Had a previous post about some lack luster performance that sorta died. Anyhoo I was messing around with the carb and I just couldn't get that good quadrajet sound so i decided to (prepare to cringe) hold the secondary flap thingies open with a piece of wire attached to the linkage. Took it for a ride, it was cold so at first it sputtered a bit, I got the RPM's up a bit then punched it..... OH-MY-GOD!! It was like someone strapped a rocket to it, made that cool sound too. I was giggling like a school girl for 30 minutes. Obviously I cant drive around like that, I put everything back to normal and same crummy performance. Someone in my previous post had mentioned the secondaries are somehow tied into the choke. Should I just go with a manual choke? Any suggestions? THanks
I know that on our 69 carbs there is a latch that keeps the secondaries from opening when the choke is shut. On mine the lever was bent and it was sticking out all the time holding the secondary flaps shut. The arrow points to it.
David, Makes sense. Any chance you have a close up picture so I can se what its supposed to look like? Thanks
Green arrow is pointing at the secondary throttle shaft. You can see that the little arm is holding it shut on the right....the choke assembly keeps this arm in this position until the car warms up. It them lets go and allows the secondary throttle plates to open up.
That is a later model Carb Adam. The 69's had this little latch that sticks out and keeps the flaps from opening.
I think they changed the style to the one you have because this setup SUCKS!!!!! :laugh: :laugh: It's always sticking. I am about ready to snip it off. :TU:
Dang! thought we were on to something. Ran out to the garage to rip the stupid thingy off and go for a little night time test ride but it looks like someone already beat me to it. Ok so what else can keep the secondaries from opening? Or maybe its time for another carb? I should also mention the carb is from a 68 430 auto
Troubleshooting tips... Here is some info.http://www.corvettefaq.com/c3/QJetTroubleshoot.mht Good Luck Bert
The next thing to check it the spring tension on those secondary flaps. If it takes more than the weight of your finger to open them then they are too tight. That will cause poor WOT performance.
Matt, I had a feeling something was keeping the air valves closed (yep, it was me!). Looks like the choke is not the culprit. How much effort does it take when you stab the valves open with your finger? Is there a "binding" feeling, or only spring tension? This is the tough part regarding internet diagnosis! There should be moderate spring tension holding the air valves closed. Not so much you have to really push to open them, not so little that they flop open with a flick of the finger. Not sure if that makes sense. I suspect something is binding, or the spring adjustment is off. The spring adjustment can be tricky, so I'm trying to get a feel for what's up before we go there. Time for ZZZzzzzzzzzz.....I'll check back tomorrow morning! Glad to hear you found some new horsepower! Devon
Devon, Your right, troubleshooting this over the internet is going to be tough because I have no idea how its suppose to feel. For what its worth I can push them open with my finger with little effort (but I'm a big boy) and if I push them all the way open i hear a click and they stay open. They make kind of a creeking sound but I think its just the spring. HMMM hold on a second..........Ok I took one of my old control arm bolts and placed it on one flap, nothing. Placed a second one on the other flap and it opened. Does that help? Bert Lots of good information there for someone who knows what a secondary airvalve or a secondary venturi bore is. That would not be me. I read the post from top to bottom twice and now my brain hurts. Time to watch a little Leno and go to bed. Thanks guys :sleep:
OK, time to play. Sounds like the air valves might be just a bit sticky. Time for some generous carb cleaner on the shaft, both right and left sides, the middle of the shaft too between the two plates. See if this helps free things up...the air valves should return to closed position even if you push them all the way open by hand. Warm up the engine and see if this helped at all. Yes? No? It may have helped, but maybe not enough? On we go. There are two main mechanisms that control the opening rate of the air valves. The reason for this is that if they just flop open when you floor it, you'd get a pretty nasty bog, or even a stall. The valves are supposed to open smoothly (but not too slowly) as the engine demands more air. The goal now is to get them to open as quickly as possible without causing a hesitation, bog or stall. We are now going to assume that the air valves, shaft and linkage are clean, not warped, folded, mutilated, trashed or spindled in any way! Mechanism #1: Air Valve Tension Spring This one is fun to play with because you can change it and change it back again. Take a look at http://www.stratagaz.com/Quadrajet/23 - Secondary spring adjustment.jpg . In this picture, youre looking at the underside of the carb top. It can also be done with the carb on the car. This adjustment requires an allen wrench (cant remember the size) and a small screwdriver. Place the screwdriver into the tension screw and hold it in placecause as soon as you loosen the allen lock screw, the tension screw will try to relax. Once you're holding the tension screw in place, loosen the allen screw. Now start by turning the tension screw 1/8 turn counterclockwise, make sure you hold it at the new position. Retighten the allen screw to lock it into place. Congrats, you just lessened the spring tension and the secondary air valves should be a little bit easier to open by hand, and should still close on their own. Time for a full throttle test drive after it's warm. No difference? Try it again, one more 1/8 turn. Drive it again. You can even go one more 1/8 turn, they should be pretty loose by now! If you get to the point that the car bogs, the air valves may be opening too quickly...time to turn the tension back up a little. If this isnt helping at all with the performance, put the tension screw back to the original setting so were only messing with one thing at a timetime to look at something different. Mechanism #2: Secondary Vacuum Break Diaphragm Take a look at the white diaphragm and how it's linked to the secondary air valves: http://www.stratagaz.com/Quadrajet/37 - Choke pulloff rod install.jpg At rest with the engine off, the diaphragm is relaxed and you can open the air valves with the push of a finger. At idle however, the diaphragm is engaged by vacuum, effectively locking out the air valves. You can note the difference yourself...try opening the air valves by hand at idle. Should be really tough to do because youre now fighting the diaphragm. Anyway, inside the stem on the white diaphragm is a very small orifice, about 0.010". You can see it if you take the little hose off and look at the end of the stem. This limits the rate at which the diaphragm can relax when you floor it. This is one way the mechanism prevents the nasty "bog". It's possible to speed up the opening rate a little bit by enlarging this orifice, but once you make the change you can't go back unless you buy a new diaphragm! Most folks start enlarging this hole in steps of 0.002". Anything over 0.025" and the damping effect of the orifice is lost completely. If the spring adjustment in #1 didnt help, try enlarging the diaphragm orifice to 0.012 and repeat the spring adjustment trial. You can try 0.014 also, but remember you can't go back! This damn well should do it. Again, if you have a wear problem or damaged/dirty parts, messing with the two mechanisms above isnt going to help. OK, Im done for the moment, I need others to check my work to make sure I didnt write anything stupid! Devon P.S. the hi-res pics came from http://www.stratagaz.com/Quadrajet/Quadrajet_7029202.htm
Devon, WOW this is just the kind of information I need. Thanks so much for taking the time to help out a newbie. I have to go to work now but I'm going to try and skip out early to play with the carb. If your ever in my neck of the woods I owe you at least a few adult beverages. Matt
Devon Great explanation of secondary tuning. Only one thing I would do differently. Warm the car up to fully open choke and disconnect and plug the vacuum line to the choke pull off/vacuum break. If the choke pull off/vacuum break is already the limiting factor, not amount of spring loosening will make a change. Adjust the spring tension with the vacuum break disconnected, then reconnect, and enlarge the hole if there is a loss in performance with the vacuum break connected.
Excellent advice. This is why I need you folks to chime in and help! I didn't consider this and it's absolutely valid. V8Buick.com rocks. Devon
I think he's going to find what's wrong. If he's wiring the secondaries open and the car takes off like a banshe then he's close. Another check to make is measuring down from the airhorn to the holes in the secondary needle arm. I think it's 41/64ths. That's a hair more than 5/8ths of an inch. If the arm is bent to far down the needles aren't pulled up out of the jets far enough. Good Luck!!! :TU:
OK Guys heres what I have to far. Used a half a can of carb cleaner and the creaking sound stopped, feels slightly better. The reason the flaps were staying open at 90* is because of a nut I had on the threaded rod for the air cleaner (hey thats the way K&N said to do it. I removed it and it no longer sticks. I can observe the diaphragm pull back when I start it up and then relax when I shut it down but even when warm (185ish) when I rev it up it doesn't budge. So I removed the hose, plugged it and went for a ride. OH BOY WAS THAT FUN. Marketable improvement although like you said it bogs a little. I havent messed with the spring tension yet because its obvious that the vacuum break is in fact the limiting factor. So heres my next round of questions; When I rev up the motor should I see the vacuum break slide in and out? Wouldnt enlarging the hole give it more vacuum and therefore pull it in more? How the heck do I enlarge the hole in 2 thousandths of an inch increments? I actually have a much better understanding of how this thing works you guys are great!
The first arrow on the left is the secondary arm. The middle arrow is where the needles hang. Look down in there and you'll see them. The third arrow points to the top of the airhorn. The hole. You need to measure from the top of the airhorn to the center of the holes in the secondary arm. Where the needles hang. :TU:
