Q-jet Idle Problems

Writen by: Damon Nickles

Quadrajet Idle Quality: Idle quality with a QJet can be tricky since they were ALL emissions carbs (except the Edelbrock's).  They can have lousy idle quality on a cammed motor because they already start out borderline-lean on a stock motor.

Here's how to fix it:  First off here's the path the fuel takes in the idle circuits:  It goes through the main jets (which are NOT a restriction for the small amount of fuel the idle circuit needs) and into the main fuel wells.  From there it goes up through the idle metering orifice.  This is basically a vertical "straw" that has a tiny restriction at its tip.  This restriction is typically ~.030".  It serves basically the same purpose as the main jet, but on a much smaller scale.

OK, from there it goes up to the top of the "straw".  There it makes a 90 degree turn and goes about 1/4" horizontally.  At that point it goes past the Idle Air Bleed that mixes the liquid fuel with a little air from the main bore area turning it into a frothy mixture that I call "cappuccino foam".  The reason for this is that a foamy mixture atomizes much better in the intake manifold than liquid fuel does.  OK, now the cappuccino foam makes another 90 degree turn and goes back down- to the idle and off idle discharge ports. But before it does it goes through one last restriction- the Idle Channel Restriction.  This is the final calibration point in the system other than the idle mixture screws themselves.

All right, we're through the Idle Channel Restriction and headed straight down to the idle discharge ports.  Before we get there we go past the off-idle discharge ports (vertical slits, really, not round holes) where we mix in a little more air and then finally we get down to the idle mixture screws and the idle discharge ports.  I think we can figure out how things work from there.

How to modify stuff:  The first place to begin modifications is at the idle discharge ports.  These are the round holes that the tip of the idle mixture screws go through.  Since the idle mixture screws are tapered you get more or less mixture flowing through them as you screw them in or out.  However, on a lot of QJets (almost all of them) the actual discharge port is considerably smaller than the mixture screw diameter.  This is done purely so that some bonehead mechanic can't back the mixture screws far enough out to over-richen the mixture by very much.  Again, all hail King Emissions!

How to fix:  Take out the mixture screw.  Measure it's maximum diameter with a micrometer or vernier caliper.  Pick a drill bit that is .030-inch smaller than that and drill those suckers out!  But first, take the carb off the engine (lets not get TOO lazy now), take the Throttle Body portion of the carb, the bottom part that has the throttle plates in it off the main body and drill the idle discharge port right through where the mixture needle screws in.  Since the original hole is already there it acts like a pilot hole and messing things up while drilling is not gonna happen unless you're smoking crack at the time.  Obviously, you don't want to leave any metal chips in there so its best to clean everything out before putting it back together and onto the engine.

One little snag you might run into (you knew there was going to be one, right?).  Some carbs, and I do mean SOME, have yet another idle mixture restriction JUST BEFORE the mixture gets to the idle mixture screws.  You have to take the throttle body part of the carb off the main body to see this restriction, so its best to check for this at the same time you are drilling out the idle discharge ports.  It is in the throttle body casting and is in the vertical "down channel" just before the mix get to the idle mixture screw (you can see the threads of the idle mixture screw from the side through it).  If this area is wide open, you're fine.  If it has a restriction in it that is ANY SMALLER than your new idle discharge port (use the drill bit to check it) then you should drill this out ALSO.

All righty, just getting warmed up now.....

Drop the carb back on the engine and adjust the idle- making sure to keep the mixture screws and EVEN number of turns out each- not 1 1/2 turn on one and 3 on the other- EVEN!  OK, Any better?  Can you back the mixture screws out far enough to make the engine slow down from an over-rich idle mix?  Do you have LESS THAN 4 turns out on the mixture screws to get an acceptable idle? NO?  Ok, then you need to go farther.

The next mod will richen ALL the idle circuits up quite a bit- BOTH the curb idle and the off-idle circuits.  It also helps cure an off-idle "sag" common to heavily cammed motors with big-runner intake manifolds.  Remember the QJet was lean EVERYWHERE in its calibration and that includes the off idle circuits!

Alright.  Take the air horn off the top of the main body of the carb- 11 screws, the choke plate pushrod, and the accelerator pump arm must be disconnected.  Now before you mess with anything lay the original gasket back onto the main body of the carb (if it isn't too torn up- then use the replacement) but make sure you have it on right, not flipped upside-down.  Its not a perfect mirror image but close enough to fool you sometimes.  There are two small "kidney bean" shaped openings in the gasket sitting right next to each primary bore.  At either end of the "kidney bean" is a little hole.  These are the idle metering orifice (the "straw") and the Idle Channel Restriction that I mentioned about 5 million words ago.

You're gonna drill out the Idle Channel Restriction a few thousandths.  Trouble is...knowing which is which.  The Idle Channel Restrictions are the holes that are slightly farther apart and farther towards the front of the carb (nearer the fuel inlet), one per side.

Now you're gonna need some drills you probably don't have.  You need what they call a "numbered drill set".  This is a set of drills that are small and very closely spaced to each other. You can buy them from JC Whitney or order them from Sears.  $30.  In a pinch just pull out your 1/16" drill bit (.063")- its gonna make everything VERY fat, but for a non-emissions high performance, weekend-warrior motor its better to go too fat than too lean.  If its a street motor and you expect ANY economy from the thing then buy the drill bits and do it right.  Remember your street car runs on the idle circuits 95% of the time!

Use the drill bits to determine the size of your stock idle channel restriction by trying to put them though the restriction on on a go/no-go basis.  Its probably gonna be about .045-.050". Pick a drill that is about 4-5 thousandths bigger than this and drill it out.  The restriction itself is about 1/4" long.  Try to drill straight down- use a drill press if you have access to one.

Why not drill out the idle metering orifice instead of the idle channel restriction?  Because its about 2 1/2" down at the tip of the "straw" and no drill bit that small will reach that far.  Otherwise, it would have exactly the same effect as drilling out the idle channel restriction.

Because of the metal filings generated its a good idea (all right- its REQUIRED) to pop off the throttle body (lower) part of the carb afterwords and clean out the channel and the idle discharge ports thoroughly before reassembling and putting back on the car.

After this mod you should notice that the mixture screws don't need to be nearly as many turns out to get an acceptable idle.  Stock they usually need about 4-5 turns.  After this you should be down to about 3.  Also, the transitions (off-idle circuits) will be richer and many engines will feel much more willing to rev on light throttle after this modification.