rochester carb problems

[oldsmobucket]

been working on my 55 olds(324 ci) had a problem with an off idle stumble with the rochester 4gc carb. so i rebuilt it took it all apart cleand everything and replaced springs , check balls etc. also cleaned my gas tank, ran new fuel line, and replaced the fuel pump and filter. i reinstalled the carb today, stated fine i shifted into drive and and barely touched the gas and the car stalled. what would cause such a thing. any help is appreciated

[enjenjo]

Vacuum leak.Check to see that no vacuum lines are disconnected, and that you used the right carb base gasket.

[wayne petty]

with the engine warm but not hot...   ... and the engine off.. can you look down into the carb under the choke... to where the accelerator pump shot is sprayed.. ...  it should be instantanious..  no delay... when you move the throttle the smallist amount...    ................

sometimes the check ball seats get corroded after years and years...

i sometimes have to rough up the seat with a phillips screwdriver stuck down the hole and twisted.. then i lightly drive the used ball of the right diameter into the seat  ... this ruins the ball.. so do not reuse it... it leaves flat spots that will screw you up later... .


see page two of this...
http://www.walkerproducts.com/downloads/catalogs/components/rochester4barrel.pdf       numbers 59,60, 61 for the discharge of the pump
and 57 and 58 for the pump inlet ball...  this is the one that allows the pump cavaity to fill and holds pressure when the pump is depressed..


the dischage ball is spring loaded and the pump pressure has to overcome this spring loading.. it is loaded to prevent the air flow from pulling fuel over at high speeds.....


i have also had to polish the pump bore...      roll up some 400 or 600 wet of dry paper and spin it in the bore...  then be sure to completely flush it out..  carb cleaner then compressed air..


it is also possable that the nozzles are clogged...    can you find a wire brush.. pull a single brissle out.. put it in a pair of needle nose vice grips and bend the end into a J or L  so you can probe the tip without needing to take the shooter out...


i guess some of you will now know that i live and breath carbs.....

[crdnblu]

The above is all very good, but also, don't forget to check the Auxiliary Throttle Valve spring tension.  Many times the set screw that holds the spring tension loosens, causing the valve to just "flap" and not hold shut.  It should only open when the the secondary throttle plates open, to help transition into the secondaries w/o a bog.  I've found that the proper spring tension will be different for  each engine, requiring 2-3 settings to get it right, then Loc-Tite the set screw.
Here's a site for 4G diagrams:
http://www.tocmp.com/manuals/Carbs/Rochester/4-Jet/64RochesterServiceMan/pages/Roch4G_0014_jpg.htm

[tomslik]

does this have the heat/exhaust passage under the carb?
might need the thin stainless plate under it....
if it needs it and you don't have it, it'll run like an egr valve is stuck open...

[Mikej]

Throttle plate and idle adjusts.

[Dave]

Make sure its not leaking around the throttle shafts too.. Id kill for a nice working Qjet. There really great carbs. Small primaries for millage and big * secondaries for gettin r done..
Dave  

[wayne petty]

there is one more thing to look at...  i may have to go out to my vw van storage and see what i have  to verify...

the idle air bleeds and the hispeed air bleeds..

decades ago on my aunts 67 caprice 396  when i rebuilt the carb i managed to blow out the tiny brass drilled cups that are just barely pressed in...    i found one.. and measured the opening.. and epoxyed them up and drilled them out..    worked much better..

the idle circut is good up till about 1500 rpms on the transision ports..


if the idle screws are out too far... you need to go smaller on the idle air bleeds.. this richens up the idle mixture... by increasing the vacuum in the emulsion mixing area...

if they are in too far... open it up a tiny amount at a time..


if the primary needles are height ajustable with a vacuum operated piston pulling them down.. the piston needs to have the lean stop raised a bit.. .... this richen up the minimum mixture...


back before l left  remember.. the chevy truck owner... who missajusted his lean stop when he rebuilt his carb and he drove it and ajusted the lean stop thorugh the hole in the qjet top cover that is usually plugged...

earlier qjets even have that ajustment.. but it is hidden in the baseplate.. between the idle mixture screws...  remember the little metal tab that sticks up from the baseplate and into the gasket area.. that lifts the bottom pin on the vacuum power piston/primary rod holder..... ..


i just went and looked at the diagram from the posted manual page.. no ajustable primary needles.. so that is Qjet mostly...


so let us know what you find...   our fingers await...

[oldsmobucket]

i went out today to work on that carb, i bent the pump rod so that it would squirt as soon as i hit the throtle, same result? checked the vacum on my gauge, at idle it is pulling 21 in. hg. took the carb off took it apart to check the pump circuit took out all the springs, check balls, etc. cleaned everything and checked all the passages and cleaned with carb cleaner and compressed air, put it back together hokked it back up to the car, same result? :?  this thing is driving me crazy because i cant figure it out. i appreciate all the help you guys have given! but i am learning more about how carbs work and im getting alot faster at taking that carb on and off

[enjenjo]

What does the vacuum do when you come off idle?

On another subject, are you still running points?, New? Old? Advance?

[chimp koose]

jusjunk,there is a quick an dirty throttle shaft bushing fix . go to the hobby store and buy a length of brass tube with the id the same as  the throttle shafts od.probably cost you around $6 . Q jets throttle bushings  are counterbored at the outer ends. If you remove the primary throttle shaft ,you can push a small length of the brass tube into the counterbore at each end . it will be a slight press fit into the counterbore and a snug fit to the throttle shaft . the part of the throttle shaft riding on the new bushing will also be new and unworn as it used to run just to the outside of the old bushing.this has worked well for me in the past and a length of the brass will probably do more carbs than you will ever own.Has anyone suggested ignition timing or a leaky vacuum advance diaphragm as a possible cause for oldsmobuckets woes?

[oldsmobucket]

have not checked the off idle vacum, but i will. how would i check for leaky throttle shaft or bad throttle shaft bushings?  running points , maybe i should go get a new set.

[oldsmobucket]

what should the off idle vacum be with the car in drive? sorry forgot to put that on the last post :shock  

[enjenjo]

what should the off idle vacum be with the car in drive? sorry forgot to put that on the last post :shock  

It will drop, I want to know how it drops, and how much.

[reborn55]

Did you check the timing, dwell and operation of vacuum advance