Q-Jet rebuild, How hard?

[porkchop]

Th topic pretty much says it all. I have a Q-jet that came off a 79 Cheby that I got for free and I want to rebuild it and put it on the wag. How hard is the carb to rebuild and put on an edelbrock 2131 intake? I know I need the adapter but are there any hiden surprises? Thanks.

[RustyJeep]

IMO, they are easy to rebuild...just as any other carb technically is. This may also be because I've have several apart and rebuilt them. You may have do work out your kickdown linkage and stuff, but it shouldn't be too much of a swap. If I wasn't going to run EFI, I'd have a Q-Jet.

[JeepFreak]

Some links that might help you out. Been years since I was in a Q-Jet.

http://duzy.8m.com/monte/ccquadrajet.htm

Here is a write on a Q-Jet as well but I couldn't get the link so here is the whole article.

Q Jet Addendum By Jim Hand

The above article is an accurate and practical review of the Q Jet, and is very much a duplicate of what we have done with our Q jets. In particular, Randy?s suggestions on secondary metering rods for a 455 are close to what I run in my wagon. Pontiac used BE rods (.041) in some applications, and they also work well. Concerning the secondary rod hangers, there are many versions that position the rods higher or lower in the secondary jets. For drag racing, they are not terribly important because we normally go from idle or low speed to full open throttle. Thus the hangers move the rods from the lean condition to full rich almost immediately. For normal driving where the secondarys may need to be partly open (such as a GM engined motor home or road racing), the hangers become very important because they determine at what point the fuel mixture changes to a richer condition via the tapered section of the metering rods. Randy advised to be sure that the secondary throttle plates open fully. Also check that the air valve opens adequately. When looking straight down from the top, the front edges of the valves should be at approximately the center of the air valve shaft when fully open. Two things that control the maximum opening are the vacuum pull-off rod, and the mechanical stop on the air valve haft at the opposite end of the pull rod slot. Bend/adjust the pull rod, and/or file the mechanical stop. My air flow bench showed about a 3% increase in air flow through the carb after this mod. As Q Jets are air-on-demand carbs, and are effectively variable size, this may not translate into an actual performance increase, but it does provide more potential air flow if our engines actually need it.

He also suggested using the air valves with "transition" slots. Most Pontiac Q-Jets have small orifices (ports) located directly in front of and at the center of the air valves, called the accelerating ports. These ports function as accelerator pumps for the secondarys. When the secondary throttle plates begin to open, a vacuum is reflected up into the carb, and this causes a small amount of air to begin to flow around and through the air valves before they begin to open. The transition slots are intended to concentrate that initial air flow past the accelerating ports. As you will all remember from your physics classes, air flow across an opening will lower the pressure at the opening. The fuel inside the accelerator wells, tubes, and orifices immediately begin to travel towards the low pressure area and is sprayed into the air stream at the transition slot. This initial shot of fuel is intended to prevent bog until the secondary metering system begins to meter fuel through the secondary discharge nozzles. This is a great system, and normally works well on stock carbs.

One of the changes Randy mentions is to open the air valve quicker, because this provides quicker secondary throttle response. At some point in the increased air valve opening rate, there may not be enough fuel from the accelerating ports (or it may not begin quickly enough) , and the dreaded bog reappears. To cure this problem, we have made the transition slots smaller. We obtained non slotted Chevy air valve plates, and cut much smaller openings in them (about 3/8" wide and 1/4" deep centered on the accelerating ports). Although the actual size is not terribly

important, both plates should be identical. The smaller openings concentrate airflow and increases the velocity at the accelerating ports, and results in much quicker and stronger fuel flow from the ports. Again, while this change may not result in measurable acceleration improvements, it does sharpen the throttle response and allows the secondarys to begin to add power almost simultaneously with the primaries. Your reaction times, and 60' times will definitely be more consistent when there is sharp and instantaneous throttle response! Before trying to remove the air valve shaft/plates retaining screws, file or grind the ends of the screws where they have been deformed. Using a large blade screwdriver, carefully remove them using a technique of twisting CCW, CW, and again CCW, while using WD-40 for lubrication. These screws may break, but with care, they can be removed intact. When all check-out is finished, a medium strength of thread lock (in case you desire to remove the screws later) should be used to secure the screws in the new plates. Concurrent with this mod., it may be necessary to drill several small holes (about .030") through the secondary well tubes. These holes will allow fuel to fill the tubes faster, and they should be located about 3/8" and 5/8" up from the lower tips of the tubes.

The two fill holes into the wells may also need to be opened up by 1 or 2 thousands. Be very conservative here because the fuel from the accelerating ports continues to feed the secondarys whenever they are open. Perform the last two mods only if a bog still exists after modifying the air valves, or if a bog occurs during a quick throttle opening immediately after doing a long burnout. The sole purpose of the accelerating ports, well tubes, and wells is to furnish the shot of fuel at initial secondary opening, and if it doesn't bog, don't fix it!

Whenever engine modifications are added that tend to adversely affect idle quality, or a later Q-Jet with more emissions controls is used, it is sometimes impossible to obtain satisfactory idle. The two Q-Jet idle "mixture" screws do not actually alter the idle fuel mixture. Instead, they allow more or less of a pre-determined mixture to be directed through the idle discharge ports. The taper angle of the pointed portion of the screws have no effect on the adjustment, other than

controlling how sharply they peak (if at all). In many cases, the mixture screws do not provide adequate fuel to obtain a smooth idle. In the pre-emission days, the Pontiac shop manuals specified to adjust the idle screws for "the fastest, purest" idle. In those earlier times, the idle screws had a range that would allow the engine to be tuned from a too lean condition through the ideal mixture, and on to a too rich mixture. The modern carbs are typically calibrated very lean at idle, and almost any change to the engine will adversely affect the idle. Idle mixture is actually determined in the Q-Jets by the "idle tube" orifice size. The idle tubes are located in the main body at the front inner edge of the primary throats. Most of the stock orifice openings range from .030" to .033". We have found that a .037 opening will provide idle adjustment range for almost every application including pure stock vehicles. It is possible to remove these tubes, but it is difficult and requires some special tools. As the actual orifice restriction is located at the lower end of the tubes, and is 1 5/8" from the top, a special 2" length drill is needed. Using a finger operated pin vise, it is possible to drill the tubes without removing them. Coat the end of the drill bit with light grease and very carefully drill through the orifice. The orifice restriction is only about .2" in length, and the passageway holding the orifice tube makes a sharp turn just below the tube. Therefore, proceed slowly and carefully to avoid bending and/or breaking the bit. The light grease will hold most of the brass hips, and the remaining brass can be flushed by spraying WD-40 (what would we do without it?) through all idle passageways in the carb body. Remember that the idle circuit functions at closed, or nearly closed throttle position. To correctly adjust the idle mixture screws, you must concurrently enrich (back out) the screws while reducing the throttle opening (idle speed set-screw turned CCW) . Otherwise, the engine will continue to run on the transition circuits and the idle circuits can not be properly adjusted. The longer drill bits were special ordered and are made by Precision Twist Drill Co. as RN 52063.

I have seen a lot of B.S. about the fuel pressure needed for Q-Jets! As the actual pressure dissipates after the fuel passes through the needle and seat, pressure has no direct effect on the carb operation. The only requirement is that the fuel bowl maintains a high enough level of fuel to correctly meter the fuel at all engine speeds. The carb will function correctly with only 2.5# of pressure providing the fuel system can maintain adequate fuel level. A stock pump generally provides as much as 6 or 7# of pressure at idle, and as PPM increases, the pressure gradually drops down to as low as 1 or 2 #. That relatively low pressure can still keep the bowl adequately full in most cases. Why not run more pressure? Additional pressure blows the float higher and richens the mixture. Excess pressure almost always causes or aggravates a hot start or flooding problem, because it tends to force fuel past the needle and seat, causing the carb to overflow fuel onto the manifold after the engine is shut off. Excess pressure can also cause fuel spewing within the carb and out of the vents.

The fuel system in my wagon is a somewhat overkill setup! The gas tank sits vertically, and I have installed a 1/2" outlet at the bottom rear. The Mallory "140" pump is located at the base of the tank such that gravity feeds the pump at all times. A 1/2" line is routed to the front to the Holley regulator, and a 3/8" line from the regulator connects the 3/8" in-line filter and the carb. This system can provide much more fuel than the engine can use, and the engine will run through the 1/4 mile perfectly with only 2 1/2# pressure. Some time ago, I was testing a non 0-Jet carb and had set the pressure at 6# for that carb. At the drag strip several days later, the wagon was off .5 second and 3 MPH. After 3 passes, I remembered the high fuel pressure, and after lowering it to 3#, the wagon picked up the lost performance. You may not see this kind of drastic change because each fuel delivery system is different. However, I strongly recommend you find out by strip testing what the lowest pressure will satisfy your vehicle, and then set the pressure about 1/2 # higher for insurance. You may be pleasantly surprised, not only with your Q-Jet but with other brands of carbs as well. Recheck for proper metering (rods and jets) after this change. For those of you willing to do a little grinding/drilling/filing/tuning on/with your Q-Jet, the ideas described above might add a bit more drivability and/or performance. KEEP ON RACING!

Jim Hand

[porkchop]

Anyone have pics of one installed on a wag with a TH400 and how they worked out the linkage? Thanks Rusty.

[porkchop]

Wow, Freak, that helps a lot thanks!

[JeepFreak]

Found some more links

This was the link I was looking for first...It is in my opinion one of the best write ups out there.

http://www.dorianyeager.com/indexquad.html

And yet another one

http://www.mts.simplenet.com/mts/tech/Quad.htm

[ April 17, 2002: Message edited by: JeepFreak ]</p>

[Tad]

No need for my input, I foung the Q-jet a very simple, capable carb for the Willys w/350.