Weird problem setting idle on '67 L79
After participating in Leif's thread about a stumble at cruise, I thought I would relate what I went through and would be interested to see if anyone else has seen the same thing. I had the same problem with a stumble at cruise as Leif. I finally solved the problem by setting the idle speed correctly. The problem was that I could set the idle at a constant 750 RPM, but as soon as I put the air cleaner on, it goes up to 850 RPM. I thought maybe the air cleaner was partially clogged, so I bought a new NAPA Gold element. No difference. Just for kicks, I orderered a K&N element, no difference. I finally screwed the idle adjustment screw in 1/4 turn and installed the air cleaner. Solid 750 RPM idle. And yes, I do have the proper gasket between the carb and air cleaner base. Is this normal?
-
-
Re: Weird problem setting idle on '67 L79
Dennis,
Doesn't sound normal. Just for kicks cup your hands over the carb just enough to slightly restrict the air when the engine is at idle. If the RPM rises you may have a vacuum leak or the idle mixture is to lean. With exception of the PCV valve, I pinched the vacuum lines one at a time to find a leak.
Some spray stuff like WD-40 at the base of the carb or other suspected areas to see if it changes the idle speed to find leaks.
DOM -
Re: Weird problem setting idle on '67 L79
With an L-79, what are we looking at for a carburetor (what model)?
Stu FoxComment
-
Re: Weird problem setting idle on '67 L79
Yes it has a List 3810 4160. It is the proper replacemment carb for the car that I purchased new about 25 years ago. I doubt it has 5000 miles on it. The car runs so good now, I hate to touch it. Although I have checked for vacuum leaks before, I am going to try what Dom suggests and go over it again. The carb has been on and off once since I checked for vacuum leaks last time, maybe.....Comment
-
Re: Weird problem setting idle on '67 L79
Dennis,
I agree with Dom, if the engine speeds up after the air cleaner is installed it sure sounds like the idle mixture is set lean. As soon as the air cleaner restricts air the engine likes it.Comment
-
Re: Weird problem setting idle on '67 L79
Tim,
I believe you are saying here that the presence or absence of the air cleaner makes a difference in the richness or leaness of the mixture. If this is really so, than what I am observing is "normal". I have the idle mixture screws set to produce maximum vacuum. When I set the idle low with the air cleaner off, it comes up to the proper idle speed when I install the air cleaner (because of the restriction), and "the engine likes it". My head is starting to hurt. As I said before, I am going to re-check for vacuum leaks, but that's probably going to be the end of this, because the car runs and drives perfectly.Comment
-
Re: Weird problem setting idle on '67 L79
Dennis,
It may be as simple as the pcv is stuck open.Comment
-
Re: Weird problem setting idle on '67 L79
Gene,
Checked the PCV valve. It rattles when shaken and passes the old "suck-'n-blow" test. BTW, it is marked :
AC SPARK PLUG
TYPE CV-7260
Is this the correct PCV valve for this engine?
DennyComment
-
Re: Weird problem setting idle on '67 L79
Dennis,
Set the correct idle speed then adjust the A/F screws trying to keep both sides the same, do this with a fully warmed engine. If the idle speed increases, lower it again to the correct RPM then set the screws again. When you install the air cleaner the idle speed should not change by much if any. By what you report it sounds like the idle screws are not adjusted correctly.Comment
-
Leif
'67 Coupe L79, M21, C60, N14, N40, J50, A31, U69, A01, QB1
Top Flight 2017 Lone Star RegionalComment
-
Re: Weird problem setting idle on '67 L79
I finally went out and dug out my Chassis Service Manual and reread the section on setting the idle. I did everything according to the instructions. adjusting the idle screws to get the highest vacuum gauge reading, then adjusting the idle speed screw to get the RPM's right. do this alternately until you get it right. I did all of this. What I had forgotten is that the manual says :"NOTE: If necessary, final adjustment the carburetor may be made with the air cleaner installed." The word "necessary" tells me that what I experienced is normal. From what I can see, it is virtually impossible to get at the adjsutment screws with the air cleaner on, so what I did was probably correct. BTW, I could find no vacuum leaks and passed Joe's PCV test. Chalk it up to being a little anal (I am an NCRS member, right?).
Thanks to all who chimed in in my futile quest for the solution to a non-problem. Back to more important things, like trying to get the interior quarter trim panels to fit on my '68.Comment
-
Re: Weird problem setting idle on '67 L79
Dennis,
The point I want to make is that anytime you change the position of the throttle blades (increase idle or reduce idle), you change the idle A/F ratio so the emulsion screws need to be checked. That's the point of the adjustable emulsion screws to trim the idle A/F ratio because the idle circuit is always richer than a engine's fuel needs at idle.
You make a good point about the service manual stating to adjust with the air cleaner installed if possible but on our cars it's impossible. I think that's why the lean roll adjustment is mentioned, because after the air cleaner is installed the slight air restriction corrects that lean roll.Comment
-
Re: Weird problem setting idle on '67 L79
To be clear, the manual specifically says to adjust for maximum vacuum. The only time the term "lean roll" is used is in reference to Rochester BV carburetors.Comment
Comment