a young man and this was my first experience with carburetor tuning. The idle screws were rich enough to cause a backfire that blew the crossover plug right off, scared the hell out of me. I did 3 things wrong, I set the idle screws to the factory position, then i basically relieved the choke adjustment spring tension, and i was adjusting the idle while my dad was in park. After a considerable amount of research i now know the more correct procedure, now i have 2 important questions that need answering before i move on with this vehicle, I'm not quite sure how the choke rich lean adjuster is set supposed to be set. I have a book from the late 60's my grandfather got when he was 16, it tells you how the choke should be adjusted (pictures attached), it also says the idle when the engine is hot is 525 in Neutral(?) I just want to double check this information. I also need to know what values am I adjusting for the choke rich lean adjuster? I also think the fast idle with the choke closed is 625, how do I go about adjusting the fast idle? I saw online you have to take measurements and bend the arms into the proper positions. Thank you readers, for your time.

[Anthony12]

To add, what I currently have the choke spring is, I relieved the spring tension and turned it counter clockwise until the marks aligned. To further specify, It's the 401 Cubic inch nailhead engine.

[Oldtech]

Just my 2 cents on the choke. If it is all working correctly - everything free and not sticky, and the heat pipe is good, and most importantly the choke is getting warm when it runs, then it SHOULD work right at the pictured setting.  If it is not warming up there is a hole for vacuum inside the choke housing that could be plugged.  It seems that often the choke can be backed off a couple of marks toward lean on older cars and it works better. might be worth a try. 

[Bloo]

I don't know why it backfired.

 

The choke should be set to the line as shown in the graphic above, assuming the choke thermostat (round thing) is the right part. Only set it richer if you need to.

 

Idle mixture (jets) and idle speed (throttle stop screw) settings need to be done with the engine completely warmed up, timing and dwell already set, and the choke fully open. If the choke is not fully open, find out why and fix it first.

 

Any "factory settings" of the jets are starting points. They're close enough to get the car started after a carb overhaul. The simple adjustment method is to start with both jets too rich, turn one in very slowly until the engine starts to miss, then back out until it doesn't. This being a pre-emission model, the threads will be pretty coarse These are small movements when you get close to the right spot. Tiny even. Now back that one out a tiny bit more. Then set the other one, except don't back it out once you find the best idle. The go back to the first one (that you backed out slightly) and set it again. Check idle speed. If it's wrong, adjust the throttle stop (idle speed screw) in the direction it needs to go. Then back the idle jets out a tiny bit and do this paragraph again. Keep doing it until you have the mixture and speed right at the same time.

 

That's the simple "pretty good" version. There's another called the "Lean Roll" method. It's sort of the same but a little bit more involved. Ideally the idle jets would be set ever so slightly leaner than the smoothest fastest spot. One of these days I'll post a thread about it. Right now I don't have the right sort of car here to use as an example.

 

Both idle jets should behave about the same when you are setting them. They should wind up set approximately the same. In other words not with one screw way in and the other way out. If you get that, or if one screw bottoms out without making the engine slow down and miss, or makes no difference, you have other things wrong and need to fix first.

 

I have to guess on the fast idle because I don't know your actual carburetor. Most have a fast idle cam. The book needs to give you more info than just "choke closed". Since the number they gave is not much above normal idle, it is pretty safe to assume it is the first step. With the car warm, and everything else set, open the throttle slightly and close the choke enough and then let the throttle close to get the fast idle screw "hooked" on the first step of whatever it contacts. What it contacts is probably the first step of a fast idle cam. It's probably located between the choke housing and the carb body. Leave it like that, caught on the first step, and set the fast idle screw. Then crack the throttle and the idle should fall back to the normal idle speed. This paragraph is completely generic. If the service manual contradicts this, follow the service manual.

 

 

Edited June 6, 2023 by Bloo (see edit history)

[Anthony12]

3 minutes ago, Bloo said:

I don't know why it backfired.

 

The choke should be set to the line as shown in the graphic above, assuming the choke thermostat (round thing) is the right part. Only set it richer if you need to.

 

Idle mixture (jets) and idle speed (throttle stop screw) settings need to be done with the engine completely warmed up, timing and dwell already set, and the choke fully open. If the choke is not fully open, find out why and fix it first.

 

Any "factory settings" of the jets are starting points. They're close enough to get the car started after a carb overhaul. The simple adjustment method is to start with both jets too rich, turn one in very slowly until the engine starts to miss, then back out until it doesn't. This being a pre-emission model, the threads will be pretty coarse These are small movements. Tiny even. Now back that one out a tiny bit more. Then set the other one, except don't back it out once you find the best idle. The go back to the first one (that you backed out slightly) and set it again. Check idle speed. If it's wrong, adjust the throttle stop (idle speed screw) in the direction it needs to go. Then back the idle jets out a tiny bit and do this paragraph again. Keep doing it until you have the mixture and speed right at the same time.

 

That's the simple "pretty good" version. There's another called the "Lean Roll" method. It's sort of the same but a little bit more involved. Ideally the idle jets would be set ever so slightly leaner than the smoothest fastest spot. One of these days I'll post a thread about it. Right now I don't have the right sort of car here to use as an example.

 

Both idle jets should behave about the same when you are setting them. They should wind up set approximately the same. In other words not with one screw way in and the other way out. If you get that, or if one screw bottoms out without making the engine slow down and miss, or makes no difference, you have other things wrong and need to fix first.

 

I have to guess on the fast idle because I don't know your actual carburetor. Most have a fast idle cam. The book needs to give you more info than just "choke closed". Since the number they gave is not much above normal idle, it is pretty safe to assume it is the first step. With the car warm, and everything else set, open the throttle slightly and close the choke enough and then let the throttle close to get the fast idle screw "hooked" on the first step of whatever it contacts. What it contacts is probably the first step of a fast idle cam. It's probably located between the choke housing and the carb body. Leave it like that, caught on the first step, and set the fast idle screw. Then crack the throttle and the idle should fall back to the normal idle speed. This paragraph is completely generic. If the service manual contradicts this, follow the service manual.

 

 

 

 

 

I have a rochester 2GC 7020048, The book says it doesnt(?) Have a fast idle. As for adjusting it I've heard that adjusting the idle screws for fastest idle, back off and repeat for right idle screw, then adjust idle screw on the side and just repeat till its all proper. And thanks for reminding me about the factory service manual, currently I can't afford it but I did plan on moving forward after consulting it, that was something very important that slipped past my mind. I've been very focused on figuring out this carbeurator thing. The choke does engage when you turn the car on but since the plug is blown I didn't run it long enough to warm up, BUT I can say previously the heat line to the choke was working properly. This is a first experience so if you need any clarification in what I'm saying just let me know.

[Bloo]

Yeah, you gotta fix that plug.... Good luck with it. We'll be here when you have questions. Welcome to the forum!

 

[m-mman]

19 hours ago, Anthony12 said:

And thanks for reminding me about the factory service manual, currently I can't afford it but I did plan on moving forward after consulting it

Service manuals don’t cost money, they save it. 😉 

 

They used to be very pricey, but nowadays you can get almost any manual on eBay for $30-$50.  

Everything that the manufacturer wanted their service personnel to know about fixing their products. 
How can you get along without one? 

Suggestion is to buy an original paper version.  The scans and reprints are ok, but commonly the photographs come out too dark and lose detail.  

[NailheadBob]

@m-mmansaid factory service manual is good information to guide you

 

This information is if you have a "Nailhead Motor"

 

I believe you may be talking about intake manifold expansion plug blew out, it's located on bottom side of intake manifold, could be one of 2 sizes. easy job to remove intake and will not loose any coolant

 

The expansion plug when blown out sounds like a very bad exhaust leak, is coming from the cross over inside the intake manifold that is sealed by expansion plug.

 

You only can reset your choke and idle mixture screws and dwell and timing properly with the expansion plug sealed up for best results

 

Dorman makes the expansion plugs #550-034 for 3" expansion plug, and #550-031 for 2 1/4" expansion plug, I have a detailed procedure to replace the expansion plug if you would like.

 

Bob

[Anthony12]

4 minutes ago, NailheadBob said:

@m-mmansaid factory service manual is good information to guide you

 

This information is if you have a "Nailhead Motor"

 

I believe you may be talking about intake manifold expansion plug blew out, it's located on bottom side of intake manifold, could be one of 2 sizes. easy job to remove intake and will not loose any coolant

 

The expansion plug when blown out sounds like a very bad exhaust leak, is coming from the cross over inside the intake manifold that is sealed by expansion plug.

 

You only can reset your choke and idle mixture screws and dwell and timing properly with the expansion plug sealed up for best results

 

Dorman makes the expansion plugs #550-034 for 3" expansion plug, and #550-031 for 2 1/4" expansion plug, I have a detailed procedure to replace the expansion plug if you would like.

 

Bob

I know, I have the intake manifold off already, I would very much like that procedure though, it would be very useful to know.

[carbking]

The automatic choke adjustment, if an integral choke with a cap and spring, may be modified as follows for best results:

 

With the engine cold, and the ambient temperature (or the temperature in your garage) between 65 and 70 degrees F.

 

(1) Work the throttle to wide-open and release; this should close the choke butterfly. If it doesn't, find out why, and correct.

(2) Loosen the 3 (sometimes 2) screws retaining the choke cap

(3) Work the throttle approximately 1/2 open AND HOLD

(4) With your other hand, rotate the choke cap until the choke butterfly hangs open, now rotate in the opposite direction until the choke butterfly JUST TOUCHES CLOSED, no tension, and HOLD.

(5) With your third hand, snug (DO NOT TIGHTEN AND STRIP!) the screws retaining the choke cap.

 

This procedure will compensate for fatigue, unless badly fatigued and needing replacement, of the choke bi-metallic spring.

 

Jon

[Anthony12]

6 minutes ago, carbking said:

The automatic choke adjustment, if an integral choke with a cap and spring, may be modified as follows for best results:

 

With the engine cold, and the ambient temperature (or the temperature in your garage) between 65 and 70 degrees F.

 

(1) Work the throttle to wide-open and release; this should close the choke butterfly. If it doesn't, find out why, and correct.

(2) Loosen the 3 (sometimes 2) screws retaining the choke cap

(3) Work the throttle approximately 1/2 open AND HOLD

(4) With your other hand, rotate the choke cap until the choke butterfly hangs open, now rotate in the opposite direction until the choke butterfly JUST TOUCHES CLOSED, no tension, and HOLD.

(5) With your third hand, snug (DO NOT TIGHTEN AND STRIP!) the screws retaining the choke cap.

 

This procedure will compensate for fatigue, unless badly fatigued and needing replacement, of the choke bi-metallic spring.

 

Jon

I appreciate the indeph procedure, This is a first time experience so I don't quite understand the procedure but this is a great start, thank you.

[carbking]

Anthony - a bit more about the choke.

 

The choke cap has attached on the inside a coiled bi-metallic spring. The spring is sensitive to heat and the free end of the spring will move as the temperature changes. Just like the bi-metallic spring in older house heating and air-conditioning thermostats.

 

When cold, movement of the free end of the spring, which is connected to the choke butterfly, will cause the butterfly to close. REGARDLESS OF WHAT YOU MAY READ ELSEWHERE, IN VIRTUALLY ALL CASES, THE SPRING DOES NOT OPEN THE CHOKE BUTTERFLY.

 

Inside the metal choke housing which is covered by the cap/spring, there is a piston in a cylinder that is connected to a vacuum source inside the carburetor. Once the engine starts, engine vacuum causes the piston to attempt to open the choke butterfly, and it will open a wee bit. As the engine warms, heat is supplied to the bi-metallic spring through the heat tube, causing the spring to relax. As the spring relaxes, the vacuum piston will continue to pull against the spring, and open the choke butterfly.

 

Once the choke is properly adjusted, there is no need to "relieve the choke spring tension".

 

It should be noted that once the choke is properly adjusted, the choke butterfly may not be completely closed on a cold engine. If the ambient is in the 80's or 90's, the spring will not completely close the butterfly.

 

Conversely, if the ambient is in the 30's or 40's; not only will the butterfly be closed; it will have tension pressing it against the choke butterfly housing.

 

The function is automatic, responding to temperature.

 

Jon

[Anthony12]

8 minutes ago, carbking said:

Anthony - a bit more about the choke.

 

The choke cap has attached on the inside a coiled bi-metallic spring. The spring is sensitive to heat and the free end of the spring will move as the temperature changes. Just like the bi-metallic spring in older house heating and air-conditioning thermostats.

 

When cold, movement of the free end of the spring, which is connected to the choke butterfly, will cause the butterfly to close. REGARDLESS OF WHAT YOU MAY READ ELSEWHERE, IN VIRTUALLY ALL CASES, THE SPRING DOES NOT OPEN THE CHOKE BUTTERFLY.

 

Inside the metal choke housing which is covered by the cap/spring, there is a piston in a cylinder that is connected to a vacuum source inside the carburetor. Once the engine starts, engine vacuum causes the piston to attempt to open the choke butterfly, and it will open a wee bit. As the engine warms, heat is supplied to the bi-metallic spring through the heat tube, causing the spring to relax. As the spring relaxes, the vacuum piston will continue to pull against the spring, and open the choke butterfly.

 

Once the choke is properly adjusted, there is no need to "relieve the choke spring tension".

 

It should be noted that once the choke is properly adjusted, the choke butterfly may not be completely closed on a cold engine. If the ambient is in the 80's or 90's, the spring will not completely close the butterfly.

 

Conversely, if the ambient is in the 30's or 40's; not only will the butterfly be closed; it will have tension pressing it against the choke butterfly housing.

 

The function is automatic, responding to temperature.

 

Jon

Thank you for explain the function! This provided me with a lot more understanding on how the choke works mechanically. So pretty much I'm adjusting the position of the choke rather than spring tension? It's only necessary to adjust the spring tension if the choke is having troubles opening/closing? I did observe the choke opening a tad when starting yhe vehicle. So what I'm gathering is when the engine is cold, ambient temp at 60-70, pull the accelerator cable back to reset the choke valve, and adjust the choke valve position till its *almost* closed? Then if it's not opening or closing correctly, I adjust the spring tension? I understand the bi-metallic properties of the spring for sure.

[carbking]

 

1 hour ago, Anthony12 said:

Then if it's not opening or closing correctly, I adjust the spring tension? I understand the bi-metallic properties of the spring for sure.

No - if the choke is set correctly, but not opening or closing correctly, determine why, and fix it. Do not adjust the tension to correct for defective parts; fix the problem.

 

Jon

[NailheadBob]

Here is the procedure I put together with the help of other AACA forum members:

 

Bob

 

[Anthony12]

On 6/6/2023 at 3:06 PM, Bloo said:

I don't know why it backfired.

 

The choke should be set to the line as shown in the graphic above, assuming the choke thermostat (round thing) is the right part. Only set it richer if you need to.

 

Idle mixture (jets) and idle speed (throttle stop screw) settings need to be done with the engine completely warmed up, timing and dwell already set, and the choke fully open. If the choke is not fully open, find out why and fix it first.

 

Any "factory settings" of the jets are starting points. They're close enough to get the car started after a carb overhaul. The simple adjustment method is to start with both jets too rich, turn one in very slowly until the engine starts to miss, then back out until it doesn't. This being a pre-emission model, the threads will be pretty coarse These are small movements when you get close to the right spot. Tiny even. Now back that one out a tiny bit more. Then set the other one, except don't back it out once you find the best idle. The go back to the first one (that you backed out slightly) and set it again. Check idle speed. If it's wrong, adjust the throttle stop (idle speed screw) in the direction it needs to go. Then back the idle jets out a tiny bit and do this paragraph again. Keep doing it until you have the mixture and speed right at the same time.

 

That's the simple "pretty good" version. There's another called the "Lean Roll" method. It's sort of the same but a little bit more involved. Ideally the idle jets would be set ever so slightly leaner than the smoothest fastest spot. One of these days I'll post a thread about it. Right now I don't have the right sort of car here to use as an example.

 

Both idle jets should behave about the same when you are setting them. They should wind up set approximately the same. In other words not with one screw way in and the other way out. If you get that, or if one screw bottoms out without making the engine slow down and miss, or makes no difference, you have other things wrong and need to fix first.

 

I have to guess on the fast idle because I don't know your actual carburetor. Most have a fast idle cam. The book needs to give you more info than just "choke closed". Since the number they gave is not much above normal idle, it is pretty safe to assume it is the first step. With the car warm, and everything else set, open the throttle slightly and close the choke enough and then let the throttle close to get the fast idle screw "hooked" on the first step of whatever it contacts. What it contacts is probably the first step of a fast idle cam. It's probably located between the choke housing and the carb body. Leave it like that, caught on the first step, and set the fast idle screw. Then crack the throttle and the idle should fall back to the normal idle speed. This paragraph is completely generic. If the service manual contradicts this, follow the service manual.

 

 

 

On 6/7/2023 at 4:41 PM, carbking said:

Anthony - a bit more about the choke.

 

The choke cap has attached on the inside a coiled bi-metallic spring. The spring is sensitive to heat and the free end of the spring will move as the temperature changes. Just like the bi-metallic spring in older house heating and air-conditioning thermostats.

 

When cold, movement of the free end of the spring, which is connected to the choke butterfly, will cause the butterfly to close. REGARDLESS OF WHAT YOU MAY READ ELSEWHERE, IN VIRTUALLY ALL CASES, THE SPRING DOES NOT OPEN THE CHOKE BUTTERFLY.

 

Inside the metal choke housing which is covered by the cap/spring, there is a piston in a cylinder that is connected to a vacuum source inside the carburetor. Once the engine starts, engine vacuum causes the piston to attempt to open the choke butterfly, and it will open a wee bit. As the engine warms, heat is supplied to the bi-metallic spring through the heat tube, causing the spring to relax. As the spring relaxes, the vacuum piston will continue to pull against the spring, and open the choke butterfly.

 

Once the choke is properly adjusted, there is no need to "relieve the choke spring tension".

 

It should be noted that once the choke is properly adjusted, the choke butterfly may not be completely closed on a cold engine. If the ambient is in the 80's or 90's, the spring will not completely close the butterfly.

 

Conversely, if the ambient is in the 30's or 40's; not only will the butterfly be closed; it will have tension pressing it against the choke butterfly housing.

 

The function is automatic, responding to temperature.

 

Jon

 

 

I got an update, I patched up the plug and have the engine running ok, I certainly have to go back and continue tuning the idle mixture screws. I tried setting the choke with my interpretation of the book and the choke is always open. I understand how the choke functions a lot more now and I would appreciate if you could answer some questions I got! So I have the 2gc 7020048, In the photos I posted, is "choke rod" referring to the gap when the choke plate is closed when it's cold? Also, for setting the spring tension, Do I set the tension until the choke valve is at the desired gap? I live in michigan so when it gets cold I want to avoid the hell of holding the choke close till the engine warms up haha. Also, the choke properly engages when the engine turns on, is there a way to accolade the movement of the valve for when the engine turns on, vacuum engages, and the plate opens slightly? Thank you very much.

[carbking]

12 hours ago, Anthony12 said:

Once the choke is properly adjusted, there is no need to "relieve the choke spring tension".

 

It should be noted that once the choke is properly adjusted, the choke butterfly may not be completely closed on a cold engine. If the ambient is in the 80's or 90's, the spring will not completely close the butterfly.

 

Conversely, if the ambient is in the 30's or 40's; not only will the butterfly be closed; it will have tension pressing it against the choke butterfly housing.

 

The function is automatic, responding to temperature.

 

Please note the above quote from my previous post - the choke reacts to the ambient temperature. Set it when the ambient is between 65~70 degrees F. and forget it!

 

The choke rod is the linkage item which connects externally to the choke arm.

 

Jon

Edited June 17, 2023 by carbking (see edit history)

[Anthony12]

4 minutes ago, carbking said:

 

Please note the above quote from my previous post - the choke reacts to the ambient temperature. Set it when the ambient is between 65~70 degrees F. and forget it!

 

The choke rod is the linkage item which connects externally to the choke arm.

 

Jon

Yeah I plan on doing so on a day with this weather,  I'm wondering how wide is the gap supposed to be from the wall to the plate,  I saw somewhere it was like 0.125 or something? Other places it said the plate it flush against the horn.

[rodneybeauchamp]

Hi, if you wish to delve deeper into the setup, you will find online the complete Rochester Service manual for your carburettor. They are comprehensive and give you diagrams and technical specs for settings and step by step procedures. The tab on the carburettor should give you the details for the exact carburettor you are working with.

 

I found one for the 4GC for my ‘63 Buick Skylark and printed off the pages for the choke set up. It is much more comprehensive than the Buick Shop manual.
 

Mine had been converted to a manual choke and was difficult to start. Conversion back to the original automatic choke makes it easier.

 

Search Rochester 2GC guides or manuals - you will be surprised with what is out there!

cheers

Rodney 😀😀😀😀😀

 

PS I also saved the download as a favourite so I could easily find it from time to time

[carbking]

One of 5 steps from my 7 June post: " With your other hand, rotate the choke cap until the choke butterfly hangs open, now rotate in the opposite direction until the choke butterfly JUST TOUCHES CLOSED, no tension, and HOLD."

 

WHEN THE CHOKE IS SET CORRECTLY, BETWEEN 65~70 DEGREES F - THERE IS NO GAP!

 

The gap is for the unloader circuit, totally different function. Get the choke functioning correctly first; then if you are concerned about starting a flooded cold engine worry about the gap.

 

Jon

[Anthony12]

1 minute ago, carbking said:

One of 5 steps from my 7 June post: " With your other hand, rotate the choke cap until the choke butterfly hangs open, now rotate in the opposite direction until the choke butterfly JUST TOUCHES CLOSED, no tension, and HOLD."

 

WHEN THE CHOKE IS SET CORRECTLY, BETWEEN 65~70 DEGREES F - THERE IS NO GAP!

 

The gap is for the unloader circuit, totally different function. Get the choke functioning correctly first; then if you are concerned about starting a flooded cold engine worry about the gap.

 

Jon

Thank you so much, this clarifies everything. Thank you.