My Q-jet Tips
Written by: Mike Ervin
To start this off, I just wanted to tell a little bit about the stock Q-jet that came on my truck (1985 C-10).  1985/86 Q-jets in U.S. trucks have dual capacity accelerator pumps.  When the engine is cold, more fuel is needed for transition from idle to part-throttle.  When the engine is warm, less fuel is required.  Dual capacity pump solenoid (plug on top right front of carb) is activated by coolant temperature sensor (sensor in thermostat housing).  At 170 degrees pump solenoid energizes, dual capacity pump valve opens and pump capacity reduces by about one-half.

I had performed some of the mods in this article on my stock carb.  After it started leaking around the air horn where it had been over-tightened at the carb mounting bolts, I decided to replace it with an Edelbrock Q-jet.  The stock Q-jet is an emissions carb as most Q-jets in the early to mid 80's are.  These carbs aren't computer controlled as many people think.  The only thing electrical about them is the dual capacity pump.  The carb has to be plugged to the sensor in the thermostat housing in order to work right.  If it isn't, the pump solenoid will stay closed all the time and waste fuel.

I used the articles by Damon on Peformance Tune Your Secondaries, Q-jet Secondaries Part 2, and Q-jet Idle Problems, to help with the tuning of my Edelbrock #1904 Q-jet.  The first thing I did was to check the choke pull off/AV damper and see how fast it opened.  Well as it turned out it opened real close to one second.  Great.  The air valves already opened fully also.  Next I pulled the sec. metering rods and hanger to see what I had.  Edelbrock says on their site the 1904 comes with DR rods.  I wanted to make sure.  It turns out it was a M hanger, along with the DR rods.  I pulled the carb off of the engine to make sure I set the AV spring tension right.  Damon recommends to use a B to G hanger.  I happened to have an E hanger laying around so I used it and set the tension to zero plus 1/8 of a turn just to be on the safe side.  This E hanger was old and a little worn but worked great.  I have since replaced it with a new Edelbrock G hanger.  No further adjustments had to be made with the change.  I found out any hanger from B to G will work just fine.

I have read where people say on the primary side, the primary rods need to be around .030 smaller than the main jets, but this recommendation is for pre-75 Q-jets, without the APT feature.  The Edelbrock 1904 Q-jet came with .050 rods and .073 jets.  As you see this is a difference of only .023.  I was having problems with the exhaust being so hot it was killing my catalytic converter and muffler.  After a lot of tuning and guess work I came up with a good combination of a cooler exhaust and good emissions.  I kept the 73 jets and installed a set of .048 primary rods and kept the factory setting on the APT at 2 turns out.  This stopped the lean condition I was having in the cruise range causing the hot exhaust.  I tried a setting of the same jets and primary rods, with the APT set at 2-1/2 turns out.  This seemed to work fairly well in cool weather, but as soon as it started warming up, I started having problems.  The hotter the weather got, the worst my stumbles got.  It would load up while sitting at a light.  I went back to the factory setting of 2, and all is fine in truck land once again.  By the way, 1975 and later Q-jets used different primary rods than 74' and earlier Q-jets.  The 75' and later Q-jets use part numbers that start with 170 (74' and earlier ones start with 70).  The part numbers for 75' and later primary rods start with 170513xx, as in the .048 rods I used would be 17051348.  The last two numbers are the size of the rods.

The APT (Adjustable Part Throttle) that the Edelbrock Q-jets have is set at 2 turns out from the factory.  What the APT does is control how far the primary rods can be pulled down in jets.  The power piston has a pin made on it that hits the APT screw and stops it from going any farther down into the jets.  The farther out you turn the APT screw, the farther out of the jets the rods will stay and the richer it will run.  It is sort of like installing bigger jets because primary rods get smaller the farther up they are held.  What you are trying to do here is get a part throttle that is crisp and responsive, without any surging or hesitation.  This is not for performance, that is accomplished with the secondary side.  You don't want to go too far with this adjustment, because it will just waste gas.  Also, if you must pass emissions testing, be real careful.  This will increase your emission levels.

There is a way to adjust the APT without having to remove the top of the carb every time.  If you look at the air horn just in front of the choke tower where the air cleaner gasket sits, you will see an aluminum plug.  To get the plug out you will need to remove the air horn.  Once the air horn is off you can knock the plug out from the inside.  In order to plug the hole and be able to get to the APT once the air horn is back on the carb, I tapped the hole for a 3/8" x 24 thread allen head plug.  This will sit flush with the air horn and not interfere with the air cleaner.  Don't tap all the way to the bottom of the hole, you won't the plug to sit flush, so check the depth as you are tapping the hole.  Also, you will need to make a tool to fit the APT screw.  It needs to be sort of rectangular in shape.  I used a piece of 5/16" steel gas line.  Just use a pair of pliers to squeeze it into an oval shape, then use a screwdriver to finish shaping it into a rectangle.  Test fit until it fits without too much loose motion.  You may have to grind down the outside of the piece to fit in the space around the adjustment screw.

On another note, I had to drill out the Idle Discharge Ports (the holes in the baseplate for the idle mixture screws) so I could have an adjustment with the screws.  In photo #3 below, the black arrow points to the actual discharge port, inside the throttle bore.  This is the hole the pointed end of the mixture screw goes in.  This will be what you will drill out bigger.

idle_discharge_port.jpg (8250 bytes)

Chevy Truck Magazine
This is an insert from an article on Q-jet Tuning Tips that appeared in the January 2000 issue.

The holes in the baseplate where the idle mixture screws are located are called the idle discharge ports.  These ports, together with the mixture screws, control how much fuel enters the engine at idle and won't need alteration with a stock engine.  However, once the engine is modified with a bigger cam, these ports often need to be enlarged to avoid a lean miss at idle.  Stock ports are usually 0.050-0.081 inch, and Murphy (Sean Murphy from Jet Performance Fuel Systems) says they can be enlarged up to 0.093 inch, according to engine needs (anything beyond .0093 inch, and you start losing idle-adjustment capability).  Although application needs vary, Murphy recommends 0.081-0.086 inch for cams with 204 to 210 degrees of duration at 0.050, 0.086-0.089 for cams between 210 and 216 degrees of duration, and a maximum of 0.093 for cams above 216 degrees of duration.  Big-block engines have different recommended specs.

I read the above insert in the article.  My CompCams XE268H cam has 224 degrees of intake duration, so I said why not.  I checked the holes and found out they were .088 inch.  So I gathered up the nerve and drilled them out to 0.093 inch.  Worked like a charm.  Now I have adjustment with the screws.  Before drilling them bigger, I could turn the screws all the way in and not affect the idle.  I had to turn them out about 4 to 5 turns to achieve a decent idle mixture.  I highly recommend you giving this a try if you have no adjustment with the idle mixture screws.  Just for the heck of it, I checked the holes in the old stock Q-jet that I had removed and found out these were, get ready for this, 0.055 inch.  That's right, no wonder we can't get a decent idle after we change cams.


Setting The Air Valve Spring Tension

In the photos below are the screws you need to use to adjust the AV spring tension.  In photo #1, green arrow, is the actual adjusting screw.  Also in photo #1, pink arrow, you can see the rod and lockscrew at the bottom.  Photo #2, red arrow, shows the allen head lockscrew.  Photo #2, yellow arrow, you see the hook of the spring wrapped around the rod.  The rod is attached to the air valve shaft (green) and is how it puts tension on the flaps.

av_adjust_screw.jpg (13776 bytes) av_lock_screw.jpg (14120 bytes)

To set the tension properly you need to make sure of zero tension, by this I mean you need to make sure when the air valves are held closed, the hook of the spring is just making contact with the rod that is sticking out of the AV shaft.  You have to loosen the allen head lockscrew while holding the adjustment screw, because the adjustment screw can and will come out, believe me.  The adjustment screw does not screw into anything, it just tightens the spring, so it will slip out if you're not careful.  The best way of doing this is with the carb removed from the engine.  This way you can see the rod and spring and will know exactly when the spring makes contact.  Anyway, according to how much spring tension you are wanting, once it just makes contact (zero tension), turn the adjusting screw the amount you need and while holding the adjusting screw, tighten the allen head lockscrew.  Never tighten the adjusting screw more than about 1-1/8 to 1-1/4 turn or you risk over-tightening and breaking the spring.  A real good idea is to make a reference mark on the carb when you find zero tension, inline with the screw slot, so you will know where it is if you need to do this again.

If you don't' want to remove the carb from the engine, you can sometimes get close to zero tension by this method.  Loosen the allen head lockscrew while holding the adjusting screw.  After you have it loose, turn the adjusting screw counter-clockwise until the A/V flaps fall open.  Do this slowly.  After the flaps are loose, slowly tighten the adjusting screw until the flaps just close.  This is as close to zero tension you will be able to get doing it this way.  Now tighten the amount you need to go and while holding the adjusting screw, tighten the lockscrew.  That's all there is to it.  Our carbs factory setting is 7/8" of a turn past zero tension, if you need a reference or need to set it back to factory specs.  (Why would you want it at factory specs?)

My Comments On This Mod

I have run Quadrajets on my cars and trucks for most of the time I have been driving.  I have never had one perform like this one does.  As Damon says , "It will hit like it has fuel injection".  I highly recommend trying this mod to your truck.  I guarantee you will love it as much as I do.  It is the best thing I have done to my carb and the cheapest.  My total cost of the above mod, $2.00 for the Edelbrock G hanger.  Well, let me change that, I did have to buy the carb in the first place, but that don't count.

BTW!  The Q-jets that came on the L-69 engines (305 HO's that came in early 80's Camaros and Firebirds, along with Monte Carlo SS's) came stock with M hangers and DR rods, most of them anyway.  The LG4 engines (basic 305's) had P hangers and DP rods.  If you try to find these rods in a junkyard look for these cars for the best chance of finding them.

The part numbers for DR rods is GM17053659.  The GM dealer is the only place (besides a junkyard) that you are likely to find these at.  They are not cheap, $8.40 each is the price I got from the dealer in my area.  On the hangers, the best place I have found is to buy the Edelbrock ones.  You can get these at Jeg's and Summit and other places on the net.  The part numbers for the Edelbrock hangers are as follows: B hanger #1960, G hanger #1961, K hanger #1962.  They have more but these are not worth mentioning.

If anyone needs more info on rods and hangers, as to what all this means, or what size are the metering rods they have, I have all this info if you need it.  A good book that everyone who has a Q-jet should have is Doug Roe's Rochester Carburetors.  This is The Bible for Rochester carbs, hands down.  I can't stress this enough, if you want more performance out of your Q-jet or just want a great how to rebuild reference, buy this book.  It is an invaluable piece of information that you shouldn't pass up.

I know that a lot of this will make no sense to you because it didn't to me at one time.  I had no idea that you could make a Q-jet perform this much better than stock.  As you can tell, I love Q-jet's, it has got to be one of the best, if not the best carb ever built for the street.  When properly tuned it will give you many years of great performance and economy.  Now start adjusting.