excessive lean condition?

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Mar 1, 2013 | 02:22 PM

#16

djmichael24951

Tuned

Joined: Feb 2013

Posts: 258

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From: Peterstown

Quote:

Originally Posted by nlareau

Looking for advice. 77 300 I-6 with Carter YFA.

The engine idles rough. There is a persistent miss. When I pull a plug it is very very light brown, leaning towards grey. Spark plug is telling me that things are on the lean side.

I am assuming that the reason for the rough idle / missing is due to a lean condition (I'm told that what it usually is). Most common reason for a lean condition is a vacuum leak, right? I can't find it.

20" of vacuum on the vacuum gauge, pretty stable.
Plugs gapped right, distributor new and clean on the inside. Coil is less than a year old. Spark box is Motorcraft, less than a year old.

Set the timing using a vacuum gauge because I think the harmonic balancer has slipped some. If I set it by the grid on the balancer the spark is WAY too early according to the vacuum gauge.

Rebuilt the carb about 6 months ago. It should be pretty clean.

Sprayed brake fluid around and didn't notice an RPM change, so I didn't find an obvious leak.

I don't want to spend $300 on a new harmonic balancer only to find out that isn't the problem.

If I had to guess where there might be a leak, I would guess it is on the intake manifold, if anyplace.

What is the most logical way to troubleshoot this problem?

I had a 1989 Dodge with a 318 in it one time that did the exact same thing that you are describing. And I, like you, did everything I could possibly think of, but nothing corrected it. I checked the EGR. The plunger was operating correctly, and it was getting vacuum, at the correct time. Finally, I just decided that I was going to replace the EGR anyway and hope that it would not be a waste of forty bucks. When I got it off, I inspected it visually, and guess what I found. The piston inside of it (you can see it through one of the exhaust holes on the bottom) was completely gone. I guess it must have burnt up or rusted away. Either way, it was gone and I had exhaust gases going through my carb at all times. Anyway, I replaced the EGR with a new one, and problem solved. It ran like a brand new one. So, before you do anything else expensive, check that EGR. Worst case scenario (if it isn't bad) is, you'll have to spend 2 bucks for a new EGR gasket.

Mar 1, 2013 | 04:18 PM

#17

nlareau

Thread Starter

Senior User

Joined: Jun 2005

Posts: 321

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From: Atlanta

$)(@!!/ son of a b... I am such a knucklehead.

Used the piston stop to find TDC, marked 0, 10, 20, 30. Unhooked the advance and stuck on the timing light.

I was at +40 advanced at idle. No wonder it was a little lean.

You'll be shocked to know that when I set it to about +8, the idle was fine. Reattached the advance and checked the total advance, it went to mid-30s like I expected.

Never knew that piston stop trick before. Note to self, don't set timing by ear (and vacuum).

THANKS GUYS.

Mar 1, 2013 | 04:57 PM

#18

fmc400

MSEE

Joined: Apr 2004

Posts: 10,386

Likes: 35

From: Austin, TX

Awesome, glad to hear you got it worked out.

Mar 21, 2013 | 07:36 AM

#19

nlareau

Thread Starter

Senior User

Joined: Jun 2005

Posts: 321

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From: Atlanta

The End of the Story

Yes, I had the timing wrong. No, the harmonic balancer had not slipped (I finally found the original markings, they were tiny).

The last piece of the puzzle I figured out last weekend. Carter YFA. I had the accelerator pump installed wrong. It will fit in two positions. One of them is dead wrong. I had no function of the accelerator pump.

Wow! what a difference. Runs like a real motor now. Now all I have to do is undo all the stupid stuff I did to burn up the transmission (see my new thread) and I'm good-to-go.

What I find interesting about rebuilding old cars is that I am constantly amazed at how much I don't know.