stupid question

[Stage1 Jeff]

on a q-jet, there are 2 vacuum diaphragms, which one is the pulloff for the choke? the one in the front,or the one in the back?? :Do No:

[LARRY70GS]

on a q-jet, there are 2 vacuum diaphragms, which one is the pulloff for the choke? the one in the front,or the one in the back?? :Do No:

No such thing as a stupid question. They both do. The front one opens the choke a set amount, from fully closed, as the engine starts. This enables the engine to run without loading up. The back one has an air bleed that gives it a delayed resonse, and it opens the choke a little more. From there the choke coil controls the opening of the choke valve. The front vacuum break also has a link to the secondary air valves. It slows their opening, to prevent bog, as the secondary throttle valves open.

[RAbarrett]

Correct on both counts, with one niggle. The secondary choke pull off actually operates based on air cleaner temperature. As the air cleaner reaches 80-100 deg F, the secondary choke pull off widens the opening to further lean the mixture. There are no stupid questions except the one you don't ask. Ray

[msc66]

Correct on both counts, with one niggle. The secondary choke pull off actually operates based on air cleaner temperature. As the air cleaner reaches 80-100 deg F, the secondary choke pull off widens the opening to further lean the mixture. There are no stupid questions except the one you don't ask. Ray

OK, so what happens if you aren't running a stock air cleaner?

[RAbarrett]

No problem. The secondary choke pull-off will simply not operate the choke. Though this may cause a slight enrichment in the choke mixtures, that will not be a problem since the colder air will require the additional fuel. Just be sure that the mixture delivered is rich enough to allow for the cold air when completely warm. If it is not, you might encounter some stumbling, surging, or hesitation. This should disappear at the weather warms. There may also be a problem with carb icing. This occurs in cool temps. As the fuel vaporizes, it draws a tremendous amount of heat out of the mixture, allowing a certain amount of vapor in the air to condense on the throttle blades, eventually freezing,and stalling the engine. This should clear as the engine warms sufficiently to prevent this. Ray

[Stage1 Jeff]

i am getting a cold engine stumble,goes away when warmed up.
i checked the rear pulloff,by pushing it in,and pinching off the hose,would not stay in.
front pulloff held ok. so would the rear one be my problem? otherwise runs very well.
the thermac is working too

[RAbarrett]

The best way to test a choke pull-off is to disconnect the hose, push the diaphragm in until seated, then cover the end. It should come back slightly, then hold. Though it depends on how the vacuum is routed, the primary is typically the front unit, used to control the air valves as well. The primary is the one connected to manifold vacuum. The secondary typically runs to a port on the air cleaner. It is possible that the stumble you feel is due to adjustments, but let's fix the original complaint first. Ray

[Nicholas Sloop]

The secondary typically runs to a port on the air cleaner. Ray

On all the 70-74 Buick carbs I've played with the secondary vacuum break gets manifold vacuum straight from the carb base, without involving the air cleaner at all.

[Stage1 Jeff]

that is what i did,the rear one failed to hold,while the front one did hold

[LARRY70GS]

that is what i did,the rear one failed to hold,while the front one did hold

The rear one typically has an air bleed incorporated into it. Make sure there is vacuum at the hose. Some carb gaskets will block the vacuum to that port. If you apply vacuum to it from another fitting on the engine, you'll see it has a delayed response due to the air bleed. The only way to test them is to apply a good vacuum source to them, you can't compress them and pinch off the hose. I've also never seen the rear vacuum break routed to the air cleaner on any 74 and down Buick I've ever worked on. Maybe this is a later year thing Ray??

[Stage1 Jeff]

mine is going to carb vacuum.

[LARRY70GS]

mine is going to carb vacuum.

Yes, but is there vacuum at the hose with the engine running????

[LARRY70GS]

This is what I'm talking about. You have to supply the secondary vacuum break with a good manifold source, and watch it. It has a delayed action.

[RAbarrett]

I think we may be getting a little off track here. BOTH choke pull-offs receive manifold vacuum; the issue is timing. Typically, the front pull-off is the one unloading the choke after a cold start. It also controls the air valve for the secondaries. The rear is typically the secondary pull-off, responding to manifold vacuum from a thermostatically controlled source. One source typically used is in the air cleaner. When the air cleaner temp reaches about 100 deg, the thermostatically controlled unit allows vacuum to pull the secondary pull-off to further widen the choke opening, further leaning the cold mixture. It is very easy to find a unit having been modified from stock, and the result can be confusion. The bottom line is this: the primary pull-off controls two things. It controls the secondary air valves and the primary choke pull off; it will recieve manifold vacuum any time the engine runs, regardless of the temperature. The secondary controls the choke opening after a period of time after the engine starts, typically based on air cleaner temp. If the air cleaner is modified, or removed, or altered, or tampered with, changed in any way, it may have been reinstalled in a different manner. The bottom line is simple: the choke pull-offs control choke blade opening. It is this choke opening that controls the engine's performance cold. If the engine is not performaing properly cold, the choke's operation becomes a possible factor. Questions?

[RAbarrett]

One other thing to consider here is the possibility of another orifice on the secondary pull-ff. Make sure that the pull-off has any and all orifices plugged before testing, or a good unit may be condemned. This will require careful observation. Ray

[LARRY70GS]

Hotrod84Buick,
Might help if we knew what Q-jet you are actually running. As I stated before, I've never seen a thermostat controlled secondary choke pull off on any 74 and down Q-jet. They may have been used on later model engines. If you have an emissions era Q-jet(carb # starting with 170), have you rejetted it? Keep in mind that those carbs were calibrated to run a leaner mixture in conjunction with a heated air cleaner. Some later model carbs actually have an adjustment on the primary choke pull off. If you have this on your carb, you might try adjusting it so that it doesn't open the choke as much. If the carb has an integral choke, try adjusting the choke thermostat one notch richer. Bottom line, even if everything is on the money, in very cold weather, the car may be a lttle cold blooded between the time the choke starts to come off, and the time the engine is fully warm. Sometimes I think we get a little spoiled by modern day engine controls, and EFI.

[Stage1 Jeff]

larry,the carb#i have is:7045244, i checked the rear pull off, by placing thumb ofer the air bleed hole,and by pinching the hose off, did not hold vacuum.the choke is set at 1 notch rich.

[LARRY70GS]

larry,the carb#i have is:7045244, i checked the rear pull off, by placing thumb ofer the air bleed hole,and by pinching the hose off, did not hold vacuum.the choke is set at 1 notch rich.

OK, that's a 75 carb. Have you rejetted it? If not, it's probably lean. You can't check those pull offs like that. The only way is to put a constant vacuum source to them. Start the motor, then run a vacuum hose from a manifold vacuum source to the pull off. If it doesn't slowly pull in within 3-4 seconds, it's bad. Have you tried going one notch richer on the choke?

[Stage1 Jeff]

carb has never been off the engine,has never been apart.i have not messed with the choke. will try to go a notch richer on the choke.that will open it up a bit more? bogs until warmed up.the rear pulloff is going to a port on the rear carb base plate

[LARRY70GS]

carb has never been off the engine,has never been apart.i have not messed with the choke. will try to go a notch richer on the choke.that will open it up a bit more? bogs until warmed up.the rear pulloff is going to a port on the rear carb base plate

Going a notch richer on the choke will increase the tension which will tend to keep the choke closed longer. See if it helps. Start the engine, then pull off the vacuum hose from the rear pull off. See if there is vacuum there at all. Some carb gaskets will cut off the vacuum supply to that rear port. You may not even have vacuum there. If there is vacuum, the pull off is bad.

[Stage1 Jeff]

just for my reference, wha is the choke setting for my carb?
would be nice to have it for future use.had another possibility, would that efe valve cause this? it moves freely

[LARRY70GS]

just for my reference, wha is the choke setting for my carb?
No such thing, set it to what works best for your climate. The spring may lose tension, that's why there is an adjustment.






would be nice to have it for future use.had another possibility, would that efe valve cause this? it moves freely

EFE valve, like a heat riser right? Which car are we talking about? It warms the bottom of the intake manifold, like an exhaust crossover. If it isn't doing that, it can cause problems with cold operation, especially with a carb with stock jetting.

[Stage1 Jeff]

1975 le sabre, 350 4bbl. mine is vacuum operated. if it's bad, would it be ok with out it
until i found another?

[LARRY70GS]

1975 le sabre, 350 4bbl. mine is vacuum operated. if it's bad, would it be ok with out it
until i found another?

Yes, but it should be easier just to check it. It should close with the engine cold. Check to see if there is vacuum present, and whether the vacuum diaphram that operates it works.

[Stage1 Jeff]

ok will check things out,(if i want to brave single digit temps in my garage) and let ya know. thanks for the answers!!

[Stage1 Jeff]

forgot something... i did notice that the port on the vacuum switch that supplies the efe valve is blocked. is not possible to unblock it without breaking off the nipple.
is there another source of ported vacuum i could tap into?

[LARRY70GS]

forgot something... i did notice that the port on the vacuum switch that supplies the efe valve is blocked. is not possible to unblock it without breaking off the nipple.
is there another source of ported vacuum i could tap into?

No, that is a thermovacuum switch. It regulates vacuum to the EFE valve by coolant temperature. Replace the thermovacuum switch. See if your parts store stocks it, if not try GM

[Stage1 Jeff]

my mistake, it was not blocked,was dark in the garage.
took that rear pulloff to test it with an outside source of vacuum,will not hold at all.
comes right back out.so good thing i have a new one to put on!!
any other adjustments should i do ,besides going a notch richer on the choke?

[LARRY70GS]

my mistake, it was not blocked,was dark in the garage.

any other adjustments should i do ,besides going a notch richer on the choke?

Nope, try that first, see if it makes a difference. As long as your air cleaner door is working correctly, that should do it. Is the EFE valve closing when the engine is cold?

[Stage1 Jeff]

yes,it is, checked that first(despite cold fingers) is closed when cold, and freely moves. will see what it does when i drive it next.how long,after the engine is running does it open? :Do No: where in the world would i find another if it's bad? :Do No:
i want to keep it there, to preserve the originality of the car

[LARRY70GS]

yes,it is, checked that first(despite cold fingers) is closed when cold, and freely moves. will see what it does when i drive it next.how long,after the engine is running does it open? :Do No: where in the world would i find another if it's bad? :Do No:
i want to keep it there, to preserve the originality of the car

Couldn't tell you, you should have a 75 Buick chassis manual. That would answer all your questions. Theres one on E-bay for 29.00 here:
http://stores.ebay.ca/Ken-McGee-Auto-Literature_W0QQcolZ2QQdirZQ2d1QQdptZ0QQpZ6QQsclZal lQQsotimedisplayZ2QQtZkm


http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=7929975372&category=6761

[Stage1 Jeff]

THANKS!!will check it out!!

[LARRY70GS]

THANKS!!will check it out!!

Hey Hotrod, put your first name in your signature, I feel like I'm talking to a cartoon character(your avatar) :laugh: :laugh: :laugh: :laugh: :laugh:

[Stage1 Jeff]

sorry,had a brain fart, i fixed it!

[LARRY70GS]

sorry,had a brain fart, i fixed it!

:laugh: :laugh: :laugh: Sorry Jeff, I liked the avatar, I meant put your name in your signature where you have your cars :TU:

[Stage1 Jeff]

ok,it's back!
i couldn't budge the choke spring housing,so will have to let it be for now.could a linkage adjustment be used instead?
if my spring is weak,might see if napa might have new ones

[RAbarrett]

There are several things to consider here. One is that the carb is likely metered correctly if it was the original. The lack of heat can cause the problems you are encountering. Question: does the engine idle properly right after starting? If not, the issue becomes one of idle speed and choke pull-off adjustment. Slight hesitations right after start up can be cured by adjusting the pull-off slightly richer. Look at the linkage carefully. The '75 carb may have a bendable linkage or a screw adjustment. Make SLIGHT adjustments, then evaluate the following morning. If the engine develops hestiations or surging during warm up, but not right after starting, the problem is the result of the choke opening too fast. Note; after getting the choke properly adjusted, you may find that the idle speeds cold may be adjusted lower. The factory used idle speed to compensate for lean mixtures for emissions standards. If there is any question regarding the condition of the pull-off diaphragm, buy a new one before attempting adjustments. Also, be sure you are adjusting the primary pull off, the one connected directly connected to vacuum. The secondary unit (if used), will not be connected to vacuum directly. There will be some sort of control in between the vacuum and the diaphragm. If two pull-offs are used, there will be a control for the secondary unit.

[LARRY70GS]

ok,it's back!
i couldn't budge the choke spring housing,so will have to let it be for now.could a linkage adjustment be used instead?
if my spring is weak,might see if napa might have new ones

There's usually 3 screws that you have to loosen. On some originals it's rivets. The replacement choke coils come with screws, and you drill the rivets out. Which do you have?

[Stage1 Jeff]

i have screws.they came loose, but housing was stuck tight(carb has never been touched)

[LARRY70GS]

i have screws.they came loose, but housing was stuck tight(carb has never been touched)

Maybe a little gumout, and let it soak in, then tap it lightly with a small wrench. probably just carboned up. It's plastic. You can get new ones if you break it. Or try Rays suggestions. If the primary pull off has a screw adjustment, try that.

[Stage1 Jeff]

well,before i shelled out $$ for the new one,i had the secondary one double checked,it held vacuuum,so i now have the front one off to check it.
also looked in a friend's motor manual, choke setting is set righr, at 1 rich. i was told by a local mechanic to set it at the index mark,or 1 or 2 lean,to open it up more. this sounds like a good idea to me, if i was to set it richer,my problem would be worse. :Do No:

[LARRY70GS]

well,before i shelled out $$ for the new one,i had the secondary one double checked,it held vacuuum,so i now have the front one off to check it.
also looked in a friend's motor manual, choke setting is set righr, at 1 rich. i was told by a local mechanic to set it at the index mark,or 1 or 2 lean,to open it up more. this sounds like a good idea to me, if i was to set it richer,my problem would be worse.

OK Jeff, no harm in trying, you'll know if you go the wrong way. Let us know how you make out.

[Stage1 Jeff]

i'm just going by what a guy here told me,i'm always reluctant to mess with these q-jet chokes.

[LARRY70GS]

i'm just going by what a guy here told me,i'm always reluctant to mess with these q-jet chokes.

Jeff,
The only way to learn is to take a chance. If it doesn't work, you can put it back to the way it was. It's hard to tell over the internet whether the choke is too lean or too rich. Go lean a notch, see if the engine likes it. If it doesn't, go one notch richer. Let us know :TU:

[RAbarrett]

Some things to consider here: 1. As springs age, they lose their tension. This includes the springs on the choke. In this case, they typically create a leaner condition. You might try bending the link on the choke blade itself. The newer chokes were riveted to the housing. Slightly bending the link counterclockwise with enrich the choke without messing with the housing itself. The link is on the passenger side directly connected to the tang on the choke blade. The idea is to bend the link a little more to enrich the setting, and to bend it more straight to lean the choke. Note: this works for slight adjustments, and only if you know exactly what you are doing. As with anything metallic, the constant bending will break a part very difficult to replace. Make the adjustments carefully, and be aware that SMALL adjustments are all that are needed. Ray

[Stage1 Jeff]

will try that. fortunately,napa has the spring & the plastic housing new for
$10,so i may pop for a new one,to have on hand.

[RAbarrett]

A wise decision. When and if you install the new one, be careful to properly install the tang on the spring in the actuator on the choke. It will need to go into the opening. It is very easy to miss the opening, which is a slot, and have a choke that either will not open or will not close. If you use the new one, I suggest setting it to the original setting, as the spring will weaken with age. Also expect the choke pull-off to fail soon as the additional tension provided by the new spring will likely cause it to fail. That will cause an after start-up flooding condition. This will also be the perfect time to check the linkage for binding, and to check the fast idle settings. Also make sure to check the vacuum advance diaphragm as well. A defective vacuum advance will cause poor driveability, a vacuum leak, and a slow fast idle. Ray

[Stage1 Jeff]

went 1 notch richer, the bog went away.
i let it run for a couple minutes, then backed it out, gave it some gas, no bog, seemed better.now,when it warms up, do i need to put it back where it was?

[LARRY70GS]

went 1 notch richer, the bog went away.
i let it run for a couple minutes, then backed it out, gave it some gas, no bog, seemed better.now,when it warms up, do i need to put it back where it was?

No, leave it. Warm the car to operating temperature, remove the air cleaner lid, and verify that the choke is opened all the way. That's it.

[Stage1 Jeff]

no, i meant put it back when the weather warms up in the spring.
watched choke ,it opened ok so, i think i have it fixed :) :TU:

[LARRY70GS]

No, leave it. As long as it opens the choke all the way now, it will do so in the Spring.

[RAbarrett]

Actually, no. Leave it. The reason is simple. As the weather warms, the spring will warm as well. In other words, the tension on the spring is dependent on temperature. As the temperature rises during the warmer weather, the spring will lose some tension when cold. This will reduce the closing tension on the choke. The spring will then be easier to warm as the engine warms, and the result will be an open choke with a warm engine, regardless of the cold position. It is very likely that the one notch richer will assist the warmer weather operation as well. One thing to be aware of is that the engine will tend to run at an elevated idle if the choke does not open completely. One notch should make no difference. One other thing to remember is that, if your choke uses hot air for heating, the lack of a heated air cleaner will affect the opening of the choke. In that case, it might be advisable to use an electrically heated unit from a later carb. If that becomes an issue, we can cover that at a later date. Let me know... Ray

[Stage1 Jeff]

am using 87 octane now, sometimes 89 octane which is best?
have been tempted to use some of that 10%ethanol, but am worried about what it will do to my fuel system? choke problem is better now,opens fully

[LARRY70GS]

am using 87 octane now, sometimes 89 octane which is best?
have been tempted to use some of that 10%ethanol, but am worried about what it will do to my fuel system? choke problem is better now,opens fully

If you are using 87 octane now, and it does not ping, then there is no advantage to using 89. Using higher octane than an engine requires is nothing but a waste of money. Are you sure that there isn't any ethanol in the fuel you are using? In N.Y., all the gasoline has 10% ethanol. I have been using it for years with no problems.

[Stage1 Jeff]

it is available at only one place here where i live, is only 2 cents higher than regular.($1.71, regular is$1.69)
i rarely let my tank go below 3/4 tank, so 5 gallons or so shouldn't hurt