Technical Quadrajet - idle mixture screws do nothing

Here is info on primary metering rod height setting. If this quality is not adequate, PM me your email address and I will send you the .pdf file.

 

Get ready for a novel, everyone.

I bought it as a STOCK reman quite a few years ago, I don't remember what company rebuilt it. It definitely seems like a reman and not a new knockoff, as there's small nicks and scratches you'd see in an older carb, that I know for a fact I did NOT put there.

Here's the stuff you asked about:
At idle, it had 16-17" of vacuum. I ***ume the reason is the cam, as the known good qjet had the same readings at idle and performed fine.

I can't seem to find any ID numbers on the carb, so I am unsure of which one I have. Is there a way to ID them? I've included a picture of the back. Hopefully that helps.

The front jets are #75

I did pull the float again and it DOES float in gasoline. Actually kinda looks like a fishing bobber.

And no, there appears to be no aftermarket retainer.

Also, I cannot for the life of me figure out what TQ means.

I'm mostly new to all this, having only rebuilt a 1 barrel Carter and an Edelbrock 1405 in the past, so I'm sorry if there's any misinfo or mistakes in what I said.

I will order a new float anyway, but I would like to continue troubleshooting while it ships.
SO FAR, I have:
Taken apart and cleaned the carburetor
adjusted the APT yoke
drilled and tapped an 1/8" NPT plug hole for the APT
adjusted the float level to ~.400" (slipped my mind to measure it before readjustment, but it was higher before). That's it so far.

I never noticed any vapor coming out, and it was NEVER hard to restart hot. Besides the EXTREME richness, this carb gave an excellent throttle response and had excellent driveability. You wouldn't guess anything was wrong until you saw the fouled spark plugs, felt your eyes burning, or tried to turn the idle mixture screws.

I'll grab a picture once the carb is back together and post it.

 

Excellent. Thanks for posting it.

 

Quadrajet identification numbers are on the carb body on the throttle linkage side of the carb.

Here is a site where you can then identify which one you have
https://www.chevy********.com/tech-stories/choose-your-quadrajet-number-identification-guide/

 

TQ means Thermoquad. The Carter spread bore carb that was used in the mid 70s on many Mopar engines. It has a plastic center body, to insulate from heat, that's where the name comes from.

can you take a picture of your carb so we can see the whole thing, put together? The RP logo on it looks like one of the later ones. Also look for a circle with a year number in it, and dots around it. Might be on the base throttle body casting.

 

Ah, if that's the case, mine has been filed/ground off, for whatever reason.

 

Everything is back together. The idle mixture screws are 2 1/2 turns out. Gonna go install it on the car now and see what happens

And correct me if I'm wrong, but doesn't the air horn look like it originally went to a CCC quadrajet? I thought that 'blanking plate'(bottom left when you're looking at the front of the carb) was where one of the sensors/solenoid went.

 

Is this what you're talking about?

 

yeah, could be 1985. It looks too new to be 1975, since it does have the solenoid block off. They used non computer carbs on heavier duty engines, like one ton trucks, up to 86 I believe. Every thing on it looks 1980s to me.

 

Yepper. Makes the APT adjustable from the outside without tearing top off.

 

The idle mixture screws actually respond now! They're about 4 turns out currently, is that a bit much or is it just me?

New symptom. A loud, regular pop through the carb. Vacuum gauge drops in sync with said pop. Infact the vacuum gauge reading is so erratic now it's almost impossible to read. Timing seems to have no effect on this pop.

Thoughts?

Edit: actually, I'm not sure where the pop is coming from, but there is a backfire through the carb when you whack the gas. Timing also doesn't seem to affect it.

 

A lot of the imitations had no stamped identification number on the side.

With no disrespect intended, I will bow out of the thread.

Jon.

 

Don't drill those "holes" where the metering rods go (aka: jets) If you're too rich now, opening those larger will only make it richer.
Make sure the metering rods are all the way in to the thickest stage of the rods when at idle.
If you don't have that "free-floating ring" snapped in all the way down secure, the metering rods are not doing a thing. You might as well have them laying on your workbench.

 

Doesn't the gasket and carb top hold the APT valve in place?

If you remove the plug in the top cover, you should be able to determine if the APT is all the way down at idle?

Just a couple thoughts.

 

The rich idle is solved now, I believe the carb is good to go besides tuning now. That popping sound was exhaust making it into the intake manifold. Pulled off a valve cover as I suspected a dead cylinder. Exhaust lobe on #7 has decided to leave this earth.

Thanks for all the help everyone. When I get a new cam in at least the motor won't be running rich anymore. Ha.

I restaked the hole and that ring sits in there nice and snug now, and the motor WAS responding to adjustments on everything before that dreaded pop started.

 

well that's a bummer

 

*It's been a long time since I've worked on quadrajets so my advice is subject to correction.

 

That's a vapor recovery valve that has been removed..I didn't think Chevy had them but it might be a Cad carb.
I believe it has to be pre- '75.
Replacement bowls don't have a vertical number on the left side..Kind rare though, and appears to be a beat up rebuilder core.
A lot of "rebuilders" never remove the APT, so adjustment stays at factory setting.
I didn't ask specifically but the jets seem a bit large for a 318 . Depends on rod tip though...should be roughly .030 smaller.
You can check the rod hanger at idle through the front vent hole. Put a tune up screwdriver down it and on the hanger. It should be all the way down at idle. Blip the gas and it should jump up.
Good idea, Jon ;-)

 

Love it. Made me laugh. Going to order a boroscope to look at the cam lobe. I'm hoping it's a collapsed lifter and not a wiped lobe. That lifter is the only one I can pump down by hand with the pushrod. All the others are fully pumped and rock solid. If I continue to have problems with the carb itself I know where to go.

 

with torx screws? It sure looks like a late model Chevy carb...this is for an 85 C30 big block. Not exact, but pretty close.

 

I'm not sure how a boroscope is going to view a cam line on a sealed engine.

I'd drain the coolant and pull the intake manifold. Whether it's the cam or lifter, the intake needs to come off.

First, be sure it's not the valve spring.

 

If the lifter is 'pumped' with little slack but can be pushed down with much less resistance than the others.......
that does sound like a failed lifter. A cam lobe won't "push down".
You can check a cam lobe by bumping the engine a little at a time for 2 rotations or more while watching the pushrod with the valve cover off. This will be viewable even if the lifter is collapsed. You can also pull the plugs and rotate the engine while observing the pushrod(s).

 

I'll check the valve spring tomorrow for travel. Chrysler LA smallblocks have a nice "viewing" window that'll allow someone to observe the cam lobes(see pic at bottom, observe lifter valley). I know for a fact I can get the lifter out through the head(see second pic at bottom), This is also where I'll be routing the scope.

I did crank the motor over some to watch the exhaust lifter and compared it to another cylinder's lifter that actually pumped up. It's a little hard to tell just looking through the heads with a flashlight, but they do appear to be moving to roughly the same height. The thing that has me worried is, when I pulled the valve cover, the pushrod was loose when the rocker rail was still bolted on. There is still a little spring left in the lifter plunger which should have been enough to take the lash out. I guess I'll find out if it's the cam or not tomorrow with the scope. Really don't wanna pull apart the motor if the cam lobe is still OK.

firetruck spelled backwards is also firetruck.

 

My bad, I forgot it was an LA and not a SBC.

 

My bet is a bad float as suggested . I’m not a Chebbie person anymore . All of the previous ones I have owned , were powered by the mighty QJet . Light years ahead of most of the carbs on the market even today . Most misunderstood and incorrectly tuned carb ever . You fine someone near you to install correct low speed tubes , for your idle speed , metering rods and spring for the vacuum you have and you will never look at another carb . Just my 2 cents .

 

I believe you are correct, Jim. I was looking at the front view .etc. Late looking choke pulloffs and style of choke. Non-computer controlled.
I saw the torx but seeing it went through a rebuilder anything is possible there.
Wonder why it doesn't have an 8 digit number on the side,..or any number?
Did any BBC's have the vapor valve that was done away with?
It has to use a vacuum enrichment system with that style top...I think ;-)

 

Reworked right they are hard to beat for a street engine, Problem is most people don't understand them or know how to work on them

 

318 Q-jet..Side inlet .. Didn't know that.
It doesn't have the GM style stud for the cable.
Doubt it would have a GM part number on the side though.

 

ground off by rebuilder, who knows why?