Tune up?

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Aug 30, 2015 | 06:14 PM

#1

Jsmithblue

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Tune up?

I'm working in my 78 f100, 300, c6

I've got it timed and carb air/fuel set to peak with a vacuum guage....
The problems,
1. There's one hell of a spark knock from cylinder 4
2. The highest I can set the carb idle, in gear the rpms drop to 400ish and vacuum drops to 4hg....and it sounds like it wants to die... When I put it back in park, idle spikes to 1900rpm.

With the spark knock, if I remove all advance (distributor timing to 0-TDC and vacuum) the knock goes away...but dies instantly put into gear...

Any ideas?

-new spark plugs, wires, cap, rotor, coil,
-professionally rebuilt carb 800-1000 miles ago

Last edited by Jsmithblue; Aug 30, 2015 at 06:15 PM. Reason: Mispellings..

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Old

Aug 30, 2015 | 08:04 PM

#2

The Frenchtown Flyer

Fleet Mechanic

Joined: Nov 2002

Posts: 1,643

Likes: 69

SOUNDS LIKE A VACUUM LEAK

PLUS

I would put a timing light on the #4 cylinder (mark the damper accordingly) to verify it is a spark knock. It may just be carbon build-up or a broken piston.

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Old

Aug 30, 2015 | 08:07 PM

#3

Jsmithblue

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I ran a can of seafoam through it today...a lot of smoke and very minimal change.

When I unplug the spark plug on #4, the knock goes away.

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Old

Aug 31, 2015 | 05:36 PM

#4

fordman75

Lead Driver

Joined: Dec 2000

Posts: 6,298

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From: South central, Minnesota

You might want to check for a vacuum leak at the #4 cylinder intake port at the head.

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Old

Sep 1, 2015 | 10:29 AM

#5

seattle smitty

Senior User

Joined: Nov 2010

Posts: 458

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Check #4 compression, even leakdown if you have the guage (what Flyer said about a possible broken piston, one of the few weak points of this engine). Is the vacuum guage anything other than a steady needle?

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