Rodchester 4bbl carb vac


#1

I have an 86 chevy 5L with 4bbl rebuilt Rodchester carb. The passenger side ported and non ported vacuum lines have no vac. Even when racing the throttle. My power brakes are working. I pinched off brake booster line and still notta. Also, the back barrels dont open. I’m kind of stumped. Any suggestions?


#2

You get the most vacuum when the throttle plates are closed–not when you,are racing the engine. I remember riding the school bus as a,kid. The wipers went really fast when the driver let up on the accelerator, thus closing the throttle plate. When you are braking, you take your foot off the accelerator, thus creating the higher vacuum for the brake booster. Now I,don’t know,what activates the secondary barrels on your carburetor. Maybe someone else can help you out here.


#3

Are you sure you used the right gaskets on the rebuild? Using a wrong one can cover up the internal vacuum passages to the ports.

This carb doesn’t have vacuum secondaries, it uses an air flapper valve that opens when enough airflow passes through the primaries to pull it open against the secondary spring. And that spring is adjustable, too tight and the secondaries won’t open easily. Too light and they’ll flop open and bog the engine.


#4

Vacuum lines originating from a carb can come from 3 different functional points , and each will have vacuum measuring from nothing to a lot depending on the throttle position and engine load. So we’d have to know at which of those three points those lines your refer are attached. Near the venturi , just above the throttle plate, or just below the throttle plate for example. Maybe what you are experiencing is what is supposed to occur.

The vacuum for the brake booster usually comes from the intake manifold, not the carb. But it might come from the carb just below the throttle valves too I suppose.

I think if I had this problem I’d take a visit to my local Barnes and Noble bookstore. In the transportation section they have a pretty good technical book on how they work and how to rebuild Rochester carbs. I think that was the title of the book I saw there. No harm done to take a look on your next visit anyway.


#5

A 1986 model Quadrajet can be a nightmare with it’s feedback (computer controlled) fuel metering system…These smog monsters had miles of vacuum lines controlling the ignition advance and smog pump and transmission shifting…Mustangman is correct about the secondary throttle bores. They open as needed…If you adjust the barn door to open sooner, you will turn your Quadrajet into a Quadrabog…


#6

Does the engine idle ok? Ported vacuum is near impossible to determine with a fingertip. Mustangman has a great point about the gaskets also. I seem to remember that some carb kits have several additional gaskets so as to allow fitment on different models of QJs.

As to the secondaries not opening, the choke must be fully open and any choke linkage adjusted correctly as a linkage problem can keep the secondaries closed.
My fuzzy memory seems to recall that some or all QJs have a lockout pin of sorts in the linkage and if that pin becomes dislodged by even a 1/16 of an inch that can also lock the secondaries out.

If I had a QJ in front of me I could probably be at least somewhat precise with any info but it’s probably been 10 or 15 years since I’ve dinked around with one of those.
They’re great carbs if set up right.


#7

I was using a vacuum guage above and below the throttle plate. I didn’t rebuild it, I bought it. It didn’t idle correctly until I put manifold vacuum on the distributor.


#8

With manifold vacuum on the distributor engine power will BOGG significantly when accelerating.


#9
I didn't rebuild it, I bought it.
Jeez, you'd HOPE the rebuilders would not install the wrong gasket, but stranger things have happened. There are a great many versions of this carb. You may have to return it, or pull it apart yourself.

FWIW, some engines are happy to have the direct vacuum ported to the distributor. Most of my older cars were like that rather than vacuum only with the throttle open.

Check under the carb, there are passages that can get gummed up between the manifold and the carb body. Especially the direct vacuum port. Like this pic. Too much, or any, RTV can clog these up.


#10

Have you worked on setting the idle correctly? It may be set too low, or the mixtures may be off (get rid of the plastic limiter caps, if it has them).

Have you worked with carbs much?


#11

Not meaning to be rude (out of character for me) but using the correct spelling for Rochester carbs Google had many articles and videos. There might be one that can help.


#12

I think you’re going to have to remove the carb and take a look on the bench to resolve the lack of vacuum signal, esp from the point below the throttle plates. There should be no problem getting a big vacuum from that location on the carb at idle. There must be something plugged up. Removing the carb – given the photo above – it is just four bolts, right? Off it comes, then you can see what’s going on your work bench w/good lighting. It’s probably something very simple to fix.

I’ve got a carb problem with my Ford 302 V8 truck and just purchased a rebuild kit for it. The kit comes with 8 pages of instructions, parts list, complete ass’y diagram, how to set-up the choke, float, other stuff, a dozen or more gaskets, the power valve, idle needles, various other assorted gadgets, all this for a paltry $25. With the problems you are having, that might be a good investment to make too. Carbs are a lot more difficult for a group of people to talk about without actually having one, than just figuring them out by putting it on the work bench in front of you. It’s usually pretty obvious what needs to be done when you are staring at it on the work bench and you have the diagrams & specs for making the necessary adjustments.

My advice should be considered a “do as I say, not as I do”. Just today I removed the idle screws and fuel input fitting and sprayed them out with carb cleaner while the carb was still attached to the engine. And it was really awkward. At least I remember to wear goggles … lol …