Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

idle issues on my Qjet 7040203

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monteholic:
Hello all!

I'm new here, and all my research kept pointing me to this forum, sure hope you can help me

near finishing a frame off resto on my '70 monte, 4 speed bench seat car. It is a basically stock 350, right down to exhaust manifols, intake and still running the original points distributor

my issue with my numbers correct carb (7040203) is the idle mixture keeps changing, not the screws, but I get it adjusted, and the idle starts changing to where I have to close the mixture screws all the way to keep it idling, then it will change back to where I need to open them back up or it dies too. sometimes to won't change for a while, sometimes this change happens after a test drive, other times while I'm backing it out of the garage, even while it's sitting there idling, the idle just starts to degrade to where it stalls, it will start and run above a ilde just fine, but to get it to idle again, I have to either close or open it mixture screws

I have plugged the brake booster and tested it-same results (its vacuum is pulled from the manifold, not the carb)
I did find a bad vacuun can on the distributor and replaced it and still have the same results
I have checked for all other possible vacuum leaks
I have checked the air cleaner vacuum can/heat switch
the only other vacuum is the PCV
it is pulling 18-19" of vacuum @700-800 rpm (usually 19")
running about 10 degrees of timing
I have also changed to float and checked the float level numerous times (set at 5/16")

hoping you can help getting this monte running as it should!

thank you,

Cliff Ruggles:
A heavy float will cause this.

Leaking at the fuel inlet seat.

Leaking under the fuel seat gasket.

Too much play at the primary throttle shaft.

Weak distributor springs or worn parts in the mechanical advance mechanism.

Small hole in the vacuum advance if manifold vacuum is being used.

I've also seen this happen when the secondary throttle plates are not correctly indexed/centered in the bores, or the linkage is too tight holding them open slightly.....Cliff

monteholic:
I will be calling to order a few parts, I got the carb from Monzon carbs in Arizona and it's never been right, even after sending it back to them

I think I'll start with a correct kit and float
 the throttle shafts don't seem to have any slop in them
 the throttle plates are closing
 I replaced the vacuum advance can
 and the mechanical advance is free and seems to be working well
 the way it is going rich/lean I will start with the needle and seat and make sure it has the correct float and gaskets in it
it currently has a 74/40 jet/rod in it, which should be a little richer for cruise, stock should be a 76/44, I just received a set of 76 jets and have a set of 45 rods which I will install and see what happens from there

thank you for your help! :D

monteholic:
great talking with you Cliff!

looking forward to getting the parts and making this run 8)

monteholic:
same issues
I will be calling you on monday, at times I can completely close the mixture screws, hardly effects the idle
then it won't idle, have to open the mixture screws to approx 3 1/2 turns

I believe there is an issue with the throttle shafts, I see evidence of it leaking vacuum past them, I think they have been rebushed before

secondary shaft seems ok, opening manually while running has no effect

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