Hi all,

so I’ve finally came to a point to try and fine tune the carb. It’s L82 from 1979. Reason why is I can really smell the gasoline from the exhaust to the point it’s agressive to my nose and eyes. I cannot read rpms cause my elecronic board is still in the mail.

so this is my carb (slightly different from what I’ve seen here on the forum, filter is perpendicular to the carb and I have thos DD screws for idle mixture):




I’m guessing it’s running rich based on my nose smelling the gas and this black residue below my tail pipes:




Now, vacuum gauge is connected to PCV valve port and during idle it jumps violently between 11 and 15 in/Hg. If it’s under load it’s at steady 20+.

now I’m confused. This is with idle mixture screws opened 2 1/4 turns. Everybody here and on youtube videos state 3 to 3 1/2 turns is a good starting point but wouldn’t that make it even richer?

also my timing is super steady at 12 BTC during idle goes to 30 something plus while under load.

I’ve never touched spark plugs but I guess bubba did cause not all spark plug cables are the same, it could easily be the case where they weren’t changed for a decade. Also the rotor I’m guessing was never changed. Should I start with that before messing with the carb?

What am I missing?

Thanks

 

1st thing I see is your rear hot air choke tube is missing, red rubber plug on rear top of carb, & PCV line looks disconnected ?

 

What I see in these photos is a mess real similar to what my 77 looked like when new.
vacuum reading bouncing is a bad thing. But then I'm not certain about your connection point.
This looks like a question for Lars. He's the expert here.
Me, I ripped all this junk off and went the keep it simple route. But years ago I had never heard of Lars.

 

Hot air choke tube? Where does it connect? Ok one side is on red plug but the other… where does it go?

second, my pcv was disconnected because vacum gauge was connected to it’s place. Normally it is connected.

 

First, verify 36 degrees @ 3000 rpm with vacuum advance unplugged.....lock it down and then record the initial base (idle) timing. Ok now disregard what you tube videos are saying and do the throttle drop method on the idle screws.......don't worry about where they have them, worry about where yours needs to be.......if your idle screws are not responding then it is time for a rebuild......the base to main body gasket may have had the weenie......or other factors. Turn the idle screws in 1/4 turn, one at a time until the idle starts to slightly drop, turn it back and thats it.....now if you turn the screws all the way in and no change then the gasket is bad or there is something causing the throttle to hang open too far (you need to tight your adapter for the throttle cable...it is loose).......and the idle circuit is defeated at that point. Your carb looks pretty original......if you can't get it sorted.....contact Lars and have him go through it.

Jebby

 

I verified the base timing is where it needs to be. Idle screws i will try to choke the engine and turn them in 1/4 turns as you suggested. If it doesn't do anything I'll order the rebuild kit. I am a bit worried why my needle is jumping while idling. I was taking a measure with the cold engine, I don't know does it affect anything, I'll try with the warm engine later today. I'll try and post a video. Also I've contacted Lars (sent him a private message) and hope he will respond.

Thanks Jebby.

 

Always start with ignition system before blaming carburetor. Pull the plugs, see if they are the correct ones and correctly gapped. Check the resistance of the plug wires, 1k max per foot of wire. Check cap and rotor condition and free movement of advance weights. After that go back and adjust mixture screws to best lean idle. If automatic, it's done in Drive with wheels chocked.

 

Don't verify base timing....verify total timing and the base just is what it is......you need 36 degrees total......

Jebby

 

The fact that you can smell your exhaust is not an indicator of running rich. CO is odorless. Hydrocarbons small "rich." Hydrocarbons are caused by timing - not carb.

First, your carb is correct in its appearance. All Chevy carbs have the filter perpendicular to the carb, and all '79 carbs have those idle mixture screws. Your carb looks exactly like any other '79 Chevy Q-Jet - nothing different or special about it.

You need to start by fixing your timing. 12 degree initial is not indicative of the timing being set correctly. Set your total timing to 36 degrees and let the initial fall where it may.

Then test and check your vacuum advance. Suck on the hose and verify that it is not ruptured, and verify that timing advances when you suck on the hose.

If your vacuum advance is working, bypass the Thermal Vacuum switch in the vacuum advance circuit and attach the hose directly to the driver's side forward vacuum nipple. This is straight manifold vacuum on the '79 carb. Once you have your timing correctly set and you then hook up the vacuum advance, you should end up with actual timing at idle of about 25-30 degrees.

You need to repair your hot air choke system. If that rear tube is missing, and if the rear hole in your manifold heat exchanger plate is plugged, your choke is not operating correctly, and the carb will run rich. Remove the forward tube that is attached to your choke housing and blow through the rear hole in the heat exchanger plate and verify that the air comes out the forward hole. Once verified, hook up both forward and rear choke tubes to make the system operable.

With the engine cold, crack the throttle open and verify that the choke snaps completely closed. Then, remove the rubber hose off your choke pulloff and attach a longer hose to it. Suck on the hose and verify that the pulloff retracts. It should retract to the point that it opens your choke 1/4" as measured from the forward lower edge of the choke blade to the forward wall of your choke tower. If it does not retract, replace it. If it retracts but the choke does not open 1/4", adjust it.

Hooking up your vacuum gauge to the PCV line is not a good idea. Your PCV is rattling around doing its thing, and will have variable vacuum induced to the PCV line. If you want to play with a vacuum gauge, attach it to a manifold vacuum source on the carb or the manifold - not to the PCV line. All fittings on your Q-Jet are manifold vacuum except the fitting in the lower throttle plate on the forward passenger side sticking out at a 45-degree angle. That fitting is ported vacuum going to your EGR TVS.

With the engine warmed up and running, screw your idle mixture screws in and verify that you can kill the engine by screwing them in. If the engine stays running with the screws seated, you have an inoperable and plugged idle system in the carb. If the idle system is plugged, it means that you are idling and running on the transition circuit with an excessive throttle opening. This will cause you to run very rich. To repair this, the carb must be removed, disassembled, and the idle fuel restrictor (IFR) tubes must be "rodded out" as I have shown in my recent Q-Jet rebuilding posts here on the Forum.

If the idle mixture screws will kill the engine, they are operable, and you should be able to correctly set the idle mixture. There is no spec for the idle mixture screw setting - on a '79 the "correct" setting can be anywhere from 2 turns to 9 turns out from seated, with the most common setting being between 4 and 6. Idle mixture must be set after the timing has been properly set as outlined above, and with your vacuum advance operable and hooked up as noted.

With engine off, turn both screws all the way in until seated. Then screw them both out 6 turns. Start the engine and make sure it's warmed up with the choke wide open. Set the idle speed to the desired rpm - usually around 750 rpm. Then, turn both screws in 1/2 turn at a time and continue to do so until you hear and feel the engine change its tone and you observe a 50 rpm drop in idle speed. At that point, back out the screws 1/4 to 1/2 turn to restore the idle quality. If the idle quality and rpm immediately starts to change as soon as you start screwing the screws in from the 6-turn setting, you need to back the screws out another 2 turns and start this process from the 8-turn position.


Lars

 

Awesome Lars thank you. Additional finds after closer inspection:

1. One of the tubes is missing from here to rear carb inlet for the hot air choke (if anyone has part number I’d be thrilled):




2. EGR seams to be broken. Can it leak/suck from this hole? Here’s a pic:




3. can someone post a pic or tell me how this exhaust flap needs to be positioned so the flap stays opened. Bubba did the emissions delete but this stayed loose. This is from heat riser.





Thanks to all, I’ll redo these and report back.

 

Heat riser positions...


Fresh Air Tube that looks to be missing ZIP LINK: https://www.zip-corvette.com/75-80-c...hoke-tube.html

 

So… an update.

I’ve managed to find some 6mm steel pipe and sand it just enough to slide into that hole. I applied a bit of high temp high pressure RTV around the pipe just to ensure tight fit.





Also I’ve deleted the broken EGR valve, which was deleted before, found a blocking plate underneath so I just cleaned it up a bit.





Now I found a vacuum line tee’d into the EGR line connected to carbon canister. Can I plug that one into ported vacuum where EGR TVS was connected (the one on the passenger side under an angle) or should I just block the hose and carburetor port, each one separately? …or tee it in manifold vacum on the PCV perhaps? Thanks.

 

If you want your EVAP system operational, you have to hook up the purge valve trigger signal hose to ported vacuum - you cannot run the purge valve trigger off of manifold vacuum or your purge valve will be open and produce a massive vacuum leak at idle. If you want the system operational, the 1979 system must be routed and hooked up like this:

You can bypass the EGR TVS and run the EVAP purge line directly to the ported vacuum port on the forward passenger side corner of the carb as shown in the diagram.

All this information is right on your emissions decal in your engine compartment. If you don't have one, buy one and install it. It is also in your Service Manual in the "Emisions" section of the manual, as well as in your AIM.

 

Another update.


First of all thanks Lars, I hooked it up as you suggested, everything seams to be in order.

Additional find, my kick-down cable was unhooked, hooked it back up, works like a charm.

Now, my vacuum needle is rock steady at 15 in/hg. This is the maximum I’ve managed to get by fiddling with the screws. I just dont get one thing. What is the sequence of closing / opening the screws? I’ve opened both to 6 turns and I was geting twichy engine with no more than 10 in/hg, started closing one up, ended closin for full 3 and half turns, got 13 max. I went to the other one, played with it got 15 max. Went back to the first one and it started droping while turning in either direction, so I just left it at 15. Is there an order on how to turn them and when?


In 2 weeks my new cables, spark plugs, rotor and coil is coming so after that I will set the timing right and report back.

 

you should go back and forth on the two scew adjustments, if you turn in on one screw 1 turn switch to the other till you are turning each one in a 1/4 turn at a time

 

You can't adjust them one at a time like that and optimize vacuum with one screw and then adjust the other. That will get you a grossly misbalanced adjustment. Both screws must be adjusted equally at all times. I outlined the procedure for you in detail in my post above. Follow my instructions and do it correctly. Throw your silly vacuum gauge in the trash and hook up a tach. Adjust the two screws eually, 1/2 turn at a time, as I outlined and told you. Listen to the engine and watch the rpm. Follow the instruction I gave you above for a correct setup.
Lars

 

Yes sir, got it. By the way screws kill the engine so I think they are operational.

I’ll set the total timing at 36 degrees and let the idle fall where it may.
i’ll set the idle to 750 rpm.
I’ll make another double D tool, hook them both up and do the 6 turn screw in procedure on the carb as you instructed.
I’ll report back.

Just one short question, when you said to trash the vacuum gauge, is it because it’s missleading, vacuum level not important or just rpm procedure will set it to optimal position and vacuum will be what it’ll be?

Thank you sir once again. Once I’m done I’ll send you a crate of beer.

 

You don't need 2 idle mixture tools. You use one tool, adjust one side 1/2 turn, go to the other side and adjust 1/2 turn. Go back to the first side and do it again. Make note of engine sound and rpm as noted in my first post to find the sweet spot where both screws are equally adjusted. This may occur at more than 6 turns out, as I have noted.

The vacuum gauge is irrelevant for doing mixture adjustment. Use it to trobleshoot engine problems. Idle mixture is adjusted using the 50 rpm lean-drop method as outlined in the Service Manual. Whether you have 15 inches of vacuum or 18 is completely irrelevant. Use a tach. Ditch the vacuum gauge. Or ditch both: You should be able to hear and feel when the best quality idle is achieved. Use your senses and stop relying on gauges that don't mean anything. During my Tuning For Beer Tour seminars we never used a vacuum gauge or a tach to do any of the tuning on any car.

Lars

 

Lars,
Will the below procedure apply to a 78 L42???
With engine off, turn both screws all the way in until seated. Then screw them both out 6 turns. Start the engine and make sure it's warmed up with the choke wide open. Set the idle speed to the desired rpm - usually around 750 rpm. Then, turn both screws in 1/2 turn at a time and continue to do so until you hear and feel the engine change its tone and you observe a 50 rpm drop in idle speed. At that point, back out the screws 1/4 to 1/2 turn to restore the idle quality. If the idle quality and rpm immediately starts to change as soon as you start screwing the screws in from the 6-turn setting, you need to back the screws out another 2 turns and start this process from the 8-turn position.

 

Works for any carburetor but the idle speed is dependent on whether automatic or manual and automatic is done in Drive with wheels chocked and hopefully a working parking brake.