I tore down and rebuilt my q-jet in December '05 and since then, the car has driven like a dream. I just cruise around with my car and never really get on it too hard. Well, now I just got my g-tech pro so I wanted to get a rough idea of HP and 1/4 ET. I figured out that my secondaries were set to tight because they never opened up during WOT so I loosened them up. Aaargh! Now they're too loose and the spring barely holds them shut. So I tightened them up again. Now they open, shut, open, shut during my run and the car bucks like a bull. I can get a pretty good run in if I don't push the pedal all the way down but it wasn't like that before the rebuild. When are they supposed to open up? I have a 73 Regal but a 72 stock motor and carb. MARK
They're supposed to open as soon as the engine can use the extra fuel and air. What you have to do is tighten them, from where they're set now, slowly a 1/8th turn at a time. Every time you tighten them drive the car and nail the throttle at a roll, when it gets to where it doesn't bog when you do this your there. Course you could add another 1/8th turn just to make sure it won't bog on you if the weather conditions change. You can actually make the adjustments in larger increments until your near where you need to be then make smaller adjustments to zero in on where you need to be. Good luck! Dave B
I used to be good at these, of course it's been a long time. I found that if you loosen the set screw and turn the screw until the flaps drop then slowly tighten until they just close. From there 1/2 turn, then thghten the set screw. Should get you real close. Rich
Okay, I'll try this out. So, in general, they should never open when you punch it from a stand-still but actually as the speed/rpm increases they would slowly open?
Usually, the opening and closing is the result of the diaphragm not dampening the opening. You might check it for operation or whether it is missing, or the link is missing. Ray
Are you talking about the front vacuum pull-off? It is there and working. I own the Rochester - Doug Roe - book as well but it's at home.
Exactly, the air valve opens as the engine demands more and more airflow. Now, unless the choke is engaged, the throttle blades will open with the throttle linkage, and all that does is enable the secondary side to come into play, the air valve actually controls the secondary side. (There's a boot-shaped lock-out link on the choke side to keep the throttle plates closed while the choke is active, engaging it is a good way to check your tuning of the primary side, fwiw.) Set everything to the stock baseline, then you can fine tune from there, often the stock setting is 90% of perfect. Standard setting on the air valve spring is 1/2 turn past contact, like mentioned above. The secondary vacuum diaphragm (the front one) also keeps the air valve closed under vacuum (low demand) conditions, and it has a calibrated opening that delays the dashpot from releasing the air valve, so it opens smoothly. It's adjusted by bending the rod connecting it to the air valve, then if you want to get real tricky, you can drill the orifice bigger (see the Doug Roe book). Once you get the air valve opening spring and dashpot link set, the first/main/key thing to check/tune is the secondary rods, but you prolly knew that...
The secondary airvalve spring will fatigue after years of service. A good idea to replace this part any time a rebuild is necessary. Jon.
Oops, I didn't replace the spring during the rebuild. I'll get that when I start performance tuning if not sooner. This is great information. Thank you very much.
The two parts normally neglected that should always be replaced when rebuilding a Q-Jet are the secondary airvalve spring, and the secondary airvalve cam (this is the funny shaped plastic cam located between the two secondary air valve plates which lifts the secondary metering rods. Jon.
Okay I finally got to test it out today. I tightened it up to stock specs but it bogged right before it shift ed in to second. So this means I have to go a little tighter or is it not getting air when it needs it and I have to losen it? MARK
The thing to remember is that the diaphragm is there to control the opening, slow it in relationship to air demand. If the diaphragm does not control the opening, you will experience exactly what you describe. Be sure the diaphragm is opening, and that the link controlling the air valve is properly responding. Ray
Okay I think I have it close now. It still bogs just a tiny bit but I lifted the throttle ever so slightly and it went away. Then I pushed it down again and kept going just fine. The diaphram looks like it's working good as well. Now that I got it working, I'm going to mess it up. I just bought some new secondary rods (CX to replace my CT). They have the long tip length and .0567. After I put these in, am I going to have to mess with the airvalve spring again? MARK
You wont have to touch them again. Why don't you go smaller on the secondary rods Stock Stage-1 used AU which are .0527 or go really small like CC which are considered mediium however they are only .0300. You can't hurt anything going to rich..Sometimes they make a big difference. Also try different hangers. What letter do you have. A is the richest E thru J probably the best. its fun to experiment and only a 2 minute change.
I'm not sure on the hanger. I figured going from .0774 to .0567 would be a pretty good first try. I'm going to get some DX rods too .0300. They're on eBay for $5 and $1.25 shipping.
Oh, man. I just rechecked my rods. I actually have BZ rods which means I just purchased about the same thing with the CX ones. My hanger is a "K". I'll check my parts carb to what hanger it has.
