First off, buying a rebuilt is hit and miss. I've fixed plenty of rebuilt and recalibrated Qjets from reputable builders that had a lot of problems with them. Did you remove the carb to adjust the secondary throttle plates? Did you turn the carb upside down? If you did the needle might have dislodged in the seat. When it is idling look down the carb throat and see if fuel is spilling into the engine.

 

If the little arm behind the choke linkage doesn’t drop down when the choke opens all the way then the secondaries won’t open Quadrajet carbs are designed to not allow the secondaries at all until the choke is fully open. I’m concerned that your understanding of a Quadrajet is too limited for you to be making adjustments. I’ve been running Quadrajet carbs since 1971 and have always enjoyed success in the operation of them. There is a sequence of 14 critical adjustments to be made when rebuilding a Quadrajet. If everything is done right they will give you many years of trouble free operation. I ran one on my 78 K-10 for 16 trouble free years and it was still flawless when I traded the truck. Sorry to hear that you took such drastic measures with yours. Suspect that the secondary throttle plates are no longer closing tightly.

 

What did you do to ," get them to open" ? ( I'm almost afraid to ask )..

 

Me too!
Just a little heavy handed bending of linkage with a big pair of pliers perhaps.

 

1) yes, reman carbs SUCK, as i now realize, but that's another topic.

2) No, i did not take the carb off the engine to adjust the secondary opening; i used needle nose pliers to bend the contact tab from the primary throttle arm which contacts the secondary throttle linkage. Can post a pic if necessary.

i'm now leaning towards the secondary plates not closing completely. What else could it be? Because that's the only thing that was changed?

 

I think you answered your own question.
Other possibility is trash in the needle.

 

Picture would be helpful..
Needle & seat leaking , PRI. Power valve sticking , metering rods sticking , bent , secondary air flaps not closing leaving sec metering rods up , sec throttle blades not closing ...

 

I went thru a Q-jet for a 69 BBC Corvette that our friend was having issues with. It was a purchased rebuild from some Corvette jobber. As it turns out it was a combination of different carbs. The piston didn’t even contact the cylinder walls. Meter rods were wong. I was able to come up with correct pieces required. As a result of the mismatched parts the wrong main gasket was also used also contributing to the issues. Holes and oriface‘s didn’t match up. Once correct parts and gasket were assembled it’s running fine.

So from this one experience would I ever seek a commercial rebuild carburetor from who knows were? No definite no.

 

This is kind of a double blind test. While adjusting the pull rod so the secondaries are fully open with the primaries,,,, you also have to check that they fully close.

Bogie

 

The secondaries do close, so i'm baffled at this point.

i will try trash in the needle+seat (and i doubt that is the problem either) next.

It really can only be like 4 things. The idle tubes couldn't have gotten clogged that fast from monday night to tuesday morning?

 

Power valve and primary needles move freely---tested thru the vent.

Float was 12/32 per spec, i even lowered it to 18/32 (ok per specs) and no change.

i am happy, and surprised at the increased performance with the secondaries, but that's not important now----if i can't get it to idle and re-start.....

 

I've seen emulsion tubes fall out !

 

If it's flooding, then fuel level is too high, or air bleeds are clogged. Check float to see if it's gone bad suddenly. Give it a aerosol overhaul and blow out the bleeds and pull the idle screws and blow car cleaner through the screw holes

 

i think i found it: The choke wasn't opening; The plastic plug part broke off and i was just using the metal clip part----not the right way to do it. i may have jarred it when adjusting the secondaries OR extra vibration from the spirited testing may have moved it. So that's likely why the symptoms suddenly happened after monday night...?

Apparently an engine can run rich at higher rpms, not so well under 800rpm?

How stupid can you get.....

Sorry for wasting everyone's time.

i will order a proper plug end from Summit and do it the right way.

 

As I mentioned in my extended response. The choke must open completely for the secondaries to open. If you have tried to defeat this feature by bending the linkage, try your best to restore the bends to original. Then pursue getting the choke fully operational including making sure that the little weighted arm in the choke linkage drops down when the choke is full open. This is necessary for the secondaries to open.

 

No need to be sorry, it all got you looking around, I assumed, as everyone else I suspect that the choke was OK giver you were in there. I been through more than one rain dance only to find it was something simple stupid that got overlooked.

Bogie

 

Another problem in addition to the ones i listed above is since q-jets haven't been produced for 30-40 years, we only have salvage yard cores or other sources for original cores, such as sitting on someone's shelf. The reman cores have, for the most part, been ruined.

At least with holley you can get a 100% brand new carb----well, if they would carry them; i think holley too realizes the demand for spread bores is dropping......

One source for a good core that hasn't been snatched up yet MIGHT be the 4.3 262cid v-6 q-jet. i think this one is actually rated at 850cfm. These seem plentiful in salvage yards. The trucks, astro and full sized vans had these for two years.

 

I have a local carburetor shop that does a great job on them. Myself, I am more miss than hit at rebuilding them. The time it takes me to get one back together and sorted, I can make the $$ to have the shop do it. Carry the unit in, get it back and bring the car back so they can tune it. Everyone wins.

 

If the top of the carb is wet more than likely the needle & seat is leaking , either from dirt , damage , mal adjustment , bad float , something ! The top of the carb should be dry ! I don't know where you're located , but you need to find a scrap yard that doesn't crush everything older than 2010 ...other than the internal choke lever & link , qjets aren't any more difficult than any other carb IMO...

 

Another problem that I have run into is the power valve piston sticking in the up position. This can happen for various reasons, most often gumming up of the power valve piston or bore, sometimes caused by carbon buildup, but can also be caused by the carb being on the shelf for an extended period of time. Wide open throttle drops engine vacuum and the spring under the power piston pushes the piston all the way up raising the metering rods which allows more fuel to flow into the primary venturies, which would produce the exact problem that you are experiencing. You have to pull the top of the carburetor off of the carburetor to access the power valve piston. I’ve only had this happen a few times, but it does happen. As I said, the symptoms of a power valve piston stuck in the up position ARE EXACTLY what you are experiencing. It’s an easy fix for someone experienced with Quadrajet carbs. Let us know.

 

I don’t need to pull the carb lid off to check power piston movement.
I just use a small screwdriver and check for movement thru the vent tube.
Small pencil works too.

 

I don’t need to pull the carb lid off to check power piston movement.
I just use a small screwdriver and check for movement thru the vent tube.
Small pencil works too.
That’s great. I wasn’t sure if your carburetor had that access. Keep in mind that it has been almost 11 years since I last worked on a Quadrajet carburetor. With the engine not running you should be able to push the piston down and the spring will push the piston back up, if it isn’t stuck.

 

If it’s leaking out of the vents around gaskets and the throttle shaft then that is float bowl overflow from the fill valve not shutting the flow into the bowl off.

Possible causes :

• Fuel pressure too high, 5 to 7 psi is plenty.
• Float level set incorrectly.
• Float is leaking (brass) or soaking (plastic) fuel making it heavy and will not shut the inlet valve.
• The metal tang that hinges the float and controls the inlet needle valve is loose at its attachment to the float.
• The shut off valve is leaking across its seat this can be the valve body or the needle.
• The gasket/washer that goes under the valve body is missing or the sealing surface of the carb body, washer, or valve body is marred or dirty.

There are a lot of combinations of parts out there this carb being front line production over many GM car lines and several other companies used this carb on some models including American Motors, Chrysler and Ford. GM alone used about 7 different floats. Meeting production criteria is not the same as Holley or Edelbrock supplies to the hot rod community.

Bogie

 

Should the plastic O ring go above or below the gasket?

 

the plastic ring should press fit into the housing---stake it lightly if you have to.
gasket goes on top.

 

the plastic ring should press fit into the housing---stake it lightly if you have to.
gasket goes on top.

Uh oh.............

So, just to be clear, the white, plastic ring should be flush with the fuel baffle (usually black)?

 

Should look like this, except White

Quadrajet Power Piston (Page 1) - Line.17QQ.com

This page is about Quadrajet Power Piston,contains Used Power Piston, Longer Pin,Power Piston, Used, later style,Power Valve Assembly,Power Piston Spring Assortment and more...

line.17qq.com

 

Ok. i took the horn off and the o ring worked itself below the gasket (i had installed it ABOVE the gasket---because that's how i found it the first time not thinking it was right, but not knowing better....) Perhaps the airhorn pushed it down when i re-installed it?

Anyways, the metering rods themselves seem to be working right, so i don't think that was the problem.

i think i'm going to order a rebuild kit and do a quick rebuild, then measure pressure---my gauge may not have been right; See this writeup----my gauge is liquid filled and i don't trust it---can't get it to zero out. Going to order a non-liquid filled gauge tonight---likely the AutoGage #2311.

https://aeromotiveinc.com/wp-content/uploads/2014/12/TB_903_Liquid_Filled_Pressure_Gauges_01.pdf

i'd like to install a new needle/seat/float, and new gaskets and make sure the idle passages are not plugged although i don't see how they could get plugged in less than 24 hours... Then test for leaks and fuel pressure. If that STILL doesn't work then try another q-jet, preferrably from a good rebuilder such as Cliff Ruggles. If THAT still doesn't work (can't imagine it won't), then move to a 4175 because i think i'm eventually going to go holley anyways since i like them too.

i'll start taking pics. icloud wasn't letting me transfer pictures from phone to computer, but i fixed it.

 

Are you able to determine fuel pressure ? past 5-7 pounds the pressure can push past the needle and seat.
when you use an electric pump you should incorporate a pressure regulator and a fuel pressure gauge to make sure you are not overflowing past the needle and seat.......... just my 2 cents

 

Are you able to determine fuel pressure ? past 5-7 pounds the pressure can push past the needle and seat.
when you use an electric pump you should incorporate a pressure regulator and a fuel pressure gauge to make sure you are not overflowing past the needle and seat.......... just my 2 cents

The more i think about it, i'm thinking pressure may be the problem----i don't see what else it could be. When i was using a mechanical pump, i don't recall any problems. It's only when i switched to electric then i've got runablilty problems.

As you may know, chevy stopped putting the fuel pump hole and fuel pump mounting holes in blocks after around 87----and i'm running an 87 block wherase previously an 86.

 

Not totally sure about this block, but I did the same thing and just a heads up it may be externally balanced and some of your rough running may be an imbalance if and in the event you may have an externally balanced engine and not realized it

 

This 4.3 v-6 does NOT have a balance shaft and yes it does NOT idle smooth, but this is NOT the problem.....

 

Your thoughts on:

Larger needle+seat being riskier towards flooding? Or does it not matter how big as long as the needle, seat and float are good and fuel pressure is correct?