Idle Mixture Screw (1 Viewer)

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Pin_Head said:
If you disconnected the vacuum hose to make a reading then yes, it is not the booster. If you used a T, then the booster could be bad.

The EGR can be a source of vacuum leak, but only for 79 and up where the EGR connects below the throttle plate.

Edit: I noticed that you have a '79 in your sig line, but this does not show in my iPad app. If it starts idling rough, rap the top of the EGR valve with the handle of a screwdriver to try to make it seat. If it is leaking and seats, it will idle better right away.
Click to expand...

Fingers crossed I hope it's the egr. I'll have to source a compression tester from someone. The compression was within spec last June. I hope it hasn't changed.
 
It's the EGR. It got so bad that the engine wouldn't even run without keeping my foot on the throttle. I finally screwed in the idle screw far enough that it would run long enough for me to get to the engine. I plugged the two vacuum lines on the EGRVM and started hitting the EGR. Every time I hit it, the idle got better, it took several good whacks. The vacuum is now a steady 17 and the idle is way smoother. There is still a stumble off idle sometimes and when accelerating, it seem the engines power doesn't increase smoothly. Here is picture of the two vacuum lines I plugged and the plugs I replaced. They are dry and very sooty. I don't know if the bad EGR caused this. Anybody else care to make a guess. At least now it runs well enough to drive it down to Jim's and have a pro take a look at it.
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Why won't the stupid thing upload more then one pic:mad:
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The VM monitors the EGR exhaust gas pressure, so at least one of the lines should have a sooty residue.

The EGR valve may need cleaning and lubrication or replacement. You want to verify that it shuts completely.

Once there are no vacuum leaks and you get the idle adjusted properly, the off idle stumble should disappear.
 
If there are no other vacuum leaks, you are likely going to need to remove that EGR valve and clean it up to get it to close completely and keep the modulator to EGR valve vacuum line blocked with a BB to keep it closed, or better yet replace the EGR valve with a good unit. With it off you can apply vacuum and check its movement and if the diaphragm inside is cracked/leaking. Also check the larger vacuum line that goes to the bottom of the EGR modulator - if its loose or falls off it creates similar symptoms where the off idle stumble and engine dying at stops will continue.
 
I drove it around on Saturday then tried to drive it on Sunday and it was doing the same thing. Tapping on the EGR did do anything this time. I soaked all the bolts with PB Blaster and will be removing it tomorrow for a good cleaning and lub. Anyone know if need and gaskets? The parts diagram doesn't show any, but I find it hard to believe there is nothing used. I hope I don't break any bolts.
 
I believe there is a gasket at the intake manifold. To clean it, dip the valve end (not the rubber diaphragm end on top) in carb dip (Berryman's chem dip) for a few hours and rinse it out with spray cleaner. You can use it with Teflon cable lube used for motorcycle cables.
 
Depends on what it's made out of and how particular people want them . I've re-seamed cans of similar design and made a hidden joint on the inside that is welded to hide the repair . Almost need to start a biz fixing nla parts for older stuff since new vehicles are so expensive .
Have an example of said can ? I have a '77 and know little about the early version emission setups .
Sarge
 
Tuesday, I pulled the EGR and gave it a good cleaning as Pinhead suggested. It came off super easy and was pretty clean. The diaphragm was functional as well. After installing it, I plugged all the vacuum lines coming out of the EGRVM and it's idling well with 18 in Hg. The vacuum gauge is not "glued" on 18 though, it is probably +- . 5 inches. I have not driven yet, but there is still a stumble off idle....Sweet!!!!
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I'm starting to think the EGRVM is the problem. Mine doesn't look like the one in the 81 emissions manual. I don't have the "R" port. Would I test it the same or should buy the 79 emissions manual from SOR?
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Are the EGRVM and EGRV fully reinstalled and connected? All other smog components intact?

If yes, then the EGRVM can commonly cause a light throttle surge and a tip-in hesitation. To verify if EGR is the problem, plug the vac hose going to EGRValve. If problem goes away, then it is probably EGRVM related. New ones are unavailable, so leaving a BB in the hose will "fix" the problem indefinitely for no $$.
 
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Yes all the smog stuff is intact. Port P, Q, and the bottom one are plugged. The air cleaner vcv has failed but still pulling a vacuum through the distributor cap.
 
If hose Q is plugged, and drivability problems persist, then EGR is not the source of problems now.
Cleaning/unsticking the EGRV may have helped, but there are also other problems beyond the EGR system.
 
I saw where Jim said the Idle mixture screw at the start of this thread was ok - but, does the slight curve shown in the picture of mine, below, matter?

Thanks!

image.jpg
 
Yes, if the shaft is distorted, that can cause problems. That screw has been run in hard.

When it is bent as pictured, what happens is the base idle will be OK, but off-idle will be wrong. Run the screw in, back it out 4 turns, idles fine. But there will be a dip in the AFR curve a little above base idle, because the tapered shoulder is now too far into the idle passage, which obstructs the flow of fuel at the idle slot.
 
FJ40Jim said:
Yes, if the shaft is distorted, that can cause problems. That screw has been run in hard.

When it is bent as pictured, what happens is the base idle will be OK, but off-idle will be wrong. Run the screw in, back it out 4 turns, idles fine. But there will be a dip in the AFR curve a little above base idle, because the tapered shoulder is now too far into the idle passage, which obstructs the flow of fuel at the idle slot.
Click to expand...


Thanks Jim - I suspected you would say that... I read that these are no longer available... is there an acceptable replacement available somewhere? Or do I need to try PnP places?

Also, I submitted a query to you, via your webpage 'contact' form - is that the best way of contacting you for carb rebuild and performance?

Thanks!
 
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