Hey gents...crankit here...this has been quite the adventure...I can build you a car out of bondo but I'll be damned if I can get this 455 to idle!!.. new rebuild rochester throttle bushings and all new hei..running 30 35degrees all in.. 24in/vac at 1100 rpm if I turn idle screw to say 850 900 rpm it idles poorly and wants to stall so I turn it back up I believe it's not pulling from the idle circuit...but at 1000 1100 rpm it runs!!!! Pulls the second aries like butter till about 4000...motor is brand new and I've been chasing this prob for a while..has a mild cam??would to big a cam cause the idle issue...idles like theirs a friggin thumper cam in it..sorry for the long winded post but trying to cover some of the possible questions... carb has been off three times checking float level..gasket fitment and so on..bought a 3406 4 hole spacer from holley the fast idle cam binds on the side!!! No reply from holley imagine that!! They say fits all gm jets 67 75 ya right...could the carb off 350 olds not be right for 455...7042250 cliffs says cadillac??? So that's the rebuild kit that was used..ok gents that enough bitchen ..for now.. any thoughts will be greatly appreciated..even from joe..ohh and congrats on the new stripes...crank.it

 

You stated "24in/vac". I have no idea what that even means. Normal measured vacuum should be between 15"Hg - 22"Hg and those readings should be rock solid "steady".
If you're attempting to state you have 24"Hg - that's enough vacuum to suck the chrome off the bumper. Is that supposed measured reading of 24"Hg rock solid steady w/ absolutely no bouncy bouncy of the gauge? If so, you have a vacuum restriction "somewhere".

 

I'll add that 7042250 is not the most appealing carburetor for a 455 BBO. Yeah, the rebuild kit is fine for a 7042250 sitting on a 350 SBO. I'd strongly suggest you use a carburetor designed for a 455 BBO (7042251).

 

No cheif... that's correct 24hg at about 3500 rpm but.. does drop to about 16 at 1100 but when I turn idle down runs ruff like a lumpy cam and vac goes to 8 10HG lol ...idle fluctuates and idle air has no effect on vacume... also pulled a plug and burning rich plug all sooty black ...

 

Let's ensure you're adjusting A/F ratio correctly and in the correct order:
(1) Set dwell;
(2) Set timing (unhook vacuum advance from distributor, plug vacuum inlets, set timing, hook vacuum advance back up);
(3) Adjust A/F mixture screws w/ vacuum gauge. Adjust EACH A/F mixture screw (one at a time) to achieve the highest vacuum possible - again, one at a time.
(4) Adjust idle screw to appropriate RPM.

 

Perform the test in the BIG RED BOX. You should be able to pass the BIG RED BOX test - hands-down. You're looking for a steady vacuum reading when you make the A/F mixture screw(s) adjustment(s). Be mindful of any vagaries in vacuum needle.
Even a tiny/little loping cam should lower vacuum not increase vacuum.

 

Ok chief thanks for the input...like I rambled about I have vacume gauge plumped into intake..the idle screws do nothing..I've seated them all the way in and no vacume diff...then I turn them out 1/2 a turn at a time and still nothing...running hei so no dwell adjustment....if the idle circuits are plugged would that cause this lack of idle????I noticed today when checking if secondaries were opening fuel looked as if it was coming from two little holes were the air cleaner stud goes???? Normal ???

 

Yes. Dirty clogged idle circuit is a good jumping off point. I'd begin to ensure you cleaned the idle circuit passageways - thoroughly. Next, would be the secondaries need to open correctly to provide the "air gap". The air gap is critical. Without the addition of air (provided when the secondaries open) the engine will burn rich (as a pig, often).

 

I failed to add this. If the idle circuit passages are plugged, fuel will not navigate the idle circuit passageway(s) & instead drip straight down into the manifold. Essentially, fuel enters, then rises to a point higher than the fuel float bowl before it falls into the A/F passage. These carburetors are considered "downdraft" carburetors; but first the fuel rises before it falls into the manifold. This can/will lead to a fuel rich condition, as well.

 

Thanks...iam leaning that way clogged primaries...maybe my thoroughly clean carb "isn't " I thought I read somewhere the carb will pull fuel from the secondary that would explain how good and ( rich) it runs when I turn the idle up running of secondaries??? Ok gonna due another vacume test and then yank the carb for the fourth time!! Thanks cheif for the insight...I'll post back with new info...crankit

 

I use "safety wire" to clean idle circuit passage - I have a small roll of it. It's the type used on (mostly) aircraft engines to secure nuts/bolts w/ a safety wire twisting pliers. You can also pull a long single wire strand from something like an electrical wire depending on gauge.

 

I tend to think it's the primary idle circuit passage, as well; since, you can't achieve ANY adjustment with the A/F mixture screws; albeit, fuel is sufficiently bypassing the idle circuit and dripping into the manifold. The fuel has to enter the idle circuit first before it can exist into the A/F mixture passages to be able to be regulated via the A/F mixture screws (which you evidently cannot do).

 

My .02 is that a 250 carb, on a 455, especially with a cam, does not have the idle circuit capacity to supply enough air and thus fuel, to idle properly. Get Ruggle’s book and use the appropriate sizing.

These notes could be decades apart & the nomenclature may vary based on what I call them vs what Roe or Ruggles calls them. I haven’t even compared them so just putting a warning and disclaimer. I hope they don’t just cause confusion or prove useless.

7040258 Ballpark. These are finished size for my 434, with stock sizes in parentheses.

Qjet 7040258 (70 W30 AT)72 Prmaryi Jets w/ 48B Rods

CV (.527) Secondary Rods on “D” Hanger
Inlet Valve .125

Main Air Bleed .070

Idle Tube .040

Down Channel .055 (.050)

Uppr Idle Bleed .0535 (.0465)

Lwr Idle Bleed .0635 (.055)

Idle Bypass Air .950 (.078)

Mix Screw Holes .096

Sec Well (POE) .040

Accel Pump Discharge .028


I MIGHT have notes on a 7043252 & some other carbs but I’ll have to look. Not sure what I have in my archives.


FWIW
These are notes I have for a 7041250 I built for a low compression Chevy 350.
Orifice sizes are stock.

7041250- 4MC 71 350 applications (In Use)

Stock 70/49B .00196271

AS(CG) 077

Inlet .116”(?)

67/50B .00156216 (in use)

G DA .440 (in use)

4.5 Second Choke Pull Off

Primary Spring 220 grams

.125 Inlet

Pull Off 4 seconds

.044 Main Nozzle (in Venturi)

.040 Idle Air (next to tapered screws in choke horn)

.053 Idle Tubes (Rearward @ base of Venturi boss)

.054 Idle Down Channel Restriction (Forward @ Venturi boss)

.049 Idle Air Bleed (45* on Venturi leg)

.035 Inboard Tube (smaller)

.076 Outboard Tube (fatter)

.073 Pump Discharge

.080 Idle Discharge (mixture screws)

.058 Bypass Air (underside of base plate>uppr hole in bore above slot)

​​​​​​….

 

Hi bccan,I dont know the history on the carb,when I rebuilt it I failed to check for correct size...but iam almost certain the primary flaps have been drilled giving more credit to your not enough air to pull the fuel out of a to small idle circuit...I may take the carb off again(4th)time and pull the top to check jet size...maybe drill the idle circuit if that's possible..but any advice is well taken..like I told chief I'll post back with the results..good or bad...crankit

 

Now that the hour is more conducive to lucidity, I think that 258 recipe might help. I put the 250 specs there more for comparison but have never bothered comparing them. HOPEFULLY the descriptions of location on the 250 help & don’t just act as confusing gibberish that doesn’t align w/ the 258 nomenclature. When I do carbs for myself or other people, all that is usually written down on scraps of paper and only gets “archived” occasionally.

Highly recommend Ruggle’s Qjet book, Roe’s is good too but when Rug book came out I always found it easier to follow.

All this said, you can work successfully with your 250, it just needs minor mods to handle the displacement & vacuum of your engine. The “holy” primary throttle plates are something I like on most Qjet builds with a cam.

The other alternative is to source 455 application carb and work with that, but modding that 250 will be inexpensive. After all this, if you want to really optimize the carb, source “Quadrajet tuning paper by Lars” on the net, it’s my bible for tuning, along with an Air:Fuel gauge in place of a stopwatch. These carbs can really be nailed for a combination of drivability, performance & economy without compromise.

The biggest niggle of Qjet tuning is that you get really good at R&I and “popping the top.” I can have one off, disassembled, reassembled, back on & running in under 15 minutes, mildly miffed every time I have to do it! But that’s the labor of Qjet love. Then it’s back out for a test drive & A:F gauge monitoring. Sometimes I get them in a few rounds, sometimes it goes a full 12, literally!
​​​​​​….

 

Excellent feedback by bccan. You'll certainly learn a great deal if you perform the mods to the 250 & as stated it should be fine. Yet, a 251 is designed to ride on a 455.

 

Thanks bbcan..I will definitely take the advice...I dont think the motor has an aggressive cam in it it sounds that way at idle real loppy...but it does run strong after1100 1200 rpm..motor has never been on the road,it was broken in last summer..but had various problems due to my erros... it's taken a year to sort through things...carb was the last thing to do and well it's running 100% better than it was..I put an offy intake..points dist...but couldn't get it to run at all burp fart stall repeat...gave up and went back to stock,intake and hei...runs much better...buuuut not right .. their for all the questions..iam gonna look up the cam again who knows could be a radical thumper which explains the crappy idle...thanks for the imput....crankit