Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
off idle stumble
clumsum:
--- Quote from: 73 Z28 on October 10, 2012, 10:23:21 AM ---Just out of curiousity, what size engine are you running 350 , 400 or 455?
Are you getting black smoke out of the tailpipe? Do you know what the engine timing is set at?
This info will help to diagnose the problem your having.
Paul
--- End quote ---
It is a 400 ram air III. No black smoke out the tailpipe. The timing is set at 9 degrees before tdc at idle, I've never checked the total. Thing is, it doesn't stumble when the choke is on and had pretty much the same problems before the rebuild.
73 Z28:
The key piece of info to solve the problem you just provided. That being that with the choke on there is no stumble. This indicates that the carb is too lean on the idle circuit. During the rebuild, did you remove the idle tubes? If you did remove them, did you increase the size of the hole in the idle tubes and the down channel restriction? If not, I would recommend increasing the idle tube hole to .038-.039 and the down channel restriction to .062 With a 400 cubic inch or larger engine, the idle circuit needs are much greater than for a 350. The other thing I would do is to increase the jet size to a minumum 73 jet and test drive the new calibration. This should improve your vehicles performance and eliminate the stumble.
Paul
73 Z28:
One more thing when you have the carb apart, remove the idle mixture screws and check the hole size using small drill bits as a go-nogo gauge. On a 400 I would use a minimum hole diameter of .095 to .099 then reinstall the screws and set them to 4 turns out from fully seated to start with.
Paul
clumsum:
I didn't remove the idle tubes (was too scared that I might screw them up). I didn't really want to start drilling on this carb since it's the stock one for the car but I guess I could. Being that this is the stock carb for the car and the engine hasn't been modified shouldn't it run ok without drilling passages out?
73 Z28:
The reason for pulling the idle tubes is that they have a small opening that can get clogged over time reducing the idle fuel flow to the engine. Pulling them allows you to clean them out and by opening up the passageway allows for better fuel flow to the engine, the mixture screws will still be the final control over how much idle fuel the engine gets. If you aren't sure about pulling the tubes, I would find a practice carb to try it on first, but it really is easy to do. I believe that in an earlier post you said that you had another carb that was doing the same thing, if you still have it, I would use it as a practice carb first.
Paul
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