Need Q-Jet tuning help...

[PhoenixZorn]

I'd like to get my vacuum secondaries to open only after 70mph to conserve gas... of course I'd like them to open earlier for quick accelleration, but I think they just open every time I drop the pedal to the floor... it also bogs ever so slightly, so slightly that I can feel it, but can't hear it.

Can someone post the links to that Q-jet tuning sheet or give me some times on getting the secondaries to open only when I'm "Passing" or doing a quick acceleration?

 

[PhoenixZorn]

....

 

[dyeager535]

I don't know as anyone has done this, so I think it would be fun/educational for you to try.

Wire the secondary air valve (the secondary butterflies) shut, and go drive it. Report back how you like the way it drives.

I really think that doing what you suggest is going to make a huge difference in the way it drives all around, and you won't like it.

 

[Hogback Fabrication]

Agreed, there is no real way to do what you are asking. If you don't want the secondaries to open while on the highway, don't give it more than half throttle. The primaries are fully open at about half throttle, and if you could get your secondaries to stay shut, going to WOT would not give you any more power. If you need the secondaries to stay at 70 mph up hills, so be it. Use the secondaries or slow down. To get rid of the bog, there is a small screw on the passenger side of the carb where the secondary butterfly (the vacuum one at the top) is. Directly under that (still on the uppermost part) is an allen bolt. Put a screwdriver in the screw and loosen the allen bolt. Tighten the screw a small amount (1/8 of a turn at the most). Drive it, and keep doing this until the bog goes away (it probably won't take much if yours isn't bogging much).

 

[PhoenixZorn]

Perhaps I worded that incorrectly... I'd like the secondaries to stay closed unless I'm above 2500rpm or thereabouts... cruising along at 65 with hardly any throttle with the secondaries wide open really wastes gas. Honestly, I don't know at what rpm my secondaries open... I can't hear it or feel the power boost when they do open.... If I floor it, I obviously want them to open up, but if I'm cruising, having them open is pointless.

 

[dyeager535]

Honestly, I don't know at what rpm my secondaries open... I can't hear it or feel the power boost when they do open...

Thats the way it should be. Seemless transition from cruise to acceleration. (essentially)

You can tighten up the secondary air valve, but as I say, wire the secondaries shut and check what happens. If you decrease the ease with which the secondaries open, you haven't changed the engine demand...you still need the same amount of gas to push the vehicle.

Tighten up the secondary air valve spring, and if it helps at cruise, great. More than likely, you will end up having to push the gas pedal further at the same vehicle conditions (speed), which means nothing really has changed.

 

[Hogback Fabrication]

Perhaps I worded that incorrectly... I'd like the secondaries to stay closed unless I'm above 2500rpm or thereabouts... cruising along at 65 with hardly any throttle with the secondaries wide open really wastes gas. Honestly, I don't know at what rpm my secondaries open... I can't hear it or feel the power boost when they do open.... If I floor it, I obviously want them to open up, but if I'm cruising, having them open is pointless.

The secondaries open only if the butterflies at the bottom are open (they open at about 2/3 throttle and are not based on RPM). Once the secondaries begin to open, the butterfly at the top will open based on engine load and requirements. The secondaries WILL NOT be open unless you are at 2/3 throttle or more, no matter what.

 

[diesel4me]

easier way??

Since the Q-jet uses a vacuum pull off to hold the secondary air valves from opening,couldn't you just tee it into a ported vacuum source in addition to the manifold vacuum source??--I'd think that would hold them shut pretty much all the time,unless its completely "floored"....

I disconnected the secondary barrels on my edelbrock performer when I had it on my 250 six,and it actually ran better if you ask me!(carb WAS a bit big for such a small motor! )--the 305 in it now with the same carb acts the same way,like its getting TOO MUCH gas with the 4 barrels kicked in,and it "flattens" out,seems to pick up speed when you back out of the gas more than flooring it does...

I might just disconnect the secondaries again,and leave it that way!--I could spend hours and lots of money changing jets,metering rods,etc--but I dont have the cash to play with it!..can hardly afford the gas to drive the damm truck period!...either that or my 2 bbl Holley "Economaster" is going on it with an adapter--then I'll have an automatic choke and fast idle too,instead of a hand choke on an edelbrock,with a missing fast idle cam...

 

[dyeager535]

Yes, guess you could tee it into ported vacuum, hold it shut at cruise, but the reality is that vacuum drops off VERY quickly with throttle, and that won't be any different than using manifold vacuum.

As was mentioned, unless someone monkeyed with the linkage, the secondary throttle plate doesn't move until the primaries are open X percentage (not arguing the percentage, just not sure what it is exactly) so if you are cruising and not into the throttle much, you won't be using them anyway.

Again, reality is, if you need the throttle open X distance, you need it open that much. Restricting fuel/air at a time when it's needed is counterproductive since supply is based on demand. Making up numbers for an example, if you need 80% primary open and 30% secondary open to go 60MPH during cruise while towing or heavily loaded, limit the secondaries and you will lose power after the primaries are opened as far as they go and engine demand exceeds fuel/air supply.

 

[PhoenixZorn]

How does engine RPM not affect when the secondaries open?? It's entirely dependent on engine vaccuum, which drops sharply as more gas is added, and subesquently you get to higher rpms... the faster the engine turns, the more air it needs, at which point the secondaries should pretty much correspond to the RPM of the motor because of the relation between rpm and vaccuum. I understand that sitting still in my driveway, I'll never get the secondaries to open because while there is a supply, there is no demand while not under load... that's why you set timing with no vaccuum.

I'm sitting in my driveway at 1500rpm... I have only the primaries running, and the secondaries are shut tight... I rev it up to 3000 and the secondaries stay shut... I do the same thing on the road in each gear, Low through 3rd, and it will open the secondaries at about 3000 in each gear. By the way, I did feel the secondaries open a few minutes ago... no bog, but a huge jump in acceleration.

So with that in mind, how can't engine RPM affect when the secondaries open?

 

[dyeager535]

How does engine RPM not affect when the secondaries open??

What he is talking about is the "mechanical" portion of the secondaries. Go out to your vehicle with the engine warm. Take off the air cleaner if you have to, to see the linkage well.(unless you can picture it) Move the throttle linkage like you were using the gas pedal. You will see at a certain point that the secondary throttle plate shaft starts to move at exactly the same point every time. *That* mechanical function is unaffected by RPM.

To use your analogy, if you run the engine at 1500RPM in park, and you only push the gas pedal 1/3 of the way to "WOT", the secondary throttle plate won't move. If you push the pedal 2/3 of the way down, it WILL move. As you noted though, engine load at that RPM in park is not enough for the engine to NEED the additional air/fuel the secondaries could deliver. So vacuum keeps the secondary air *valve* (upper plates) shut, even though the accelerator has opened the secondary throttle plates.

Make sense?

 

[PhoenixZorn]

Yup... thanks for clarifying. =)