Q-Jet Misbehaving, Seems To Not Be Using Idle Circuit
 

I was fortunate to have three roommates at General Motors Institute that did their co-op at Rochester Products in the 70's. So I learned a lot from them on the Quadrajet design and tuning features, and had access to a good variety of jets and rods. One of them got me an 800cfm model for a Buick GS455 Stage-I that I modified the throttle arm on to fit my Firebird linkage. I ran the carb for many years on my 1970 350 without issue.
For the restoration of the Firebird, I had the carb refurbished and rejetted for the upgraded engine. Although a bit on the rich side, it ran very well on the dyno. Three years later (after covid interruptions), it's finally in the car and runs terrible. It starts fine and runs well as the choke pulls off on the fast idle cam, but refuses to idle when fully warmed up. I have to set the idle stop screw at the point where the engine is at 1500 rpm to run. It doesn't matter where the idle mixture screws are set. I can close them all the way or take them out all the way so the engine must be running on the main metering all the time.
I've even put the lightest power piston spring in from Cliff R to try and eliminate any issue there, but to no avail. Here's what is in the carb right now:

74 jets with 45B rods
CH Secondary rods on an "R" hanger
Air valve spring set at 3/4 turn tension
Power piston spring is Cliff's lightest

Engine runs extremely well from 2000 to 6000 rpm.

Engine:

1970 350 block bored .060 over
4.25" Eagle cast crank and 6.8" forged H-beam rods
Ross forged pistons, flat top, set at 0.0
6x-4 heads 9.3 compression ratio
Comp XE276HR 224/230 @ .050 110LSA
1.5 ratio rockers for .510 total lift
1-3/4" X 30" Hedman headers
MSD billet dist with 23 degrees mechanical set at 12 degrees initial

I'm thinking my next step to be a complete teardown of the throttle body but will gladly accept any suggestions.

You need to list the carb number, so the folks here can help you better...

Good point - sorry I missed that: 7041242

Don’t run a new motor pig rich, you’re just waring it out from the start.

Take the carb off now, remove the air horn and confirm that the power piston is not hanging up.

Also confirm that the air horn gasket is not out of place or mangled such that it’s restricting the free movement of the metering rods themselves.

After that you might find some find enough copper wire to run into the idle air bleed passages in case crap from sitting has them clogged.
I find Floral wire to be a useful size or strands of copper out of a big enough gauge copper wire.

Don’t not use strands from a Aluminum wire.

I did have it apart to confirm the power piston movement, but now that you mentioned it, the gasket did seem very difficult to un-thread from the metering rods and to get back on. I have a new rebuild kit coming so I'll definitely look at that as well as a thorough flushing of all passages. Thanks for the tip.

What's the vacuum reading at idle?
Is it steady?


:confused:

If Cliff happens to scan over this, I'd appreciate his thoughts on the jets and rods chosen for this engine combination which is coupled to a 2800 stall convertor, turbo 350 and 3.23 posi rearend.

Possible that you bent the primary rod hanger arms?
The hanger can come loose from the power piston as well.
Make sure when you tear into it the hanger arms are level with each other and the main housing.
And give them a few gentle taps to ensure they are seated in the power piston.

What's the vacuum reading at idle? Possible vacuum leak?

Old mechanics trick when you believe that there may be dirt in one of the circuits. Rev the engine to about 2500-3000 RPM, and slam the choke blade shut as you open, and close the throttle by hand. Open the choke before the engine stalls, repeat several times. By opening and closing the throttle plates all the circuits get exposed to the high vacuum, possibly dislodging it.

The increased vacuum induced by slamming the choke shut will sometimes pull an offending piece of dirt through the passages saving the headache of tearing the carb apart to remove it manually.

If the choke blade has been removed, a shop rag can be used to close off the primaries as you try the easy way to remove the restriction. This probably works more times than not, but sometimes you still end up tearing down the carb. It's worth a try to spend 5 minutes, saving all the labor, and parts to remove the offending blockage.

Link to a guy doing it on a VW bug, but it works on any carb:

https://www.youtube.com/watch?v=oaYADALqQnY

I could have used this trick a month ago...thx sirrotica

:thumbup: :welcome:

Sounds like you have some debris or deposits in the idle system somewhere. Remove the idle tubes and clean them. Clean the cavity where they are pressed in with some carb cleaner and compressed air. Get some cleaner and compressed air into the idle down channels as well. You might want to disassemble the carb at this point, and ensure there isn't any other deposits anywhere else. Some .028 or .030 spring wire is stiff and small enough to get into most of the places where it could be plugged.

"For the restoration of the Firebird, I had the carb refurbished and rejetted for the upgraded engine."

Who did the work and what did they do to it?

You also mentioned having a rebuild kit coming for it. Where is it coming from and what is in the kit?

It was done by Quadrajet Power. Standard restoration service plus level 2 modification for my engine specs. Out of the box, it seemed to run fine when the engine was on the dyno other than the secondaries being too rich. I changed hangers for the last pull and that helped some. Here is that pull where it is idling nicely:

https://www.google.com/search?client...so2r9udv8,st:0

Rebuild kit is coming from Carbs Unlimited as they did show one specific to the carb part number.

The Buick 455 Stage I carb takes special parts, highly unlikely Carbs Unlimited will be supplying those in a generic Walker based kit.

I would have spec'd out different rebuild and tuning parts for what you are doing. High flow N/S assembly, HP accl pump (lifetime warranty), jets, metering rods, IFR's, etc.

That carb is a factory "hot rod" and really does't need much help for what you are doing. Not sure how or why it doesn't have the original calibration still in it or isn't working well for what you are doing. Going from memory the only tuning parts still original are the 45B primary rods, jet and secondary rods have been changed........

Cliff, if you can put together a recommended set of parts, I'll be glad to make the purchase from you.

Not a problem. The early Buick 455 carburetors, 1971-74 are 850cfm units and among the very best starting points for a really nice high performance unit, especially for big CID builds and higher power levels. Almost all of the FAST Class Buick and Pontiac carbs I've built use those as the foundation and flow 897cfm when I'm finished with them. Some of those engines are close to 2hp/CID and the cars are into the 9's on bias ply tires.

So for sure a correctly set-up 7041242 Stage I unit would be about as good as it gets for what you are doing. Just have to figure out if it's a fundamental issue, parts issue, or a little of both.......

Thanks Cliff. I'll be tackling this after Labor Day Weekend since I need to recover after all my laboring at my daytime job :) and let you know my findings.

I'm only working till noon today, then taking off for a long weekend. I will have a LOT of questions for you if you want me to get involved.

We have to make sure the issues aren't parts related first, fundamental second. Since the carb has been "modified" at some point, and was "rich" right from the start when it shouldn't be, I'm leaning towards a fundamental issue as the root cause. In any case with things like this it is best to scrap just about everything inside the carb (that's what I do when one shows up here with a muddied history) and start from scratch.

First order of business is to strip it down and measure all the "players". Pressure/leak test the casting, verify no cross-leaking between the fuel bowl and area below the DCR's, throttle plates all FULLY closed'/tightly sealed in the bores when closed, no miss-matched components installed (different top/bottom), no lead plugs driving into bypass air holes, or any bleeds or orifices made larger or smaller than the stock size over the years.

One has to keep in mind with these things that over the last 50 years many of these carburetors have been apart and monkeyed with mulitiple times. In almost all cases something will be modified or mangled, like the last owners "guru" buddy that ran a hand drill thru the jets to give it some "more power", or drilled huge holes in the throttle plates so it will "idle better, or simply "twisted" the throttle shaft not taking the time to hold the fast idle parts when removing the retaining screw. I could go on for days here. I've found over the years after having thousands of carbs sent here and custom tuning troubled units that you can leave no stone un-turned with these things and assume nothing.

In other words it's simply best in all cases, to pretend you know nothing about the carbs history and start from a clean slate. I'll also add here than more times than not you are a LOT further ahead to be working with a "virgin" unit vs one that's had a lot of hands on it as needed modifications will be simple and predictable.

I just ran into this a couple of days ago. I sent a customer EVERYTHING required to set his carb up EXACTLY for the application (late 1970's Corette with mild modifcations to the orignal 350 engine).. He owned the vehicle from almost new and was certian the carb had never been touched. So he installs all the parts I sent and makes all the settings we discussed. He also installs an A/F meter. The carb actually works pretty good but really rich right off idle, normal driving and heavy/WOT throttle. I'm talking down in the 10 to 11 to 1 A/F range. So we go back and forth with emails, I have him make multiple adjustments, and it's just not responding like it should. I finally went back to "basics" and had him send me some pics of the airhorn and main casting.

Low and behold someone had driven .045" main airbleeds into the air horn and main casting, stock size for that carb is .120".. How or why we'll never know but I'll remember the aggrevation it caused and the hours I lost of my life trying to help figure all that out! Good news is that it's now working like it should and everyone is happy and we can chalk it up to experience and move forward.........