quadrajet shenanigans

[my kids took the truck]

This thread is to collect experiences with quadrajet behavior. Not expected behavior but the non-intuitive what the... type behavior.

There are probably a ton of you that will laugh at the newbie type experiences, but hey, share some old man wisdom.

For the record, and backed by some historical context, I am almost certain the quadrajet is an unfinished, pushed to production, prototype design. Couple the 'greatest carb design' with real world GM bean counters and some 25 years of various on-the-spot last minute design remuddling, and what you are likely holding in your hands is one quirky carburetor that did not come with instructions.

 

[dyeager535]

From my dealings with them, I haven't ever experienced any WTH moments...I experienced typical carb issues (bad electric chokes, poor running due to wrong settings, worn throttle shafts, bad pulloffs, improper settings, engine modifications, etc), which is why I no longer run carbs.

The various iterations of the Q-jet makes a one-size-fits all approach impossible, which is unfortunate as you'd have to have a lot of time with each to know what does what on each one, and there aren't many people around like that anymore.

There are a couple of books on these things, (one by Doug Roe, can't recall the other) and once you start talking about APT, EGR, and stuff like that, it quickly becomes apparent that fine-tuning any carb design, let alone a Q-jet, is no easy process to do right. I'm sure plenty of people get them "good", and I think I fell into that category, but without monitoring the exhaust gas composition I'm not sure how it can ever be claimed to be "perfect". The fact that GM used as many different parts on these carbs as they did is an indication they DID spend the time to calibrate them extensively based on application, not many of us have those types of resources.

If anything, ethanol is probably throwing a few people fits. I have no idea how the float and any of the seals in the q-jet handle ethanol, but since later stuff (early GM EFI) doesn't handle it well at all, I'd expect the Q-jet to have issues, and those would probably be frustrating to deal with.

 

[my kids took the truck]

-design airflow

Yesterday, I drive down the highway in the 74 K5 and the quadrajet is pulling nearly 20 inches of mercury and seems happy at 60 mph on the level. The quadrajet is a '7' prefix 77 C10 version cfm unknown with a recent rebuild and in decent shape.

But there is a problem, the idle is erratic. At each stop light, I can roll the dice if it will settle somewhere between 600 and 2,000 rpm. If I stomp on the gas a few times, it might settle back down to 600 rpm. I isolate the problem to the primary blades binding and a vacuum leak along the front base. This is resolved easily enough with the discovery that the local shop has what looks to be holley spreadbore gaskets in the quadrajet gasket stack, and by luck I grabbed one and failed to check my gasket. A throttle blade was catching the edge of the gasket in a sissor pinch. Retune, vacume (how do you spell vacume) steady at 22 in-hg, dial in for highest vacume and idle and then find the lean drop, richen up a bit - idle screws are a quarter turn from base, dial down float level but no change, weird but works, reset idle to 600 rpm, carb cleaner spray around the base to check for leaks, and put some propane down the bore to check for a rich-drop or lean rise. No rise, no drop. Everything is dialed in.

So we hit the highway. It drives like a dog. It has 15 inches of mercury at 55 and seems to want to be at 50 mph. What the... it liked the vacuum leak? So it is rich now? Why didn't the propane give an rpm drop?

Then I remember a chevytalk post I read while trying to understand the erratic idle. It warned that the carb is designed to have air flowing through not just the bores but also the PCV, hot air choke, and vapor canister. The idle circuit band is not sized for just the bore airflow. It has a baseline airflow. So, I throw a hose from the clean air port on the back over to the hot air choke that was plugged off. I just added a 'vacuum leak'.

It loves it and drives like yesterday, 20 inches of mercury and steady at 60 mph.

Is this what it seams or did I miss something?

 

[diesel4me]

Some Q-jets had a "hot idle compensator" that is a bi-metal "spring" leaf that opens under a high temparature and opens up a vent passage,the leaf has a rubber cone thing on the underside that seals off the vent...

Maybe that thing is causing it?..

You can have "internal" vacuum leaks in carbs also..one thing I hated about rebuilding carbs,is the fact the kit gives you several different "top bowl" gaskets,and some had holes where others were blocked off,and you never could tell which of them BELONGED in your particular carb,and matching the old one up wasn't always a good idea,as someone else could have put the "wrong" gasket on,and you copy their mistake...I took one carb apart three times and tried all 3 gaskets before I found the "best" one it ran halfway decent with!..

Edit...it is true the carb is jetted "rich" at idle to compensate for the PCV valve and other things like the choke,etc you listed...

Some guys who blocked off all the "emission crap" had troubles with carbs running too rich at idle..

I recall having a few 2 bbl carbs from old small blocks that were on pre-PCV engines with road draft tubes that refused to idle smoothly and carboned up the plugs if you idled it too long..

If you look close at some Rochester 2 bbl carbs,there is actually a drilled hole in the throttle butterfly casting,a pre-determined "vacuum leak"--if you blocked it off,the carb wouldn't idle for crap..

 

[dyeager535]

I think that goes back to all the different components that were used over the years...not the carbs fault, just a reflection of the changing emissions demands, and of course GM's desire to not adopt fuel injection for as long as possible.

 

[my kids took the truck]

my rebuild kit only had two top-bowl gaskets, one was clearly wrong and the other lined up; I will look into the third gasket, sounds like a good check to make

This article in highperformancepontiac has an image and short function description of the "hot idle compensator" but not much more. The 'old car manuals project' has a diagram of the function. I will look into this one more.

 

[dyeager535]

TBI, TBI, TBI. lol

 

[my kids took the truck]

This quadrajet does not have the "hot idle compensator" it is open in the back and the port is plugged, or was not drilled. There is an amazing lack of images of the compensator. And no videos describing the function.

I think it would not apply in this case. We were driving late at night in cool bay weather. Probably mid 60 deg, and the engine was steady at 180 deg. I have no inner fenders so the engine bay is well cooled.

Over on GMCforum they have this, a quote of a quote but well explained

The hot idle compensator is a thermostatically controlled valve that helps to prevent engine stalling when idling in very hot weather. Long periods of engine idle cause an excessive amount of vaporization of gasoline in the float bowl. These vapors will find their way into the carburetor throat and cause an overly rich mixture. The hot idle compensator consists of a bimetallic strip of metal which operates a valve that controls an air passage ending under the throttle valve. The bimetallic strip, which consists of two pieces of dissimilar metal with different expansion rates, will curl upwards as the temperature increases, opening the valve. This will, in turn, admit air under the throttle valve compensating for the overly rich mixture.

I understand your saying that if the rubber stopper on the strip is missing it will suck air all the time and cause a difficult to trace problem since it pulls varying amounts of air depending on the temperature.

Yes, TBI. My 89 GMC 3/4 ton 4x4 has TBI. I spend vastly less time on it than the K5 - I couldn't even tell you what gears it has or if it has posi. It starts, I put a ton of stuff in the back, the springs sag, we go down the road, it gets unloaded, and I turn it off. I rebuilt it and the 700R4 in the most primitive of conditions and have both ran strong. They were fun. At one time I also had a well running '87 3/4 ton 2WD with the entire TBI. It was slated for the blazer. But, I gave my cousin the roller 350 4 bolt main and my cousin thought it was a good idea to sawzaw through the wire harness so he could take it off the engine since there were all these wires and plugs to undo. His mech engr brother was with him to help and was like 'what?'.

 

[Big Ray]

air leaks at the throttle shafts...there's bushing kits just for that.

Here is all the things you'll ever need to know about the venerable Quadrajet. It's a long read

linkity

p.s. here's some parts vendors

cliff's Q-jet , Q-jets.com (parts), Carbs unlimited

how to info

click 1, click 2,

There's many more out there. If you'll notice almost everyone recommends the book Rochester Carburetors by Doug Roe. It's considered to be the Q-jet Bible by many.

 

[my kids took the truck]

62_Fat_Guy - air leaks at the throttle is possible

The throttle shaft air leaks cause a lean condition. And drivability improved when I purposefully created a controlled vacuum leak by uncapping the hot air choke. When I sprayed carb cleaner on the throttle shafts there was no change in the engine RPM; a minor change on the passenger side secondary shaft but not by much.

This quadrajet has fairly tight throttle shafts. It has a holley reman sticker on it - so it passed inspection not too many miles ago. If this is the same carb, my brother-in-law used this one for all of a couple thousand miles before he swapping it for another carb.

Why do you suspect a vacuum leak is producing the observed power increase when leaned?

Edit: reading through your links, thank you for those, if the power increase on lean then it seems like an over rich condition. I am no pro so odds are I have/had the float set too high (I have tried two heights, but no change in behavior) and/or the power piston stop height is too high. Also, just filled the tank and we are getting 13mpg. And idle, off idle at stop light, cruising, acceleration, all is excellent and within the bounds of as good as I've driven in this K5 with a Holley or quadrajet.

 

[shady]

The big companies like Jet that rebuild these seal the heads of all the screws coming up through the bottom with a breakable (for future removal) epoxy. the screw heads can leak around them causing multiple drivability issues

 

[my kids took the truck]

shady_ in my case the screws do not have epoxy. They do have lock washers without the little screw gaskets. If they expoy those screws then there is a reason. Maybe I should be putting epoxy on them too?

 

[shady]

I looked again and its the casting plugs.... Theresa home fix kit for it.

 

[my kids took the truck]

yea, I read that the casting plugs can leak. I looked carefully at them. The well that sits under the main plugs was super clean, so they are tight. The front plugs look OK. But since they are exposed they had some soot on them - seemed like normal stuff and not caused by leaks.

From the apparently rich condition, the plugs are certainly on the close watch list with the float level, and now after reading through fat guys recommended library, the air horn warpage and power valve adjustment are on the list. I will check the later two before taking the plugs too seriously. I see 9/10 odds that anything adjustable is most likley where the problem is. The warpage and plugs come into play. The base plate is super flat, I checked it with a straight edge but that is no guarantee the top plate is flat.

Also, the 13mpg we just measured, about a third of the driving was with it running rich before I added the hose from the hot choke air tube to the air cleaner takeoff tube. So mpg is now slightly better. And, it is possible that since the vapor canister port on the carb is plugged, the idle circuit is still looking for the other 1/3 of the constant 'air leaks' it expects. If I open that port to a clean air source, it could lean things up and I find there is no source of excess fuel getting into things.

 

[my kids took the truck]

I made a csv spreadsheet file from the table of quadrajet models given by Lars on the digital corvettes forum that fat guy posted a link to.

Not sure what to do with it but here it is if anyone can make some use from it. In the file header I posted the breakdown structure, I cross checked with a couple webpages that provide the breakdown and it looks correct. It has a bit more detail than the explanations given elsewhere.

To use, download the text file and rename the extension from .txt to.csv then just double click the file to open with excel. If you don't have excel, there are free readers available that will open a csv file.

Edit: revised the q-jet spreadsheet to correct some errors and add more information on the secondary metering

 

[dyeager535]

That's pretty cool.

On the air horn warpage, I've seen a carb that was TERRIBLE due to someone really reefing on the screws. Enough that it made the secondary air valves stick. But I can see even just a little bit more torque than should be warping them a bit immediately around the bolt holes...something you wouldn't easily notice unless using a straight edge as you mention.

 

[my kids took the truck]

if these are worthwhile they should be posted to a thread dedicated to quadrajet secondary metering rods, I wanted to post the whole excel file but CK5 won't allow excel, but here they are, tables

I grabbed this data from http://www.hioutput.com/tech/qjetrod.html, and it had this acknowledgement attached "this information was generously provided by Damon Nickle.....Much Thanks!!!"

I like to see my data:

Table 1 has all the known secondary metering rods, it gives an idea of the range these covered and the various profiles. But, due to data overload, it is not much good for anything more.

Table 2 is just the most common sec metering rods. I determined this by looking at how many years the rod was used. It seems clear that the GM engineers were having trouble finding a profile that worked. They reused some and over the years modified some. And in 1974 all the profiles were revised for the new 4M4 model. At the end, each GM division has a BB and SB profile; chevy used two sbc profiles.

Table 3 are 'our' secondary metering rods that were used in factory SBC M4M version quadrajets. Though I'd think there is not a single stock configured SBC here, so who knows what secondary metering rod is actually 'tuned' but hey, even the GM engineers seemed to struggle with finding a 'universally' well behaved profile.

 

[dyeager535]

When you really get into how much things were actually "tuned" by GM, one realizes that things weren't just "thrown together". They may not have always arrived at the the best solution (hindsight being perfect) but many don't even think about the thousands of differentparts out there for just the Q-jet, or 700-R4, to name two. Just about every combination of drivetrain down to AC or not used different parts, and is why the parts manuals are so massive.

It also makes it pretty clear that as modifications go, one mod from stock is going to get you away from "ideal", if you consider factory setting ideal. In many cases it's simply the best compromise they could come up with, particularly given the technology that was accessible at the time.

That is a LOT of data BTW. The parts combinations possible are nearly limitless just for the carbs.

 

[my kids took the truck]

second that;

It also makes it pretty clear that as modifications go, one mod from stock is going to get you away from "ideal", if you consider factory setting ideal. In many cases it's simply the best compromise they could come up with, particularly given the technology that was accessible at the time.

change that to "the best compromise [we can] come up with"

I honestly have no idea how far away my current K5 setup is from optimal - it works and is drivable, sometimes enjoyable.

Just spent $25 on this. Seems the closest you can get to understanding the shenanigans. And straight from the man who designed the quadrajet himself.

"The Design Evolution of the Quadrajet Carburetor" Donald Stoltman 1966 SAE technical paper http://papers.sae.org/660127/

Flipping through it they have a section on center of bouyancy for the fuel float; a core reason CK5 is a source of quadrajet knowledge.

 

[livid4dirt]

Great thread, thanks for digging up all the info. Saving this for future reference :