4bbl

[Macbuick]

Hi,
On my 350ci is a stock Rochester 4BBL. (El Camino 1971)
How can i be sure that the 2 big barrels chokes (at the rear of the carb) really get open when i drive it ?
Must they be always open as the engine get warm ? Or ...?
The front ones works well but i can't figure how to check the big ones...
Thanks
David

[exwestracer]

Hi,
On my 350ci is a stock Rochester 4BBL. (El Camino 1971)
How can i be sure that the 2 big barrels chokes (at the rear of the carb) really get open when i drive it ?
Must they be always open as the engine get warm ? Or ...?
The front ones works well but i can't figure how to check the big ones...
Thanks
David

Those aren't choke plates :wow: , and they have nothing to do with engine temperature. Those are secondary air valves. They are forced open by airflow through the secondaries when the throttle is WFO. Otherwise, they should be completely shut. You will notice a bar running between the two plates. This is attached to a set of metering rods in the secondary jets, which control fuel flow in direct proportion to airflow.

[Macbuick]

Those aren't choke plates :wow: , and they have nothing to do with engine temperature. Those are secondary air valves. They are forced open by airflow through the secondaries when the throttle is WFO. Otherwise, they should be completely shut. You will notice a bar running between the two plates. This is attached to a set of metering rods in the secondary jets, which control fuel flow in direct proportion to airflow.

:hail: thanks to fill my lack of knowledge about those carbs.....

How is it possible to check if they work? it seems that the car doesn't pull as i kick down or open wide.
I don't want to drive it without the front hood and air filter to see it in "live" :lol:

David

[exwestracer]

:hail: thanks to fill my lack of knowledge about those carbs.....

How is it possible to check if they work? it seems that the car doesn't pull as i kick down or open wide.
I don't want to drive it without the front hood and air filter to see it in "live" :lol:

David

There really isn't, because you'd have to leave the engine at WOT long enough to get airflow up through the secondaries. I wouldn't recommend that. If the engine isn't a monster and the brakes are good, you might be able to brake-stand it long enough (if it's an auto).

Easiest thing to check is whether the secondaries are opening fully when you floor it. That's a common problem with Q jets, as the big barrels don't start to open until very late in the throttle travel, then have to catch up when the pedal hits the floor. If your linkage isn't right, the secondaries won't open fully.

Open the throttle fully by hand, push the air doors open and look in to see if the blades are straight up. Then push the pedal all the way down and have someone check again. As long as the air doors move freely, they should work fine on their own.

[Macbuick]

So, if i understand well : if the blades are fully opened by the linkage, the air doors should open easily, ....is it by the vacuum/airflow created by the engine when running?

I noticed that my air door linkage was stuck ! Perhaps was the problem there!?

Thanks again
David

[exwestracer]

So, if i understand well : if the blades are fully opened by the linkage, the air doors should open easily, ....is it by the vacuum/airflow created by the engine when running?

I noticed that my air door linkage was stuck ! Perhaps was the problem there!?

Thanks again
David

There is a piece of linkage that holds the air doors shut unless the secondaries are open. You have to have the throttle blades WOT (or close)for the air doors to unlock. From there they should operate freely. If the throttle is WO and the air doors are still stuck, yes you have a problem.