car stalls when i do this...

[mike561]

There must be a carb adjustment to correct this, i notice if i have the car in drive but completely stopped, then hammer rapidly on the gas pedal a few times the car will stall. I can start it right back up but this problem makes it not as responsive if i need to floor it hard from a sudden stop...

[Blue Bowtie]

What year? What engine? What transmission? What fuel system? What mods?

[mike561]

83 caprice stock 305 700r4 trans. i made a pic of what best describes the prob im having: im pretty sure this is a carb adjustment (4bbl qjet), but i want to know where to start, i might even just end up having the carb rebuilt.

[MagicRat]

If it does this when you quickly push the gas pedal down half-way, your accelerator pump is not delivering enough fuel.

If it only does this when you floor it, then your secondary air doors are opening too quickly. there is a quick and simple adjustment to slow-down the rate that they open.

[mike561]

any info on how to adjust the two? or at least the secondaries?

[Blue Bowtie]

Take a look at the accelerator pump cup, spring, check ball, and linkage rod. If the engine starts and runs normally otherwise, the fuel bowl level is probably close to correct.

Look at the windup adjustment on the secondary air valve for correct tension.

You can check the secondary spring windup without removing the top of the carb, but you'll have to remove the top to check the accelerator pump with the exception of the linkage rod.

[Blue Bowtie]

Setting the Secondary Air Valve (AV) Windup Spring Tension
The Secondary AV is the dual flap which covers the secondary bores. It opens with air pressure when the secondary throttle plates open and the resulting pressure differential creates flow Decreased spring tension allows the AV to open faster, with a lower pressure differential giving you full power sooner. It also allows the AV to open more easily with airflow, giving you the possibility to make more total power. Having too little tension creates too much secondary air flow and can create bogging as you describe.

The secondary air valve is held closed by two different things. One is the choke pull-off vacuum diaphragm on the right side of the carburetor. When the engine is running , the choke pull off keeps the AV closed by pulling the linkage via the rod that connects them. When you go toward WOT, vacuum to the choke pull-off is reduced and the internal spring of the pull-off allows the AV to open. The pull-off generally releases slowly, slow enough that even with no AV spring tension you shouldn't get a bog. The rate at which it pulls out is not usually adjustable.

The second thing holding the secondary AV closed is the spring at the right end of the shaft, and the tension on that spring is adjustable. To adjust it, you will need a small Allen wrench or Torx bit (depending upon year) and a flat screwdriver.

Spring Windup Adjustment Procedure

1. Turn off the engine and remove the air cleaner and any obstructions;
2. Observe the linkage on the right side of the air valve shaft. Through a slot you can see a small flat screw head. This screw will be adjusted after loosening the clamp bolt;
3. Locate the clamp bolt on the underside of the air horn at its edge. A mirror and good lighting may be helpful;
4. Loosen the clamp bolt (setscrew) slightly so that the screw at the end of the AV shaft can be turned without using excessive force;
5. Turn the adjusting screw counter-clockwise until the AV falls open under its own weight. Turn the adjusting screw clockwise very slowly until the AV just closes by the spring tension. This is the zero tension point. To adjust to factory specifications you would turn the adjusting screw the additional number of turns specified in the service manual, usually ½ to 1 turn;
6. If you are so inclined, you can lock the clamp screw at the zero point and test drive the vehicle;
7. With the engine fully warmed up come to a stop, then floor it. If the engine bogs the tension may be too low. If the engine does not bog and stumble the adjustment is finished;
8. Increase the windup spring tension only enough to prevent bogging, or about 1/8 turn at a time. Adjusting the tension beyond 1 turn can permanently distort the spring.

This must be done when the engine is warm, the state of tune is normal, and no other problems exist. If the choke pull-off is not operating, repair that before adjusting the AV spring windup. If the accelerator pump is not working adjusting the AV tension will not compensate for it.

[mike561]

Thanks for the helpful input magicrat & blue. it seems like its getting enough fuel, it doesnt do it when i press half way down, only if i stomp on it at wot, i will take a look at this and let you know how it goes

[j cAT]

Originally Posted by mike561

Thanks for the helpful input magicrat & blue. it seems like its getting enough fuel, it doesnt do it when i press half way down, only if i stomp on it at wot, i will take a look at this and let you know how it goes

make sure you have no vacuum leaks before any carb adjustments are made ..make sure the PCV valve/hoses etc are correct and secure ..all hoses going into the carb base are usually damaged ..this is where carb cleaner has eaten at the rubber..

you use a vacuum guage to find and corrrect leaks...

[mike561]

im in the process of getting putting on new rubber caps to plug those vac ports first on the carb, i've replaced these several times and they keep cracking what should i do about that?

[j cAT]

Originally Posted by mike561

im in the process of getting putting on new rubber caps to plug those vac ports first on the carb, i've replaced these several times and they keep cracking what should i do about that?

I would use vacuum line/fuel line hose then use a machine screw to seal the open end of the rubber line..I too have had these rubber caps crack..they last about a year if that ...must not be compatible with fuel ...most likely chinese crap ..

[mike561]

yeah i get even less than a year with those caps, probably from roasting in the engine bay as well

Edit: also i cannot for the life of me find the two screws that you adjust for the secondary flaps, tried searching around for them with a mirror too. if someone can get a picture of a 4bbl q-jet and draw some arrows pointing to where the screws are that would be great