Rough Idle

46 PANEL

MEMBER
1976 CADILLAC ELDORADO
8.1L
V8
2WD
AUTOMATIC
37,000 MILES

Have problem with very rough idle. Think I have looked at everything, but still can't find problem.

Compression test: All okay, about 148 psi, +/- 10%

New plugs and wires, think distributor is fine.

Checked every single vacuum hose, I think. Most now new or like new.

Sprayed carburetor cleaner all around carburetor and intake. No change in speed.

Tightened all carburetor and intake manifold bolts.

Adjusted Q-Jet idle screws, does not affect much at all.

When put hand over intake, when running, speed up a little, so maybe running lean, but screwing idle mixture screws out does not help, even if out until about fall out.

Seems to get enough fuel, runs great at high RPM.

Thanks for your help!

3 Replies

KASEKENNY

MECHANIC
18,907 POSTS

Hi 46 Panel,

Have you checked your timing?

Sounds like you addressed most all the bases but I didn't see this mentioned. If timing is a little low then try advancing it a little. I wouldn't worry about setting it to the exact spec. Let the vehicle determine where you should set it. It will start to idle real smooth and that is where you want to leave it.

Let me know if this is not it or doesn't make sense. Thanks

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Tuesday, May 21st, 2019 AT 4:58 PM

46 PANEL

MEMBER
2 POSTS

Thanks so much for the answer, does make sense.

I now have set at 6 degrees advance, but with rough running, can't get to run very slow, maybe 800 RPM, and even then stalls when put in drive. I do have vacuum off and plugged when timing. Specs. Call for 600 RPM idle, so maybe centrifugal advance is messing up. I'll try to get it slowed down, advance timing a little, idle speed screw a little, when next can, next two days, and tell you how it goes. Some folks say up to about 10 degrees is where they advance to, so I'll try that. I'll check number one at Top Dead Center, make sure timing mark, rotor, are where they belong.

I sprayed carburetor cleaner everywhere, again, carburetor base, manifold, hoses, can't find a leak that way, no speed up at all.

Vacuum to distributor is from carburetor, not manifold, as car came when new. All lines are routed as per original service manual, and now all new. But some say to put advance on manifold vacuum, so I may try that, but it won't change rough idle, maybe, unless there will then always be a little advance at idle.

Thanks again. This is a puzzler, at least so far.

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Wednesday, May 22nd, 2019 AT 5:01 AM

KASEKENNY

MECHANIC
18,907 POSTS

Okay. Sounds like you have a few things to try but just remember that we always set timing first. Then go back and set idle mixture and idle speed. Moving the timing will affect your carburetor settings. So I would get the timing advanced to about 10 like you said because 6 is the low end of the spec. Then go back and tune the carburetor.

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Wednesday, May 22nd, 2019 AT 2:12 PM