davoaz

72' 350 with a 4bbl quadrajet. Just had the carb adjusted. Shouldn't the secondaries open when revving the engine to WOT? Or does it take the dash pot a little longer to push them open then a quick rev to WOT?

MDchanic

When you say "secondaries," are you referring to the secondary throttle butterflies or to the secondary air valve?

- Eric

davoaz

yes that is what I mean. After I had carb rebuilt the mechanic made some adjustments and when he revved it to WOT you could see them starting to open. They opened pretty quickly. Since then I started studying the chassis manual and adjusted the mixture screws myself. I took it to a second mechanic (recommended) who was supposed to be good with quadrajets to see how well my adjustments were. Besides being rich he mentioned some of the linkage was not set up right to open the secondaries. He tinkered with it but I really couldn't see what he was doing. When I got it home I was running some test on the voltage regulator and started rev the engine to WOT and noticed the 2nd air valves aren't opening like they did before. I can manually pull on the dashpot linkage to open so doesn't seem like anything is binding. But revving the engine with the throttle and watching the dash pot it seems slow to start pushing it open. Perhaps I'm not revving long enough? Makes me nervous pushing the throttle back to WOT w/o having the engine under load more than a quick flip.

oldcutlass

You generally will not see the secondary air valve open the butterfly just reving it on the driveway. The engine needs to be under load.

olds 307 and 403

Unless your secondaries are set loose, you won't see them open. The air valve is velocity operatedand opens a certain amount due to flow and where the stop is set. Joe was right, I was in LA LA land. Both the AVS and Thermoquad/Street Demon also use a similar set up.

MDchanic

+2. If you can see the secondary air valves opening at idle, there is something wrong - either the secondary air valve spring is set too loose, or you're holding it open long enough to make the engine unhappy.

- Eric

joe_padavano

The secondary air valves on Qjets cause almost as many posts as Olds motor mounts and headers for Supremes.

These are NOT the secondary throttle valves. They are also NOT operated by vacuum (ironically, they are held CLOSED by vacuum in the dashpot...). They are opened by mass airflow through the secondaries and will normally not open much if you are revving the engine in park.

GEARMAN69

The choke pull off on many early quadrajets (pre 75) have up to a 2.5 second opening rate when the go to zero vacuum (full throttle) which is easier under load since you run risk of over revving to do it in park. Some early quadrajets have the faster 1 second opening rates from the pull off like the later 75 and later Qjets. The 1 second can begin to open on a throttle blip in park but the 2.5 second one not so much. The slow ones in some cases can be tuned by clipping back where the hose slips on the pull off and drilling in .001 increments the orifice hole inside but you got to be careful not to ruin it. This mod can help a lot in reducing ET in some cases better to swap to another one thats faster if you can. Just have engine off, push in plunger lever on pull off place finger over hose end to block vacuum and let go and see how long it take for the lever to extend while looking at a stop watch

BlackGold

You're right, you've got to be careful not to ruin it. Modifying the vacuum brake to open the secondary air valve faster has got to be the fastest way to make most cars go slower.

Unless you've got a strong race engine and/or a high-stall converter, there's no reason to open the secondaries fast. With the engine fighting the converter and the mass of the car, it takes a couple seconds for the RPMs to climb high enough to warrant wide-open secondaries. Most street cars with a Q-jet can be made to accelerate quicker by easing into the throttle over a second or two than by flooring it instantly. Go ahead and try it. And also note how much more responsive the throttle is when the engine's cold and the choke's still on, holding off the secondaries.

GEARMAN69

Personal tune and combo results may vary.....lol. If it goes slower on a flat punch to floor than easing into it you got tuning to do.... 1 second transition should not hurt ET on a stock car versus a 2.5 second delay. If it does you have an issue. It actually a big improvement normally

davoaz

Thanks, Not worried about increasing ET as its a 260hp 350, LOL. Just wanted to be sure its working. Seems like when I punch it on he road. The way the other mechanic had it set up before, as soon as you pushed the throttle WOT it would open.

GEARMAN69

He probably had the rod or the vacuum removed from the pull off.

cdrod

Gearman:
Thanks for this very useful tip! I'm getting ready to build a Q-Jet for my stroker small block and the info you provided here should be helpful.
Rodney

joe_padavano

That is a classic example of someone who has no clue as to how a Qjet works and who should not be allowed to work on one. You don't know what else this "mechanic" screwed up while improperly adjusting that carb.