When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Secondaries don't want to open much on Rochester 4-barrel
I rebuilt my carb after buying my '72 98 about 3 months ago. Thought I did a good job and put it back together properly, but ...it seems the arm that controls/opens the secondaries - connected to that vacuum actuated dampener - is hitting the back side of the choke housing. Can anyone tell what I've done wrong?
These are the secondary air valves. They open in response to the airflow needs of the engine. They will NOT open much if you are simply revving the engine in PARK. More people have needlessly lost sleep over this than nearly any other perceived problem.
The rod that you show is actually keeping the air valve closed until vacuum drops and the choke pull off relaxes. Keeping valve from flopping open to quick.
As Joe said, airvalve opens when needed and if set up right you shouldnt hardly feel the transition from closed to open.
More likely to hear it than feel it on a stocker.
And that would only be with a load on it.
Thank you both for educating me on how the carb works re the secondaries. But I’m still confused/concerned that the arm attached to the …shiny vacuum dashpot(?) is still hitting/rubbing on the back of the choke housing - so no matter How the secondaries are actuated - they’re not going to function properly if the arm isn’t able to move freely.
Bend it some till it clears. You will still have to bend else where to adjust.
You want the pulloff/dashpot to hold it shut when running at idle.
The pulloff then releases when vacuum drops and allows air valve to open when needed and at a controled rate.
That also works WITH the air valve spring setting to prevent it from flopping open to soon.
Oooooo. Or does it PULL the arm vs push it????? If it pulls, it should be fine. I’ll go check myself. Thx all
That rod and diaphragm is a dashpot that slows the opening of the secondary air valves to match the needs of the engine. The diaphragm is connected to manifold vacuum. High vacuum indicates light engine loads so the sedondary air valves are delayed in opening to match engine airflow needs. Low vacuum indicates wide open throttle, so the diaphragm isn't holding the air valves and they can open quickly to support engine airflow needs. The Qjet is arguably the most sophisticated 4bbl ever developed, and the complexity of these systems is why people without the patience or skill to work on them will crap on them. Read the carb section of the Chassis Service Manual, which provides pages and pages that describe the operation of all the systems in the carb.
Don't bend anything until you know how the system works. The rod didn't contact anything when it left the factory and won't now. Bending is only required to set the gap at the air valve slot. READ the CSM and FOLLOW the directions. Don't randomly bend stuff.
don't bend anything until you know how the system works. The rod didn't contact anything when it left the factory and won't now. Bending is only required to set the gap at the air valve slot. Read the csm and follow the directions. Don't randomly bend stuff.
The clearance provided between the Vacuum Break Control Rod (58) & the choke housing is roughly the diameter of a gnat's ***. These are images of a perfectly tuned & running carburetor before I elected to freshen it up w/ a rebuild. The Dashpot is actually referred to as the Vacuum Break Diaphragm Assembly (Dashpot for short). The Vacuum Break Control Rod (58) should not be hitting (running into) the carburetor housing. Do check to ensure you have adjusted the Vacuum Break Diaphragm Assembly (Dashpot) correctly & according to specifications (noted in the diagram I am providing here). If the Control Rod was working well before it was removed, then you removed it, and now it's hitting - again, ensure you did adjust it correctly - there is an established tolerance setting (noted in the diagram). It may be the case you just need to tweak (tighten, loosen, etc.) something - possibly the Vacuum Break Control Bracket isn't correctly sliding into place. Again, if it was working fine before you took it off, don't bend anything until you've exhausted other possibilities. People often go about bending carburetor linkages without understanding the purpose - be judicious and employ some caution. The tolerance is close to the diameter of a gnat's ***. Here are some images which may assist you and the diagram on adjustment(s). Good Luck.
Pictures (often) tell a thousand stories...I don't have time to decide which picture(s) are best or assign arrows, etc. - pick the several which work for you.
Follow Joe's advice and don't get bent out of shape. There have been a couple times in my life those air doors (or whatever they are called) have given me a fit. Years ago I bought a 79 Cadillac from the original owner who were an elderly couple. That thing would bog when you put your foot in it. I overhauled the Qjet and it still bogged. This was in the days before the internet and websites like this. Finally I read about adjusting the tension on the air doors. Fiddled with that and solved the problem. I never could get it right on the Qjet on my 75 Olds and finally had somebody else rebuild the carb. It was money well spent. Later he and I both found out the 75 has a one year only carb that Cliff Ruggles said he would not even fool with. It works pretty good but kind of has a mind of its own.
Oooooo. Or does it PULL the arm vs push it????? If it pulls, it should be fine. I’ll go check myself. Thx all
I’ll explain what the problem is with your set up since all the experts missed it
your dashpot bracket is either the wrong one or it’s bent…it looks bent to me. Because it’s bent downward , the dashpot arm is hitting the choke housing when the dashpot is fully extended,, it’s not 90deg to the dashpot housing.
its suppose to travel in and out at 90deg…. Through its full extension..not get jammed up at a different angle because it’s hitting the choke housing
look at this angle..it’s correct. The arm is 90deg to the dashpot. And then yours under full extension, it’s crooked
Last edited by CANADIANOLDS; July 20th, 2023 at 12:05 PM.
That rod and diaphragm is a dashpot that slows the opening of the secondary air valves to match the needs of the engine. The diaphragm is connected to manifold vacuum. High vacuum indicates light engine loads so the sedondary air valves are delayed in opening to match engine airflow needs. Low vacuum indicates wide open throttle, so the diaphragm isn't holding the air valves and they can open quickly to support engine airflow needs. The Qjet is arguably the most sophisticated 4bbl ever developed, and the complexity of these systems is why people without the patience or skill to work on them will crap on them. Read the carb section of the Chassis Service Manual, which provides pages and pages that describe the operation of all the systems in the carb.
July 19th, 2023, 02:17 PM
