Heus

I have a feeling that my secondaries are not opening on my 78 L-48. Should their be a noticeable burst of power when getting on the throttle? For a low hp engine it has decent throttle response at higher rpms but no noticeable seat of the pants feel. Can this be checked with the car not running? I also think my linkage needs adjusted, because the kick down does not engage when passing. Thanks

wrencher60

I felt the same thing about mine a couple months ago. My throttle linkage wasn't adjusted correctly and wasn't letting the throttle plates completely open. After adjusting that correctly, that made a huge difference. Then I went after the secondaries. There is a small Allen adjustment screw that you get to from the bottom side of the air horn that is locked in by a set screw. If you google Q-jet secondary adjustment you will find a good write up on it. Only want to go about an eighth of a turn adjustment at a time. I kept stepping it up until I got a bog and then backed it off an eighth of a turn. You can't check it without trying it out because there has to be a load on the motor. Trial and error, but it is fun once you find the sweet spot. I have a 4-speed car, so the kick-down is not there, but that could be a linkage problem or a solenoid problem.

aussiejohn

Unless the external linkages have fallen off, the secondary throttle butterflies are opening fully when you floor the pedal. Check this with the engine off. The Quadrajet is a mechanical secondary type carburettor. However, unlike the Holley, the Qjet has a vacuum operated air door above the secondary throttle blades that will not open under light load. When it does open, it raises the secondary metering rods to allow petrol to flow into the secondary passages, proportional to the amount of opening.

You will not see this by looking at the carb and opening the throttle. It happens under load, i.e. when overtaking. The opening of the air door is gradual and if the carb is operating correctly, you should not feel a sudden "seat-of-the-pants" surge in power. It appears that your carb is operating normally.

Regards from Down Under.

aussiejohn

doorgunner

On my '68 project the engine produced a noticeable increase in sound when I accelerated heavily/I had no problem passing cars........but there was no "kick in the pants".

According to tech threads, I checked the secondary throttle plates with the engine off---flipping them with my fingers while I held the throttle wide open----they opened fully.

After reading several threads I learned that the kickdown solenoid was not operating in the TH400 automatic transmission that the previous owner installed.

After installing a new kickdown switch on the accelerator bracket
/installing a kickdown harness (the car was a 4-speed previously)
/replacing the original kickdown solenoid inside the trans mission
/changing the old fluid and filter............

The car is now scary when the accelerator is floored....it will "put me back into the seat".................

now I don't need as much throttle to pass someone.

Shark Racer

As others have said, you should not feel the secondaries engage.

Further, on Aussiejohn's post, the lower secondaries are mechanical but many QuadraJets (by design or by wear) do not open to the correct 90* position. Some have overtravel from wear (throttle stops being worn down), others have the secondary travel limited by design. This can be adjusted by removing the throttle body from the quadrajet and tweaking the lever that engages the secondary linkage arm. Once you've done this, you'll want to readjust the throttle stops.

My 78 has a natural feeling of "getting on the powerband"(which many of my modern EFI cars have had), but nothing like "oh wow the secondaries just kicked in".

commander_47

On a stock car driven on the street the secondaries almost never come into play with a q.

Heus

Thanks guys. I looked down through the carb and fully engaged the throttle. The secondaries barely opened. I also adjusted my kickdown cable so now I at least have my passing tear working.

Paul L

You will hear the secondaries opening/sucking air. They are twice the size of the primaries.

wrencher60

Exactly, you may not feel it in the seat of your pants, but you will know when they open up, and you gain a few HP.

jnb5101

When the engine reaches the RPM range that demands enough air to open the secondaries, the power curve is (should be) starting to quickly rise. So it's not the secondaries opening that you will feel, it's the power curve. In order to tune the primaries on a qjet, wire the fast idle cam up so that the secondary lock-out arm prevents the mechanical butter-flies from opening. Run the engine like that up to about 3000 RPM and it will feel no different from not having the lock-out. Past that RPM range, it will feel like you're driving a VW Bug.

Paul L

I don't do foot to the floor on my 1974 L-48 very often, but when the TH400 shifts down and the secondaries open up the car does move pretty impressively. I was doing some tuning today and had the chance to give it a try on the highway when there was no gendarmes in sight. The L-48 is no slouch going from 50 to 80mph. I didn't press my luck beyond that. The 4-barrel intake noise is like music! Of course the 1974 has cowl induction so you can really hear it.

Heus

Shouldn't my secondaries open up mechanically when not running if I have someone put the pedal to the floor? Mine barely open.

Scottd

Wow.....maybe Im the only one with a Lars re-worked Q-JET......but when I go from a roll and MASH the pedal, it feel like Im downshifting when the secondaries kick in. I mean its like I hit the afterburners. I have no experience with a stock q-jet, but I can tell you with 100% certainty that I KNOW when they kick in.

wrencher60

Yes, with the pedal to the floor, the secondary throttle plates will open if your cable is adjusted right. if you go under your hood and move the throttle linkage until it stops, the throttle plates should both be full open. If they don't, then your linkage ain't right. If they do, then your throttle cable is not adjusted right. If the throttle plate doesn't open all the way then your secondary air horn will never open either because there won't be enough vacuum. That is exactly what was wrong with my 68, the throttle cable was not set right. The carb linkage was right, but the cable from the pedal was not pulling the throttle plate all the way open. Basically, I was driving around with a two barrel, and the gas mileage still sucked, lol.

Priya

If you can feel the secondaries kick in then your carb is not adjusted correctly. There should be a smooth transition from the primaries to the secondaries.

7T1vette

If you don't 'feel' the secondaries kick in, you've got a puny engine. In any event, you should HEAR the secondaries open.

There are many reasons why Q-Jet secondaries won't open: 1) your floor mat is too thick; 2) the secondary lockout won't release because of some problem with it or the choke on your carb; 3) the throttle cable is misajusted; 4) the accel pedal linkage is weak/flimsy and not transferring motion to the cable as designed; 5) carb linkage is misadjusted and/or binding on the secondary mechanism; etc etc

With car in driveway/garage and engine OFF, remove air cleaner, hold choke fully open, hold secondary air valve plate open, look down into secondary venturi AND have someone else in the driver's seat pushing accel pedal to the floor with their foot. If the secondary throttle plates are not completely vertical, there's a problem. Many times, they are not even beginning to open for one (or more) of the listed reasons.

Sorry, AUSSIEJOHN, you missed on this one.

dugsgms74

7T1 pretty much nailed this one but I'll add one more possibility. Wrap up spring too tight not allowing the secondary air valve to open against air pressure. But in my experience most of the time its choke/lockout related.

Heus

Can this spring be adjusted by turning the screw in the center of the spring?

Shark Racer

Far too many people have created bogging Q-Jets trying to "feel" the secondaries kick in and pulling too much tension out of their secondary air valves. A well-tuned QuadraJet will deliver the amount of power your right foot demands, with no obvious transition into secondary opening - just like any other well set up induction system.

My 451hp/485lbft 400 ci small block is puny, yes, but I don't find a specific point in acceleration where the secondaries "kick in", though like I said, it definitely has a point where I enter the meat of the powerband. Even if I had one giant throttle blade (typical of EFI) it would feel the same.

If you feel minimal acceleration for a bit of time and then suddenly the power comes on - you've adjusted the secondary air door tension too loose. The car should respond immediately to WOT - period.

dugsgms74

Yes. but you have to first loosen the lock screw.

Heres my take on secondary air door spring wrap adjustment.

You'll experience the best acceleration by setting the wrap as loose as you can w/o a bog. How loose you can go really depends on the car but typically the lighter the car, the more power and the higher the rear gear you have the looser you can go. For instance a full size car with a SB and 2.XX rear gears will need alot of wrap(around 1 full turn) while a BB Vette with 4.11s can get away with nearly no wrap. I run my 68 L36 car w/ 3.70 gears around 1/4 to 1/2 wrap and works well.