Strange idle issue

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duster360

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Last few times I drove the car (67 Dart), it started fine. Drove it 5 miles or so, shut it off, 5 minutes later restart it, idles about 700 rpm. Normally idles at 850 rpm. Driving back home (5 mile return trip) it has a definite miss. Could be a spark plug, BUT, I get home, leave it idling and open the hood to check a couple things. It isn't like the engine is just shaking real badly, but it is actually a little more stable but idling lower. There is fuel in the holley carb bowls. As I am looking things over it is idling lower and lower until it just shutsoff. During all this the AFR gauge is showing extremely lean when idling, so lean the digital gauge pegs out and wont read. Advice needed.
ALMOST FORGOT. Car is YELLOW
 
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You say it has fuel in the carb, but your AFR gauge and description of how it dies out says it's running out of gas for some reason.

Water in your fuel maybe?
 
Could be water. I bought gas from a station I don’t normally go to. I am sure the 93 octane pumps don’t get used as much as the others.
 
Just had a brainstorm. I wonder if my vacuum advance diaphragm is going bad. I run full manifold vacuum. If the diaphragm is bad that would drop my initial way down and the idle also.
But that wouldn’t explain the extreme lean condition on the AFR gauge
 
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Just had a brainstorm. I wonder if my vacuum advance diaphragm is going bad. I run full manifold vacuum. If the diaphragm is bad that would drop my initial way down and the idle also.
But that wouldn’t explain the extreme lean condition on the AFR gauge

Possible, but like you said that doesn't explain the dying.
Especially like you said "Idled lower and lower until it died".
The vacuum issue would cause it to drop all at once.
 
Possible, but like you said that doesn't explain the dying.
Especially like you said "Idled lower and lower until it died".
The vacuum issue would cause it to drop all at once.
A vacuum leak will lean it out .
 
Yeah, you need to change the color but you know that. Just make sure you remove all traces of it or the jinx stays with the car. lol
 
A vacuum leak will lean it out .
You are absolutely right. I believe that narrows it down to 2 things. Vacuum advance diaphragm or the factory style carbon carb spacer I installed a week or two ago when all this started. I am going to start with the spacer.
 
Problem solved. I just wasn’t thinking when I added the 1/4” carb spacer. Added a little more fuel to the idle and all is well.
 
how did the spacer impact the idle, can you explain ? I just did a rebuild on my holley 1920 and added a spacer...everything is great except right off idle, taking off it stumbles for a moment.
 
When adding a spacer, there will be a larger volume of air under the carb. When there is more air, more fuel is needed.
 
The issues would come and go, even after playing with the carb. I recently checked the distributor vacuum advance diaphragm, no vacuum tool, it seemed fine. Guess it must have have a small leak because the issue would come and go. Now it was all the time. Checked timing and it was low. ( I run full manifold vacuum) Rechecked diaphragm and sure enough, has holds no vacuum at all now. So I capped the carb vacuum port and and retuned everything and all is well now.
 
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