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Unsteday idle
#1
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happy
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55 400 idle variations
With engine warm and the vacuum advance disconnected and the port plugged, idle is unsteady. A vacuum gage connected to the rear port of the carburetor reads max on the gauge. Visually comparing the distributor position to a 56 400 and firing order are the same. The 56 idle is smooth and vacuum is normal. I am at a loss as to what could be causing this unsteady idle, and high vacuum. Any suggestions as always would be deeply appreciated.
Happy

Posted on: 5/26 15:04
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Re: Unsteday idle
#2
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Ross
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Best guess is that your timing is vastly advanced. You will need to check it with a timing light.

Posted on: 5/26 19:20
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Re: Unsteday idle
#3
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Thanks for the feedback. High vacuum due to my using the wrong port. I plan to remove the distributor. I wish there were one of the old Sun machines near here but, they are almost nonexistent. Timing mark unsteady. moves +/- 5degres.

Happy

Posted on: 5/31 19:19
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Re: Unsteday idle
#4
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humanpotatohybrid
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I wonder did you have it connected to the vacuum pump instead of the manifold?

I can grab some pics off my car.

Posted on: 6/1 6:27
'55 400. Needs aesthetic parts put back on, and electrical system sorted.
'55 Clipper Deluxe. Engine is stuck-ish.
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Re: Unsteday idle
#5
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humanpotatohybrid
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Pics

Attach file:



jpg  20240601_085602.jpg (3,563.85 KB)
225076_665b1a94333b8.jpg 4032X3024 px

jpg  20240601_085620.jpg (2,994.35 KB)
225076_665b1aa57d2bc.jpg 4032X3024 px

Posted on: 6/1 7:57
'55 400. Needs aesthetic parts put back on, and electrical system sorted.
'55 Clipper Deluxe. Engine is stuck-ish.
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Re: Unsteday idle
#6
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happy
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Yes, but now connected to proper manifold port. Thanks for the pics. You may recall during your visit that I have a running 56 400 that I have been using for reference while restoring the 55. The wiring order is correct, distributor orientation is the same. I have verified TDC and rotor positions are correct. With all this the RPM and timing mark on damper are unsteady. It is a puzzle.

Happy
For what it's Worth: when the engine was dissembled for rebuild (with less than 38,000 miles) number 7 piston had hole (burnt?) in the top compression ring grove. possible crossfire ?

Posted on: 6/1 9:34
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Re: Unsteday idle
#7
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TxGoat
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Detonation due to low octane fuel could cause piston failure. Another possible cause is a vacuum leak that caused that cylinder to run too lean, allowing it to detonate. Ring gaps not staggered correctly could be another cause.

Posted on: 6/1 11:58
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Re: Unsteday idle
#8
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TxGoat
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Defective points, worn distributor bushings or other worn parts in the distributor advance mechanism could cause the timing to "float". Too much friction at the distributor cam/rubbing block could also cause it. A weak spring in the point set might cause it, or a spring that was way too stiff might.
A throttle plate that is loose on the shaft or a loose throttle shaft can also cause erratic idle, and the erratic idle might affect the timing as shown with a timing light.

Posted on: 6/1 12:04
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Re: Unsteday idle
#9
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humanpotatohybrid
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There was also a change between 55 and 56 to space out the plug wires to prevent cross firing between cyls 5 and 7.

Posted on: 6/2 6:47
'55 400. Needs aesthetic parts put back on, and electrical system sorted.
'55 Clipper Deluxe. Engine is stuck-ish.
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Re: Unsteday idle
#10
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humanpotatohybrid
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Posted on: 6/2 7:59
'55 400. Needs aesthetic parts put back on, and electrical system sorted.
'55 Clipper Deluxe. Engine is stuck-ish.
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