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*Sigh* Yet another new problem after my vette came from the shop and got shipped out to me: the idle is TERRIBLE below 1,800-2,000 RPM.
Once the car warms up and the choke comes off the idle drops and then the car shutters and dies...
He's a crappy video of the carb with the air filter off while it's running and dying:
https://www.facebook.com/adamindublin/videos/10154929106561929/?l=4799614494155319250
When I take it to wide-open throttle the secondaries interestingly enough never open.
I had headers installed and a throttle position sensor installed by the transmission shop because the computer-controlled 4L60e needs it.
This is the original Quadrajet from the car (79) and it was rebuilt and rejetted in 2014.
I don't even know enough about the thing to say whether it has mechanical secondaries or vacuum secondaries but given the mini vacuuum-looking thing sticking off of it, I'm guessing vacuum. My headlights barely/ don't go up or down now either unless I really give it a lot of throttle.
My best guess is a combination of needing to rejet after the headers went on and having a vacuum leak somewhere but I don't know what I'm talking about.
If I can get it to idle decent enough to not die without me pushing on the gas pedal it will help me to T-shoot my gas leak; I don't need it perfect as it's coming off for an EFI conversion....
I'm CLUELESS about carbs and I was really hoping to stay that way and do the EFI conversion first, but I need to get it to a place that it doesn't die to help T-shoot the fuel leak. The secondaries not opening at all also concerns me...
Adam
Last edited by NewbVetteGuy; Dec 19, 2016 at 02:53 PM.
From: At my Bar drinking and wrenching in Lafayette Colorado
The Q-Jet is a mechanical secondary carb. If the throttle linkage has enough travel, the secondaries will open unless the secondary lockout is engaged due to the choke being closed. You cannot "wing" the throttle in neutral to make the secondary airvalve open, so if that's what you're doing, you're not testing it correctly.
Your poor idle is another problem though, and is indicative of a terrible tuning job having been done by the shop you hired. Main jetting does not affect idle, so even if the jetting is lean for the headers it will not cause the problems you have.
You need to do a complete tuning setup on it, starting with timing. Don't touch the carb until the timing is set up correctly. You should be running about 18 degrees initial with 36 total. Add the advance from a correctly matched vacuum advance for an additional 11-12 degrees, and you should have about 30 degrees of timing at idle. Verify that before you do anything else.
The Q-Jet is a mechanical secondary carb. If the throttle linkage has enough travel, the secondaries will open unless the secondary lockout is engaged due to the choke being closed. You cannot "wing" the throttle in neutral to make the secondary airvalve open, so if that's what you're doing, you're not testing it correctly.
Your poor idle is another problem though, and is indicative of a terrible tuning job having been done by the shop you hired. Main jetting does not affect idle, so even if the jetting is lean for the headers it will not cause the problems you have.
You need to do a complete tuning setup on it, starting with timing. Don't touch the carb until the timing is set up correctly. You should be running about 18 degrees initial with 36 total. Add the advance from a correctly matched vacuum advance for an additional 11-12 degrees, and you should have about 30 degrees of timing at idle. Verify that before you do anything else.
Lars
The secondaries aren't SUPPOSED to open in neutral? -Huh, didn't expect anyone to say that...
This is the thing: The car was running great, idle included 6 months ago. Then the car went into this shop to get the transmission swapped and headers installed- then it got put on an open carrier truck and sent to me and now it runs like crap.
Would a massive vacuum leak cause idle issues like this? (because I'm pretty sure I've got a definite vacuum leak somewhere...)
Check your timing and set up as Lars recommends. Then, go from there.
You guys haven't steered me wrong yet, but vacuum leaks cause crappy idles like this; I KNOW I have vacuum leaks and you can even hear the hissing in the video and you're telling me to start with ignition timing --particuarly knowing that the car ran great 6 months ago and then all of a sudden started idling crappy???!?
--That sounds like really bad troubleshooting and I think I'm going to look for vacuum leaks first, instead. Hope no one is horribly insulted by that but checking timing given the circumstance doesn't seem like the logical place to start.
I know Lars is the MAN when it comes to the Qjet carb, but I'm not sure how much he sees cars like mine that have entire systems that haven't been touched since 1979... -I've got some systems that have been almost completely replaced end-to-end (like the brakes) and other systems that are completely untouched.
Adam
Last edited by NewbVetteGuy; Dec 19, 2016 at 06:36 PM.
From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
It's obvious that you need to track down vacuum leaks first. But then you need to spend some quality time with your ignition before tuning the carb. Who knows what the shop did with your car before you received it.
Last edited by resdoggie; Dec 19, 2016 at 06:47 PM.
It's obvious that you need to track down vacuum leaks first. But then you need to spend some quality time with your ignition before tuning the carb. Who knows what the shop did with your car before you received it.
Sound like it needs a very good visual inspection and the use of a stethoscope to find vacuum leaks if they are not obvious.
Looking for help on getting a plan together to find the vacuum leak
While the engine's running: Get a pair of pliars and start pinching the major vacuum hoses coming out of the carb and intake one at a time- if the idle improves and the hissing sound stops, I know the general direction to look.
If #1 worked, then continue following the vacuum line to where it splits off into other directions and pinch each of them off until the idle improves- use this method until I identify the source
If #1 didn't work, start spraying WD40 with a straw around vacuum fittings and carb / intake gaskets
I'm not sure if this is relevant but when the shop got rid of my stock manifolds and put on headers, they DID get rid of the AIR/SMOG pump -I don't think it connects to engine vacuum at all, but figured I'd mention it...
Hmm... This Corvette Magazine article says that the smog pump DOES connect to the Vacuum system and has it's own "vacuum advance solenoid"... -Did it somehow get a signal from manifold vacuum and increase the amount of air being pumped into the exhaust manifold at WOT? (to keep the dilution rate steady?)
Now I'm thinking that the shop that installed the headers and removed the SMOG pump didn't plug the vacuum line that was going to it! I've got something to look for, anyway!
Yes, a large vacuum leak can cause all sorts of problems and with so many vacuum operated systems in a '79 such can be a rather daunting task.
You said that the transmission was swapped. Does the replacement, like the original use a vacuum operated modulator valve? Such gets its vacuum from a fitting at the top, rear center of the carb. It's mainly a formed metal tube with small sections of rubber at each end. Make sure that line is hooked up properly (if needed) and plugged if unneeded.
Base idle speed can bet set with all vacuum operated controls plugged (in fact you must ALWAYS plug the feed to the vacuum advance). Granted such requires plugging a lot of connections in a '79 but it can be done and will eliminate vacuum leaks as problem contributing to idle.
You said your car has "entire systems that haven't been touched since 1979". With the possible exception of the HVAC (which is easy to check individually for vacuum) leaks I suggest that you replace EVERY vacuum hose for any of those systems that are still functioning! The hoses themselves may test fine but ends get stretched and nearly every connection is probably leaking slightly. Doc Rebuild (look him up) sells GREAT replacement hose kits with excellent diagrams and first quality parts all striped like the originals.
Since you had headers installed, the A.I.R. system was almost certainly abandoned as A.I.R. system equipped headers are very rare. The A.I.R. system has a TVS (temperature operated vacuum switch) in/near the upper radiator line connector in front of the carb that should have been removed and plugged. The same TVS also operates the EFE (early fuel evaporation) system that was also almost certainly removed with the header install. That TVS should have been removed with its connection plugged. Its vacuum source is near the top front of the carb and it should be plugged.
Another TVS operates bot the EGR and EVAP systems. Even if your EGR has been removed you should keep the EVAP system working. By the way--EVAP (fuel evaporation) system hoses are the most likely to be HIDEOUS condition.
Yes, a large vacuum leak can cause all sorts of problems and with so many vacuum operated systems in a '79 such can be a rather daunting task.
You said that the transmission was swapped. Does the replacement, like the original use a vacuum operated modulator valve? Such gets its vacuum from a fitting at the top, rear center of the carb. It's mainly a formed metal tube with small sections of rubber at each end. Make sure that line is hooked up properly (if needed) and plugged if unneeded.
Base idle speed can bet set with all vacuum operated controls plugged (in fact you must ALWAYS plug the feed to the vacuum advance). Granted such requires plugging a lot of connections in a '79 but it can be done and will eliminate vacuum leaks as problem contributing to idle.
You said your car has "entire systems that haven't been touched since 1979". With the possible exception of the HVAC (which is easy to check individually for vacuum) leaks I suggest that you replace EVERY vacuum hose for any of those systems that are still functioning! The hoses themselves may test fine but ends get stretched and nearly every connection is probably leaking slightly. Doc Rebuild (look him up) sells GREAT replacement hose kits with excellent diagrams and first quality parts all striped like the originals.
Since you had headers installed, the A.I.R. system was almost certainly abandoned as A.I.R. system equipped headers are very rare. The A.I.R. system has a TVS (temperature operated vacuum switch) in/near the upper radiator line connector in front of the carb that should have been removed and plugged. The same TVS also operates the EFE (early fuel evaporation) system that was also almost certainly removed with the header install. That TVS should have been removed with its connection plugged. Its vacuum source is near the top front of the carb and it should be plugged.
Another TVS operates bot the EGR and EVAP systems. Even if your EGR has been removed you should keep the EVAP system working. By the way--EVAP (fuel evaporation) system hoses are the most likely to be HIDEOUS condition.
I have no idea; I can't believe a transmission would require a vacuum signal. My new trans is a 4L60e; does that mean anything?
I have no idea; I can't believe a transmission would require a vacuum signal. My new trans is a 4L60e; does that mean anything?
What am I actually looking for and where?
Adam
Presuming it was automatic, your original used a vacuum modulator valve to downshift and delay upshift during wide open throttle. Again its connection is top, center, rear of the carb via a tube stub. There is only a short rubber hose there before transitioning to a formed metal tube that leads down and back to the transmission where it connects via another short rubber tube.
I believe that your new 4L60e transmission uses something along the line of "electronic force motor pressure" to accomplish the same thing and requires an adapter to convert to vacuum operation. I would have thought that and decent transmission shop would have known this and made proper connections/adaptations.
Presuming it was automatic, your original used a vacuum modulator valve to downshift and delay upshift during wide open throttle. Again its connection is top, center, rear of the carb via a tube stub. There is only a short rubber hose there before transitioning to a formed metal tube that leads down and back to the transmission where it connects via another short rubber tube.
I believe that your new 4L60e transmission uses something along the line of "electronic force motor pressure" to accomplish the same thing and requires an adapter to convert to vacuum operation. I would have thought that and decent transmission shop would have known this and made proper connections/adaptations.
The shop is a self proclaimed auto care and "hot rod" shop and not a trans shop; I planned and gave the transmission swap instructions to them because I don't like to leave anything to chance and this disturbs me as I knew absolutely nothing about it... I am going to call monster transmission about it and then the shop depending upon what monster says....
The shop is a self proclaimed auto care and "hot rod" shop and not a trans shop; I planned and gave the transmission swap instructions to them because I don't like to leave anything to chance and this disturbs me as I knew absolutely nothing about it... I am going to call monster transmission about it and then the shop depending upon what monster says....
Adam
Thinking about it a bit; my best guess is that the 4L60e doesn't need such a vacuum hookup because it has the electronic throttle position sensor signal and already knows what percentage the throttle has been depressed/ is open. This vacuum thingy doesn't actually make sense; I can't even understand why something like an old-school 700r4 would need to use vacuum for this as the TV cables purpose is to provide throttle position information to the transmission; this seems like it would be an either vacuum or throttle position knowledge kinda requirement, right? What value would both bring?
Adam, we are on similar trajectories. Let me share what I've learned (which is always subject to comment, which is fine, because the wisdom of the crowd is awesome.) What everyone posted is correct. The carb tuning process is vacuum first, ignition second, carb tuning third. Period. The first two are basically stated assumptions in order to tune carb correctly.
No, the 4l60e does not use vacuum. I installed a slightly younger cousin (700r4) to replace my THM 400. Out came the vacuum line and electric kick down wire.
When I first got my 74 in 2015, i chased down massive vacuum leaks. Every stinkin' system was compromised for having sat for 15 years. The original carb was long gone, i was working with a holley 570. After replacing intake gasket, sniffing with propane and starter fluid, etc, heres what i did to find the very elusive vacuum leak.
i removed all vac hoses. When choke was on, engine idled GREAT. When choke was off, idle wouldn't hold. If i put my had over primaries, idle worked. So the leak was still there after all hoses removed and plugged, intake gasket replaced, and carb rebuilt. (Insert expletive here to completely relive the experience. Maybe throw a wrench for some good drama.)
so went to my local 7/11 and bought a $.79 Black and Mild cigar. (Nothing but the best for me, high brow stuff) i stuffed a shirt in the primaries and secondaries. All vacuum lines are blocked off. I discoonected brake booster hose, took a big pull on that black and mild, and blew it into the brake booster line that was still connected to the manifold vacuum. Low and behold, the smoke came out of the shafts of the butterfly valves on the throttle plate. Viola. I bought a new carb (holley 600) and problem was fixed. (The completely rebuilt holley 570, electric choke, vac secondaries, but needs throttle plate shaft seals, is now for sale if anyone is interested. ;-) )
Hope this helps. Good luck tracking the vac-gremlin down.
I can't believe it but my crappy idle issues are gone; she runs like she's supposed to now, just that easy!
It was NOT the vacuum going to where the smog / A.I.R. pump used to go- they properly put a vacuum cap / stopper thingy there.
It was Swampeastmike's idea that they might not have plugged the connection that used to go to the TH350 trans that made me take the dual snorkel off and check the rear of the carb- I did not find the metal pipe coming out of the back of the Qjet; I was instead greeted to the top of the manifold vacuum port being broken off. (I'll post a link to the picture as soon as I get it off of my phone.)
WTF- Is this where the connection to the trans would have gone? (It's a T fitting coming out of the intake manifold with a 3rd connection out the top and the top's hose barbs were broken off. I just put my $0.50 vacuum plug on it and VIOLA! Magic idle demons be gone!
It idles at 1,000 RPM now after it warms up, which seems a little bit high but I'm happy with it. I alternated pinching different vacuum hoses coming out of the carb and the manifold vacuum and didn't see any major idle improvements so I'd say that I have minimal vacuum leaks now, anyway!
I honestly can't believe this went this smoothly; I seriously used to dread working on cars and was thinking that there might be no way I should own an old car like this and drop all the money I'm dropping on it because it's going to **** me off when I can't fix it, but with 2 projects on 2 different days in a row I've been successful. I like how this is going so far!
Photo of the problem; can anyone confirm what is connected to that vacuum hose barb from the factory? (It's the connection to the transmission, right?)
Note: Please ignore all the chipmunk food on top fo the intake manifold; I've only had the car here for like 8 days now and the chipmunk food is low on my priority list. ;-)
Adam
Last edited by NewbVetteGuy; Dec 21, 2016 at 12:20 AM.
Really Crappy Idle & Vacuum Issues....
