[srmmmm]

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

[srmmmm]

Installed new 0.038 idle tubes and 0.055 down channel restrictors after thoroughly cleaning out all passages in the carb, carefully reassembled and put the correct base gasket on. Set the idle screws at an initial 3-1/2 turns and like magic, I've got an engine that will idle now!

https://www.youtube.com/watch?v=WEAOsbrju2U

I think I can still back off on the choke a bit as it seemed to stay on the fast idle cam longer than necessary, and I'm tempted to drop down one number on the primary jets to see if I'm still running on the rich side - an A/F meter is definitely on the wish list.

It was nice to be able to finally enjoy a Saturday night drive in "The Bird". Thanks to all the tips from members and the great forum thread history that provided the idle tube and restrictor sizing Cliff had recommended to someone with a similarly cammed 400.

[Cliff R]

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.......

[srmmmm]

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.

[Cliff R]

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.........

[srmmmm]

Originally Posted by Cliff R

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.

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.

.

Finally had the opportunity to tear down the carb and look for issues, but first, the history. I purchased the carb new in 1975 direct from Rochester Products. Up until this rebuild, the only things I ever did was to change jets and rods from those installed for a GS455 Stage-I to sizes that performed more effectively on my 350. Physical modifications were limited to polishing out casting lines and filling in linkage rod slots for more precise control of the choke plate. So I would consider this close to virgin, or maybe just a "First Base" date night unit.

The next event was the restoration work done which included new throttle shafts, an electric choke conversion, and rejetting for the 63 cu in displacement increase, 6X-4 head installation and use of a Comp XE276HR cam.

Upon disassembly, I noted the throttle plates all closed completely with no visible light showing around the edges and that the throttle shafts showed no play in the throttle body.

The float bowl did have some varnish/sludge in some crevices and the primary metering rods did seem to have some contact against the float bowl gasket. My concern was the appearance of the 2 brass idle air inlet restrictors. They seemed to be somewhat banged up, and I could not remember if there were originally 4 or just 2.

The four tubes in the air horn are not at the same height, one being about 3/16" shorter than the others. Pictures show these details. Considering the uniqueness of this unit, I certainly hope it is still usable.

[srmmmm]

Some more tweaking after a 600 mile round-trip shakedown going to the Trans Am races in Austin. Cliff R provided a tip on the distributor not coming down off the mechanical advance and indeed, when hot, the weights seem to be hanging up and I'm seeing 22 degrees at 1000 rpm when it should be at my initial 14, so I've got that to contend with. I did discover I can close the idle mixture screws all the way and the engine still idles, so the APT must have been "adjusted" and the true possible cause of the remaining issues. What's the best way of getting the epoxy off to access the scree again?

I was very pleased with the 18mpg on the first 250 miles and 19mpg on the last 250 miles running on I35 at 72-75 mph with absolutely no pinging, so I think my rod and jet combo is what it needs to be.

[Schurkey]

Originally Posted by srmmmm

What's the best way of getting the epoxy off to access the scree again?

Why is there epoxy over the screw?

[srmmmm]

Originally Posted by Schurkey

Why is there epoxy over the screw?

I suspect the carb rebuilder altered the setting as part of his "Stage II" level option taking into account my engine/cam specification, and epoxied over the opening.

[Kenth]

Originally Posted by srmmmm

I did discover I can close the idle mixture screws all the way and the engine still idles, so the APT must have been "adjusted" and the true possible cause of the remaining issues.

It is not entierly impossible that your engine runs on the off-idle slots when the idle needles are seated, or are they?

Maybe the springs are long enough to stop the needles before they hit bottom and closes off the holes?
Remove the springs and seat the needles, does it still runs?

And, adjusting the APT will do NOTHING for the idle needles position, the idle circuit is a separate stand alone circuit.

FWIW

[srmmmm]

Originally Posted by Kenth

It is not entierly impossible that your engine runs on the off-idle slots when the idle needles are seated, or are they?

Maybe the springs are long enough to stop the needles before they hit bottom and closes off the holes?
Remove the springs and seat the needles, does it still runs?

And, adjusting the APT will do NOTHING for the idle needles position, the idle circuit is a separate stand alone circuit.

FWIW

I'll pull the springs off and try. I do already see the tips of the needles protruding into the barrels right now. But I had thought the height of the power piston was possibly altered by the carb rebuilder to allow more fuel to be available to the idle circuit and transfer slots in anticipation of the cam I was running.

[Kenth]

Originally Posted by srmmmm

I'll pull the springs off and try. I do already see the tips of the needles protruding into the barrels right now. But I had thought the height of the power piston was possibly altered by the carb rebuilder to allow more fuel to be available to the idle circuit and transfer slots in anticipation of the cam I was running.

The square area between jets and rods will always be larger than the square area of the idle tube restrictions, thus it wont matter how you adjust the APT.
As said before, the idle circuit is stand alone circuit.

[Half-Inch Stud]

See 12.2 ET Sig....my 9.0:1 470 needed the 1050 to idle, and certainly needed the Choke attached to thero-Spring to Start and Idle.

Any, all Q-JETs from 65-70 that i rebuilt would not start and idle. So i tried a pig-rich mod on the idle air bleeds and saw some improvement but the 1050 seemed better.

Solution:my 2 cents; more compression or less cam(dunno your specs), or maybe a 1972-1978 Q-JET has an improved chance of good start and idle on your 9.3:1. Oh, confer with CLIFF.

[steve25]

The pickup tube that's driven in too deep is likely making the 4 cylinders that are feed fuel from that side of the Carb go lean at or around full throttle.

[Schurkey]

Originally Posted by srmmmm

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.

The FIRST thing I'd do is to determine where the idle mixture screws are set now. Remove them. Grab a rubber-tip blow gun connected to compressed air, and blow into the threaded holes where the idle mixture screws were.

If you don't have compressed air, an aerosol can of carb cleaner with the red "straw" attached, could work.

This may blow any clogs of the idle passages back out the air bleeds.

Reinstall idle mixture screws to whatever position they were when you began. Start engine, see if you can adjust idle mixture screws and idle speed to something more reasonable.

This is the fastest, easiest, least-invasive potential fix. If this works, great. If not, move on to the more-intensive repair options.

[srmmmm]

Originally Posted by Schurkey

The FIRST thing I'd do is to determine where the idle mixture screws are set now. Remove them. Grab a rubber-tip blow gun connected to compressed air, and blow into the threaded holes where the idle mixture screws were.

If you don't have compressed air, an aerosol can of carb cleaner with the red "straw" attached, could work.

This may blow any clogs of the idle passages back out the air bleeds.

Reinstall idle mixture screws to whatever position they were when you began. Start engine, see if you can adjust idle mixture screws and idle speed to something more reasonable.

This is the fastest, easiest, least-invasive potential fix. If this works, great. If not, move on to the more-intensive repair options.

Extensive cleaning will begin tonight, but I had already removed the idle mix screws to verify they were the same tip profile and had the same spring lengths. Both were set at 1-1/2 turns open.

[Pav8427]

Thise idle tubes ARE mangled. The concern I would have is that someone opened the restriction way big.
The restriction is down at the bottom of the tubes and hardly measurable as installed. I wiuld pull them and check. Cliff has new ones and will drill them for you if need be.
Time to check all the proper dimensions and make sure someone didnt screw it up at some point.
Then get some new numbers for a recipe from Cliff.

[srmmmm]

Originally Posted by Pav8427

Thise idle tubes ARE mangled. The concern I would have is that someone opened the restriction way big.
The restriction is down at the bottom of the tubes and hardly measurable as installed. I wiuld pull them and check. Cliff has new ones and will drill them for you if need be.
Time to check all the proper dimensions and make sure someone didnt screw it up at some point.
Then get some new numbers for a recipe from Cliff.

Thanks for the observation. I was afraid of that. I would think the stock tubes would be fine considering the OEM application of the carb for a GSS455 Stage-I should handle my 413 cu in.

[Pav8427]

Your carb also has the early base plate APT.
Cliff has a kit to change it to externaly adjustable.
That will help with the tine tuning once all is put back together.

[steve25]

Something often overlooked in these carbs is the pivot hanger that the float hangs off of .

This needs to be bent such that sitting down the air horn to the body keeps this part held firmly in its notch.
If not then fuel pressure can un-seat the neddle and bounce the float around.