Technical Quadrajet - idle mixture screws do nothing

Hey all,

I have a 318 LA with a Edelbrock 4bbl intake and Rochester quadrajet. It has an extremely rich, eye burning idle, with the idle mixture screws having absolutely no effect on vacuum readings or idle speed(but it does seem to idle smoother with them turned all the way in). The only way to adjust the idle speed is with the throttle adjustment screw. The motor is running so rich that it's fouling it's plugs with thick, black soot. Here's a few things I've observed during troubleshooting.

- I'm 400% certain it's not a vacuum leak on the motor side, nor is it a timing issue. I put a known good quadrajet from a Chevy 305 onto the 318 and it responded like a proper carb should. Mixture screws actually having a noticable effect on idle speed and vacuum readings, etc.

- I pulled apart the ailing quadrajet expecting to see dirt clogging the idle circuit(despite it being relatively new), but I found it was basically spotless inside. I found no clogged p***ages and the jets and needles were pristine.

I can't find any reason for the idle circuit to refuse to work. Any ideas anyone? I currently have the entire carb blown apart on the bench and plan to re-epoxy the well plugs while it's apart.

 

I should also mention the 318 is freshly rebuilt. .020" over Pistons, Hughes whiplash cam, mini headers, everything else essentially stock. Vacuum readings are on the low side of 'normal' for a stock engine(16-17") with both carbs.

 

Are the front metering rods moving up and down freely? They should be in the down position at an idle and venturi vacuum lifts them to enrichen the main jets. If they are stuck in the up position it will be rich all the time. There is an adjustment on the yoke that holds them to adjust how far down they move, it is to fine tune how far out of the main jets they are in relation to vacuum. If that has been messed with it is pretty hard to get it adjusted correctly. There is a possibility the main body casting has some pin holes in it allowing fuel to leak out. You may never find a pinhle, but I have replaced a few I suspected of that and fixed them.

 

It was pretty common for the plastic float to "sink"....it was a good idea when the carbs got to be about ten years old or so, to put a new float in with the carb kit. High fuel level will make it always rich.

edit: and the well plugs under the middle of the main body, might need epoxy, or the foam thing that is supposed to seal them up

 

I've heard that well plug leak thing only applies to early (1960's) quadrajet.
The early and later carbs have different shaped base plates, but identical bolt pattern. A late carb on early manifold can vac leak if the correct under carb gasket is not used.
I use an early gasket on early manifold, then a 1/4" phenolic spacer, then late style base gasket, then my late model q'jet.

Just throwing out ideas.
Let's see if @carbking has any suggestions.

 

They do move freely. Everything is clean inside the carb. Is there a ballpark I should shoot for in the number of turns for that adjustment yoke? The carb always ran a little rich, but I chocked the extra carbon in the exhaust to oil burning before I rebuilt the motor. but now running it inside my garage for extended periods, that characteristic eye burning idle is unmistakable. And I certainly hope it's not a pinhole.

I did just submerge the float in a bowl full of hot water for a few minutes. It still seems to be sealing/floating with nothing swishing inside when shaken, and this carb is only a 3 or 4 years old. It was bought as a reman and sat on my shelf for a year or so before I finally used it for the 318. I do plan on re-epoxying the plugs, but to the best of my eyesight I couldn't see them leaking when I sprayed carb cleaner in the bowls and watched.

 

Here's a shot of the front metering rods. There's a little free floating ring around the top of the plunger. If it was rubber I'd say it's pretty well petrified now. Can anyone clue me in on it's purpose?

 

At idle, do you see fuel dripping from the main fuel nozzles on the primary side? Or the secondary as well for that matter.
Are the secondary throttle blades fully closed at idle?

 

Are the metering rod tips bent or broken?

 

That free floating ring gets pressed down into the body of the carb and friction fit holds it there.

Looks like someone has already smacked the body with a chisel in order to tighten up the hole in hopes of get the floating ring to stay in position.

It probably just popped out when you removed the gasket.

It is shown at 10:54 in this video ...

 

I had a quick peek down the carb before I took it off and opened it up and don't recall seeing anything of the sort. I didn't look down the secondaries, I do know the secondaries were fully closed though.

Here's a picture of the metering rods. They look to be in good shape.

 

I was backwards on my vacuum statement. Vacuum pulls that yoke down at idle. My bad.
With the ring seated into the body push down on the yoke and see if the largest step on the rods just enters the hole in the main jet. if it does, it is real close. The adjuster is that little flat sided part in the hole just in front of the hole where the yoke slides in. There is a measurement for that but it varies by application.

 

Oh just put a Thermal quad on it..

 

APT. Adjustable part throttle. 3 1/2 turns up from bottomed out. Knock aluminum plug out of air horn and tap with 1/8 NPT. Once back in service bring RPMS to around 2000 and cup your hand over primaries.Looking for a small rise in RPMS. ABOUT 50-100. Drive and fine tune. Could take a while.
Re tune idle mixture after the 'tip in' procedure.

 

If I understood that right, then it would appear that the yoke needs some adjustment. Even at the bottom most end of travel, those shoulders are nowhere near entering. It was hard to get the pics, but here they are.
Top is bottom most travel, bottom is not touching the yoke or plunger at all, so topmost, and it seems like they're favoring one side of the jets. Is it alright to bend the yoke into adjustment or is that a no no?

 

Har har har

 

I think I'll do just that, thanks. The idea of having to pop the top off again just to retune makes me cringe.

 

Really insufficient information.

Best GUESS based on what information is posted would be a defective plastic float. The br*** floats can be tested in HOT water. Waste of time to even think about considering the possibility of testing a plastic float. Just replace it every time you open the carburetor.

Jon.

 

I know several GM guys who like the Quad.....I simply can't get past when I was in high school and we referred to them as the QuadraBOG.

 

yeah, the guys who knew nothing about carbs didn't like them, but if you took the time to understand how they work, and set them up right (mostly that means to put them back stock, as designed), they work pretty darn good on most mild engines.

 

Make sure arms that hold primary metering rods are level and straight. You want rods to be same amount into jets wherever they are. Rods being off center will be the way it is. Spring retainer holding rods to APT will always hold them off.
If you have small drills you can drill the holes under the mixture needle to around .080. That can sometimes help.

 

I've got the new epoxy on the well plugs curing overnight. I drilled and tapped an 1/8" NPT hole in the airhorn as pav8427 suggested. I'll put it back together tomorrow to see if anything changes after screwing with the APT and blowing out all the channels with carb cleaner and compressed air. I also bent the float tang to be pointing very slightly downward, versus it's very slightly upward bend it had in it.

Prob not the best to be changing multiple things at once, but I am a very impatient man.

I do have an ***ortment of small drills, but I'll make that a last resort as everything I've done so far isn't permanent besides the APT plug installation.

If you need anymore info I'd be happy to provide it. Carb will most likely go back on the car tomorrow so there will be more information. You just need to ask because I'm not sure what would be relevant info and what wouldn't be.

 

You mean a thermo - quad , good luck finding one !

 

Not hard to find

 

You can make a APT adjusting tool by flattening a small piece of tubing down to around a 1/8" ID and test the fit prior to putting airhorn back on. when tuning screw APT in until you feel a light stumble at light throttle just of idle when taking off. Screwing APT up richens mixture and you should be able to tune out stumble over time. May take a while.
like days. The more you drive it the finer you can tune.

 

I have the APT initial adjustment process/specs. I can't post them until tomorrow.

 

The carb has a couple holes on the bottom of the main body factory sealed with deformed lead ball plugs. Common for them to leak. Suggested fix is to hi-temp epoxy them. If leaking, this would definitely cause your problem.

https://www.76vette.com/2020/02/resealing-quadrajet-well-plugs-part-2.html

 

Without a lot to go on, I'd vote saturated "heavy" float.
What's the float set at ? What jets and rods are in it?
When you shut engine off, hot, does it have gas vapor rising out of the primary side? Is it then hard to re-start? Looks like and earlier model ..not the one that actually came on some late model 318's.
After you find that it's not the well plugs leaking any significant amount...show us a full pic of the carb.

 

Rochester built Q-Jets for engines (from memory) of 230 CID through 500 CID; ALL were appoximately the same size, differing in calibration. Which one do you have?

You posted it was a rebuilt; by whom? Stock, modified? If modified; rods, jets, power spring, CPO?

Looked like in one of your pictures that the power piston retaining ring that normally is pressed into the casting wasn't; and the aftermarket retainer was not present?

And if I missed it, sorry; but what is the idle vacuum number?

Probably the worst carburetor problem I ever got into was a Pontiac Q-Jet. Some "self-educated expert" had read the book, and found there were no idle air bleeds as found in the book; so he installed them. Pontiac air bleeds are different from Chevrolet air bleeds as he installed. That carb was REALLY lean with TWO sets of air bleeds.

Q-Jets ARE great carbs when calibrated for the application, but as mentioned the TQ's are not that difficult to find. Of course, the TQ needs calibration as well.

Again, with what you have posted, I would install a new float and see what happens.

EDIT: one other question, is this a real Q-Jet or one of the imitation ones sold a few years ago? The imitations had LOADS of issues.

Jon.

 

A defective plastic float will float in water - try it in gas and it will sink!!! Been there and found out the hard way.