Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
Can't get my rebuilt '78 400 to idle; runs great, but surges about 100 rpm
Shark Racer:
Typically from the factory they run .32-.35ish.
Mine were about .34, and I started at .36 and went to .38.
The idle tubes are in the main body, one on each side of the float well. There is a "path" that leads to the down channel restrictions. Basically two brass wells, ~1/8" diameter. The one that is mostly flush with the top of the main body casting is the idle tube, the restriction is about 2" deep and you'll likely have to remove them to change the size.
The down channel is the larger hole, about 1/4" below the top surface of the main casting.
The tubes are next to the corners of the float, the down channels just slightly forward.
corellian corvette:
Thanks again. I'm going to ask a bunch of Q's so I don't bugger this thing up.
I was reading an article on HPP that talked about doing this, but the condition they were dealing with was the car was running too RICH.
In my case, it appears to be running too LEAN. So wouldn't opening the idle air bleeds make the carb leaner?
Again, I *really* appreciate all the help and guidance here. I don't mind tackling this at home, but I don't want to start drilling into things without knowing exactly the outcome.
As it stands now, it appears I need to somehow improve the amount of fuel the engine gets at idle, as it's running lean. How would you order the steps of drilling out the tubes to analyze this problem?
Thanks SO much!
corellian corvette:
I should also point out, after reading several other tuning articles, it seems I need to start by upsizing my IFR (Idle Fuel Restrictors). Are those the ones in the body of the carb, or the ones that are pressed into the carb top?
Shark Racer:
They are basically a pair.
Opening up the idle air bleeds will add emulsion air and tip the scales more lean. I can honestly admit that I don't understand why a different sized bleed has different characteristics from another. IE, you can get the same mixture with bleed A and tube A, or bleed B and tube B.
At any rate, the down channel restriction may also be called an idle fuel restrictor. It works together with the idle tubes. I would up them both in pairs. There are some recipes in the book that explain different sizes. I'd start by sizing what you have and then go up a thousandth at a time, maybe more. I ended up having to go up QUITE a lot on mine.
Considering you are over 5 turns out and still seem to be running lean, I would think you need to go up a couple steps. However, it's not easy to "shrink" a hole, so ... :)
corellian corvette:
OK using Cliff's book, I went ahead and pulled the measurements for my car. My harbor freight drill set doesn't have some of the smaller size increments, so I'm going to get a better set to get more specific
Idle Tube: .046 fit, but not .052 (my next size up)
Idle Down Channel: .046 fit, but not .052 (my next size up)
Upper Idle Air Bleed: .070
Lower Idle Air Bleed: .055
Idle Bypass Air: None.
The upper air bleed seems really small based on the specs I've seen.
Any thoughts on this?
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version