more qudrajet bugs(455 sleeper)

finaly got to drive the car, (about 1.5 mile) runs real good , shifts and downshifts fine.
with choke set rich enough for good idle and warm up when cold, it takes forever (about 10-15 min ) to completly open the butterfly, and allow the secondarys to come in.
with choke set lean enough to to open after about 5-8 min of run time, it just seems lean and doesnt want to run unless i play with the accelerator pedal(just tap it now and again)
i tried raising the fast idle screw a bit on the lean choke setting but it still seems lean and doesnt run well unless i play with the pedal.
what is the fix? what am i missing?
ps - starts and runs real good on the rich choke setting(i can start when cold and rev it a couple a times and just walk away)
thanks  

hello buzz

i know these carbs well but not as well as some. it is lean, not enough fuel. members will need a lot more info to help.

did it work ok before and then you rebuilt it or put a big cam in your motor etc.

if this was a holley i would go up two sizes on the main jets. you'll need to go to a speed shop to get them. hard to find. double check your ign timing first idle 8-10, 3000 rpm around 30. also check your vacuum diaphram  on your dist.

it can also be an air leak. cap off the vacuum including to any power brakes and try it again. also spray brake cleaner carefully at base gskt to see if rpm increases. use the red nozzle it comes with. close the choke butterfly and cover the secondaries. if it still idles well its an air leak. i have repaired similar conditions literally hundreds of times hopefully this info helps.

did it change for no reason?  

barnett you might want to read some of buzzins threads. it is a 455/400th in a amc budget build.  
76 455/4spd TRANS AM
69 GRAND PRIX 406/5SPD

ohhh man rowsley. that would be like...work!

still need to know if the carb ever worked ok. that would be easier than trying to figure out what's going on, but i'll try to figure it out if i get a minute.

thanks  

hello rowsley

found something but what i found doesn't answer any of the questions i am asking him.

thanks for the tip though  

I'm having the same issue with my quadrajet.  I installed a wideband air/fuel gauge in and at idle it is fine (14.6) but just off idle it goes to 17 and stays until I accelerate.  Then the acc pump richens the mixture.  I put in a larger cam wben I rebuilt the engine so I'm pretty sure the jets need to be increased.  I went to Summit and got the following jets - 73, 74, 75, 76, 77.  The carb currently has 72's in it so I thought I would start with 75's and adjust up or down depending on the AF gauge.  Just a question - do you think it would hurt to leave the 2 screws out of the primary throat while trying the new jets?  Don't won't to risk dropping them in the engine if I don't need to!  

double post  

hello goatwool

what happens when you drive it?

is it flat off idle then suddenly accelerates? does it idle ok?

turn your idle screw up untill it seems lean then turn the idle screws out and then in and see what it does.

is the accelerator pump working?



if you are going 30 mph with steady throttle and accelerate [or attempt to] does it do the same thing as off idle. if so its definitely too lean see post above. if it is only off idle its complicated but typically fixed by increasing main jet size. it's the transition from idle to run [high speed]circuit. extremely typical even on stock cars. most cars were lean in this area from the factory. you can try increasing idle circuit air jets but i have never had to do that on a rocky. i hate them with a passion. sorry guys. give me a holly square bore leaker any day of the week. easiest carb to tune and get parts for in the world. plus the fuel that leaks out keeps the top of your manifold nice and clean.  

That is why there are different manufacturers.  Me myself, I like quadrajets.  They are easy to work on, great performers, and parts are also easy to get.  You just can't duplicate the sound when those secondary plates open!  

hello goatwool

i bet you're not lovin your quad so much now otherwise you wouldn't be askin how to fix it would ya "lol".

i know they can be made to work fairly well mainly on mild performance motors and i like the secondary sound too but where i live i can get holley parts in 5 minutes, not so with quads. pluss i've never seen one on a motor with over 600hp. i'm sure there are but they would be few and far between in my part of town. don't think many nhra or nascar guys run them.

i'm guessing by your lack of answers to my quwstions that my suggestions are no value to you therefore all i can do is whish you the best with it.

PS

i left a question on the cylinder wall thickness post for you.  

going LEAN just above IDLE is the APT adjustment..

turn it counter clockwise 1/4 turn at a time till the flat spot off idle vanishes... or you get a nice transition air fuel mixture slightly richer than idle mixture...

look at the idle mixture coming out around the tip of the idle mixture screw..




look at the idle transition coming out the transition slot...  as yes.,, i know this is not a quadrajet.. but carb theory works for any carb...




lets look at a quadrajet   the upper idle discharge hole is actually the idle transition slot...



in the image above.. do you see the primary metering rods..???   see that they are HUNG on the primary rod lifter..

the power valve piston pushes them up... when you go deep into the throttle.. but coming off idle.. the APT screw limits their downward motion into the jets.... farther down.. leaner... farther up richer...   this is where the high speed circuits start to flow...  yes... you can change the main jets.. and change the metering rods.. but it is SO MUCH easier on some quadrajets to adjust the APT  

quadrajets have a tiny primary bore.. so they come off idle transition much earlier than other cars...

i don't recall if you have an pre 75 quadrajet or a 75 up version with the APT adjustable through the plug directly under the front of the air cleaner mounting gasket...   let me find some more pictures..  

this is where the 75 up APT adjustment is if the float bowl is off..



you can see the PIN that rests down on the shoulder of the APT adjustment screw.. limiting HOW LEAN the primary main mixture can be...




did anybody know that they can measure the APT adjustment with the carb completed...

it starts at 5MM or 5/32" up the scale on the Thexton 370 dipstick tool...  

if you don't have one...  ZERO on the dipstick tool is 39MM up from the end...  so that would  put the APT MINIMUM at 39MM +5MM or 44MM down from the top of the vent tube with the tool pushing down on the metering rod holder..





this is the thexton 370 tool...   guess what.. you can also check the float level with the engine running with this tool..



its 1/8 thick and 3/16 wide


the tubular tool is what i have gotten off a snap on tool truck about a 15 years ago from borroughs special tools... its the shaft with the white cap.. its a double D tool... i forgot to measure the size of the double D head on the APT screw...

 

hello buzz

i reread your post if i interpret it right it sounds like it is just barely off idle. i'm guessing it has some type of cam. it's probably this. your cam is thirsty.

providing the ap screw that waynepp is referring to is under the carb top like i think it is then i would do what i suggested first including adjust the idle screws as mentioned. if that doesn't work then take the carb apart and install one size larger main jet [you need it anyway with a cam upgrade] and turn the ap screw that     mentioned just slightly at that time. this saves you a step. then check the idle screws again using the technique i mentioned. this should hopefully improve or completely fix it. you may need another main jet increase and ap screw adjustment. better too rich than too lean.

good thinking waynepp i never had to do this fortunately.  

yes small cam, it is a 73 carb, does it have the apt adjustment?
if not how do i adjust?
i have slightly richer primary and secondary metering rods, but stock 73 main jets.
should i try to go to 74s?
the car runs extremely well everywhere except off idle.
ps-(the exhaust pipe is real clean, it doesnt smell rich when idleing in shop),this is my first quad, i have built over 100 holleys,they always smell rich at idle.
thanks  

My carb is a 73.  In my case the engine goes lean once I go off idle and stays lean until the secondaries open (goes rich with accelerator pump of course).  It didn't do this until after the rebuild where I went to a larger cam.  I'm pretty sure going 1 or 2 jet sizes larger will fix this.  And yes - I still like quadrajets!