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?? o 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.
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
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
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
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
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.
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??
From The 72 Buick Chassis Manual 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.
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?
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
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.
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?
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.
