4bbl carb disable the secondaries

This is a Rochester Quadrajet. The vacuum diaphragm is a delay mechanism to keep the back two barrels from opening too soon and bogging the engine down. You can zip tie through the part that comes out of it to keep it retracted all the time. The bottom butterflies will still open, but the top air valve will stay shut, and the venturies will see no airflow, therefore no (or VERY little)fuel flow in the back barrels. However, your engine may run very lean during high demand, as the front jets cannot flow enough to feed the engine properly at times of high demand.

Doing this WILL NOT increase mileage as much as you think. It's better to change driving habits to keep from opening the back barrels as often. Also, talk to a local gearhead about recurving the distributor, keep it tuned properly, blah, blah.. Before any carb work on a Q-jet, replace the float, set it properly, and check the bottom of the carb for leakage arount the plugs. You will also have to remove the caps for idle mixture control, as by now the engine is worn past the point that the factory sealed and set idle screws will work properly.

Why do you want to do this?

Don't depress the gas pedal all the way to the floor.

Don

This is a really bad idea. If you try it you will find that the car belches black smoke from running very rich with full manifold vacuum imposed on the secondary jets. Take my word for it, if you even adjust the spring too tight the car will flood. You are correct that there will be little air flow but the will be LOTS of fuel flow as the secondaries are essentially fully choked.

Don

I agree with you fully here.

The carb uses fuel, has to be rebuilt every year because the power pin sticks and the jets have to be rejetted in order to get proper fuel milage.

The engine does not have many miles on it.

I am looking for an alternative to the 4bbl quad.

Thanks > chevyeng> > I would like to disable the secondaries on the 1979 Chevy 4bbl carb. I

Well if the carb is the problem ... has to be reuilt every year ???, disabling the secondaries e.g. insert a thin metal plate between the carb and the manifold, is not going to fix that. How about a new_to_you manifold (2 bbl) c/w 2 bbl carb ... or fuel injection?

Dang. What a brain fart I had. I forgot about engine vacuum, I was just thinking about airflow through the venturies. You are correct. Any opening of the secondary butterflies without opening the air doors will suck fuel. Good call Don.

I have looked for 2bbl carb for 1979 Caprice and have not found any. Then putting fuel injection on the car sounds expensive.

Would you happen to know the approx cost for intake and 2bbl carb and the approx. cost of fuel injection?

The car body otherwise is in very good condition.

bowgus wrote:

Just remove the link rod between the primary and secondary linkages...then they are inoperative.

one of the new Edelbrock AFB's or AVS's might make a nice choice. Holley makes a spreadbore that should theoretically be closer to your old QJ in terms of throttle response and economy... but it's a Holley. tuning it could become a hobby in and of itself. Alternately you could have your present carb completely gone through instead of just kitted by a carb restoration shop, in which case it will probably be good for another 200K miles or so.

good luck,

nate

chevyeng> The carb uses fuel, has to be rebuilt every year because the power pin

I see linkage on the driver sider at the bottotm of the carb. It looks like the crab might need to be removed from the car to remove the linkage. There looks like there is a spring assembly where the linkage is. If the linkage is removed the spring assembly would need to be reassembled?

Thanks > Just remove the link rod between the primary and secondary

It's been a while...but yes, I think you have to pull the carb off the manifold, flip it over and perhaps remove the throttle plate (bottom section) to be able to disengage the linkage...they have a little ear on the ends that will only allow removal from certain angles...that is without bending the rod, which you can do if you don't want to mess with all that, but no guarantee the rod can be straightened correctly if you need it again.

I'm trying to think which spring you see...if it returns the secondaries to rest position after opening, it has to be there to keep them closed. If there is no spring tension closing them with the link removed, you'll have to block the secondary throttle plate closed however you can, or they'll try to slowly fall open, and cause flooding...

The link in blue is the one you want to drop...there may be one side or the other a screw holding the bellcrank (the part the link fits into), so you can remove the bellcrank, move it around to where you can disengage the link, then put it back on...like I said it's been too long, you'll have to look.

Is the red circle the spring you spoke of? That's part of the cross shaft, it stays put...

Make sure you completely remove the link, don't just drop one end, or it will jam your throttle open (don't ask how I know this:banghead: :lol: )

The red circle is what I was talking about.

jeffcoslacker wrote:

Wrong, wrong, and WRONG.

Disabling the secondary air-door WITHOUT disabling the butterflies below it will result in, effectively, a closed choke on the secondaries. Massively over-rich, huge clouds of black smoke, enormous fuel consumption, fouled plugs, etc.

If you want to disable the secondaries (why on earth WOULD you? It won't really help mileage and will remove your ability to safely merge on an interstate) the way to do it is remove the linkage from the main butterflies to the secondary butterflies so that the secondaries never open.

It's not as bad as you might think...we had two wreckers at the shop that had secondaried disabled (because the boss was the cheapest bastard you ever met), and both would tow adequately, albiet not very swiftly...but when I opened them up on a trial basis, fuel mileage plummeted, and he made me change them back...Tried to convince him that a large 2 bbl would be a good compromise, but that would cost MONEY, see, and well, that just wasn't going to happen...

At least it was good practice. Towing a full sized conversion van with a 350 breathing through the primaries of a Q-jet only was very much like driving the 80,000 lb GVWR rigs I currently run...very slow accelerating and use your gears wisely...:lol: