douglite

1980 L48 On the passenger side of the Quadrajet there is a adjustment screw on the vacume daspot. How does this adjustment effect the opening of the secondaries? how do you know if it is adjusted correctly? would appreciate any help. Thanks

lars

The phillips-head screw on the end of the choke pulloff diaphragm linkage has no effect on secondary operation - it is the adjustment for the choke pulloff. To adjust:

Assure the choke is fully closed, either by having a cold engine or by applying finger pressure to the choke intermediate shaft. You can also loosen the 3 screws holding the black choke cover in place, and rotate it temporarily to just barely fully close the choke plate (open the throttle slightly when doing this to allow the choke to fully close). Disconnect the short vacuum hose running from the choke pulloff to the carb and attach a long hose to the pulloff. With the choke closed, suck on the vacuum hose to fully retract the pulloff. In this position, apply slight finger pressure to the forward lower edge of the choke plate (simulating the force of air on the plate). The distance from the lower, forward edge of the choke to the air horn wall should be 1/4". Measure it by using a 1/4" drill bit. Adjust the measurement by turning the screw you are referring to. This will determine how rich/lean your engine will run upon initial startup.

Secondary airvalve adjustment is made using the spring windup adjustment screw and its lock screw located at the secondary airvalve lever. Procedure is as follows:

The secondary spring windup is adjusted with a small, slotted-head screw on the passenger side of the carb, right at the top of the carb on the secondary side. The screw head points right out to the side. 90 degrees from this, on the bottom, there is an allen-head lock screw that keeps the slotted screw from turning. If you have trouble seeing it, place a mirror under the area until you spot it. With a small slotted screwdriver holding the adjustment screw, loosen the allen screw about ¼ turn. This will allow you to turn the slotted adjustment screw. Counting the turns, allow the slotted screw to slowly unwind until all spring tension is gone. You can use your mirror to see the spring disengage contact from the pin lever underneath the air horn. If the spring tension was lost after only ½ turn, the windup was too loose. Bring the spring into contact with the lever. Note when it just barely touches. From this point, wind the spring up between ¾ turn and 7/8 turn. This is a good starting point, and will prevent any bogs or hesitations due to premature secondary opening. From here, you can loosen the windup about 1/8 to 1/4 turn at a time and test-drive the car - when the car falls on its face or feels "flat" going into the secondaries, you've hit the max point. Tighten the spring back up 1/8 to 1/4 turn, and you have the quickest secondary opening rate that your car/engine will handle.


[Modified by lars, 11:07 AM 9/24/2002]


[Modified by lars, 12:07 PM 9/25/2002]

douglite

Thanks for the reply. I started messing with the adjustment because i thought it effected the sencondary opening your info was helpfull. next time i will will ask first. thanks again.

douglite

Lars, I am still confused on the secondary adjustment. i notice the choke pull off rod runs to the secondary air plate . when i start the engine with the air cleaner off the vacume holds the plate closed even when the engine is warm. when i open the throttle it will open a little bit. is this normal? Just noticed on your post that you own two Goats. I bougt a new one in 65and dragged raced it untill i got marrried in 67. not sure what was more expensive the new wife or drag racing having fun with the Vette but still looking for a 65 goat. still have the wife

derv3tt3

Hey Douglite,I bought a 65 gto convertible back in 65. :seeya

clem zahrobsky

when you floor the car the vacuum in the vacuum diaphragm goes away. the size of the bleed hole in the diaphragm determines how fast the secondary air flap opens. the larger the hole the faster it opens. the spring determines when it starts to open. :chevy


[Modified by clem zahrobsky, 10:25 PM 9/24/2002]

lars

Doug -
Your observations on the choke pulloff and secondary airvalve rod holding the airvalve closed at idle are correct. This is normal operation as Clem states. To further expand on Clem's notes:

The airvalve (the upper plate on top ofthe carb you are looking at) is spring loaded. The secondary throttle plates (the actual plates in the lower throttle body of the carb that the throttle linkage attaches to) on a Q-Jet are purely mechanical (the Q-Jet is not a "vacuum secondary" carb). If the airvalve were not there, and the large secondary throtles were allowed to be immediately mechanically opened, the car would experience a massive lean stumble due to the sudden decrease in intake velocity and due to lack of immediate fuel flow through the secondary side.

To prevent this, the airvalve is located above the throttles. The opening rate of the airvalve is determined by the spring windup and by the choke pulloff: When you go to wide open throttle, the engine looses vacuum, the choke pulloff relaxes, and the bleed hole in the pulloff allows a smooth and gradual opening of the airvalve to prevent a secondary tip-in stumble. The airvalve is opened by the airflow passing across it once the pulloff has relaxed. The spring windup restricts the cfm to a level required and needed by the engine to prevent a secondary side flat spot at WOT. Thus the Q-Jet is actually a "variable cfm" carb flowing up to 750 cfm. It is due to the operation of the secondary airvalve and this variable cfm flow that GM was able to use the Q-Jet on everything from 6-cylinders (Pontiac OHC6 and Buick V6) to 454's.

What actually happens is that the airvalve compensates for the fact that there is no secondary accelerator pump. When the secondary throttles open below the airvalve, the area below the airvalve is exposed to manifold vacuum (which is significant until the airvalve opens). Exposing the "plenum" or "venturi" area of the secondary side of the carb to low pressure with the airvalve still closed sucks fuel right out of the secondary discharge nozzles moments before the airvalve opens to provide airflow. Thus, the engine gets a shot of fuel just like an accel pump shot right down the secondary side just as the secondary airvalve starts to open. If the airvalve is allowed to open too quick, or is otherwise improperly adjusted, this shot of fuel will be altered, and secondary tip-in stumble will result.


[Modified by lars, 9:55 AM 9/25/2002]

clem zahrobsky

some Q jet have 2 holes in the carb top just above the air valve which acts as a accelerator pump discharge nozzles as they pull fuel just as the air valve starts to open to prevent a dead spot. other Q jet have the same thing in the primary side above the choke to act as a fuel enrichment jets at wide open throttle. we always use this type for race cars and supercharged engines as it is a great way to richen up the carb fuel curve at wide open throttle. :chevy

joe58

You can also make the mechanical secondaries open sooner (not the air valve) by installing an aftermarket lever kit. It works good with a higher gear like a 4:11 . Hurts mileage though.

douglite

Thanks again for the info now i understand how it works. the progressive lingage on the goats tripower was easy to adjust. hold it to the floor and hang on but this dog is geeting too old to hunt now i climb in the vette and cruise.
1980 L48 completly stock red auto 64000 origional miles. it still has the origional exaust and paint.

thamzid

Will this work on a Stock 81 QuadraJet with the Computer Control.
Duh

lars

The adjustments outlined for the choke and secondary airvalve will work exactly the same for the computer controled carb. The ECM does not control either of these parameters, so you can do these steps as outlined.

Scootin_Z

I know this is an old thread but a great post by Lars. So I have a 69 350/350 car that falls on it's face if I get on it anything under 2500 R's or so. Other than that the car runs flawlessly including choke etc, and the car is in really nice condition with a fairly fresh resto so this is not a old dirty carb at all. So I read this thread and went to the garage. First off I have 3 different sets of Allen wrenches (2 standard and 1 metric) and for the life of me I could not find a allen to fit the locking screw for the flat blade screw. So after screwing with that for a while I decided to see if the flat blade was tight. It was firm but not tight so I gave it a 1/4 turn tighter (more spring presssure because it seems as though they open too fast)
After messing around with it including backing it all the way out until the secondaries were "floating" with no spring pressure) I then went in the 3/4 - 7/8 after the butterflies had just closed (contact) That was still no good and about the best place was maybe 1/4 to half turn in from where the butterflies would close after floating. Even then it still is "boggish" under 2500 R's but it's much better than it was. Before it would just fall on it's face completely if I tried getting on it at a low RPM. I really wish I could get this thing dialed in. Is there anything else that would be causing this? If I hit it at say 1200R's it's pretty bad but will eventually get up and go, 2000 R's it much better, at 2500 ALMOST gone and if I stab it at 2800-3000 she goes like a raped ape. What else can I do here short of sending the carb to you Lars? Someone mentioned you rebuild them, but to be honest I am not sure it needs it, but what do I know. The car is clean as mentioned but it has done some sitting. BUT with that said it had ALWAYS been like this since the resto and I usually throw a tank of 100LL AV fuel in at the end of the season because it don't go bad in 6 months like auto fuel.

I normally don't beat on the car, but it is nice to be able to jump on it once in a while. By the way it is a 4 speed car with a pretty low gear in it. Not sure but based on R's most likely a 4:10 gear. Plan on a 5 speed over the winter...

Thanks,
Kurt

Shark Racer

A lean primary side mixture or weak or broken accelerator pump can cause the condition you describe.

I recommend posting a new thread for this rather than digging up an old one.

Scootin_Z

Thanks Shark Racer. I will research that and see if mine is bad ir lean on the primary side. Just have not folled a whole ton with carbs so trying to not make it worse than it already is.

As far as digging this thread up, I just figured this was a good post by Lars so why start a new thread. I've been ragged at for starting threads because there are already a million of them on the particular topic, so I try to search first.

7t9l82

accelerator pump is a common problem with the ethanol "enhanced" fuel.

Scootin_Z

Thanks. Anyone know whats involved with that job???

7t9l82

pull the top off the carb and lift the old one out drop the new one in. first time 1/2 hour second time 10 min.

Scootin_Z

That simple? This could be doable for me...lol

Shark Racer

Step 1 to verify an accelerator pump problem is to let the car warm up (choke open) and then shut it down. (well, you don't have to shut it down but it's never wise to look down a carburetor while it's runnning)

Take the air cleaner lid off and look down the air horn (forward hole with choke flap)

Give the throttle a big long pull and verify that you get two solid squirts of fuel.