400 slight rough idle
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New User
Joined: Jul 2005
Posts: 6
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From: Portsmouth Va.
400 slight rough idle
Just installed a rebuilt 400 tough one from an auto parts store. Run motor for the first 30 minutes at 1500 rpm to lube the cam bearings well during break-in-in as recommended by tom's book. I think that number 4 exhaust value may be burnt. Engine misses and rough idle, very small but detectable (small tick like a value). Motor run very strong and mad oil pressure. Changed intake manifold gasket, which had no change. Will do a compression test at the end of the month.
Spec.
1978 F-250 Camper Special 400 CID <O
</OHolly 600 Vacuum secondary
Weined Intake Mallory Unilite (LED) Pro Sidewinder wires Daynomax Headers<O</O
Spec.
1978 F-250 Camper Special 400 CID <O
</OHolly 600 Vacuum secondary Weined Intake Mallory Unilite (LED) Pro Sidewinder wires Daynomax Headers<O
</O
Posting Legend
Joined: Sep 2001
Posts: 30,255
Likes: 37
Use a vacuum gage to check the valves. There is a link in my home page links section about 2/3 of the way down with instructions on how to read a vacuum gage.
Last edited by Torque1st; Oct 18, 2005 at 12:14 AM.
Tuned
Joined: Jul 2003
Posts: 455
Likes: 3
From: Berkeley
LEAKY VALVE: needle drops 2-4 points when valve should close. short circuiting individual spark plugs will indicate cylinder in which the valve is defective, when engine idling.
would the cyclinder with the valve problem be the one that does NOT change the bouncing vacuum effect?
thanx. Paul.
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New User
Joined: Jul 2005
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From: Portsmouth Va.
Ive have already checked vacum which is 22 lb with a slight osilation of 2 pounds at idle. Already disconnected plug wires to deterime number 4 cylinder has little change compared to the rest. Number 4 plug is brownish color where the rest look new.
