FE 390 missing on Cyl 8
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FE 390 missing on Cyl 8
In our 1950 F-1 we have a 1972 FE 390 that has developed a miss on cylinder 8 after warming up and we've noticed that cold idle is rougher also.
Manual choke
Replace plugs and wire - still misses under load?
Suggestions? Never any of the other cyl.
Trace
Manual choke
Replace plugs and wire - still misses under load?
Suggestions? Never any of the other cyl.
Trace
Hotshot
Joined: Mar 2013
Posts: 14,255
Likes: 199
From: Phoenix, Az.
Off the top of my head:
1. Remove the Dizzy cap and look at the inside. Look for a "carbon track" from #8 terminal to the base of the carb. it would look like a crack.
2. Could be #8 plug wire. Get a spray bottle with water in it. At night so it's easier to see a spark. Idle engine. Being cautious of the fan, spray the water all over the plug wires. If they are leaking the spark energy to ground you will see the spark. And a snap sound.
3. Check the gap on the plug.
4. Do a wet and a dry compression test on the warmed up engine. Carb opened all the way. Post the results here.
1. Remove the Dizzy cap and look at the inside. Look for a "carbon track" from #8 terminal to the base of the carb. it would look like a crack.
2. Could be #8 plug wire. Get a spray bottle with water in it. At night so it's easier to see a spark. Idle engine. Being cautious of the fan, spray the water all over the plug wires. If they are leaking the spark energy to ground you will see the spark. And a snap sound.
3. Check the gap on the plug.
4. Do a wet and a dry compression test on the warmed up engine. Carb opened all the way. Post the results here.
Elder User
Joined: May 2017
Posts: 590
Likes: 4
Off the top of my head:
1. Remove the Dizzy cap and look at the inside. Look for a "carbon track" from #8 terminal to the base of the carb. it would look like a crack.
2. Could be #8 plug wire. Get a spray bottle with water in it. At night so it's easier to see a spark. Idle engine. Being cautious of the fan, spray the water all over the plug wires. If they are leaking the spark energy to ground you will see the spark. And a snap sound.
3. Check the gap on the plug.
4. Do a wet and a dry compression test on the warmed up engine. Carb opened all the way. Post the results here.
1. Remove the Dizzy cap and look at the inside. Look for a "carbon track" from #8 terminal to the base of the carb. it would look like a crack.
2. Could be #8 plug wire. Get a spray bottle with water in it. At night so it's easier to see a spark. Idle engine. Being cautious of the fan, spray the water all over the plug wires. If they are leaking the spark energy to ground you will see the spark. And a snap sound.
3. Check the gap on the plug.
4. Do a wet and a dry compression test on the warmed up engine. Carb opened all the way. Post the results here.
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New User
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390 Missing Cyl 8 -Compression Normal and Exhaust Valve Intact
Did a compression test, Cyl 8 had the best compression on that side, valve spring looked to be intact.
Will check the HEI for cracks and spark top ground on Tuesday.
Thanks Trace
Will check the HEI for cracks and spark top ground on Tuesday.
Thanks Trace
Fleet Mechanic
Joined: Jan 2014
Posts: 1,946
Likes: 118
Jefffafa has good suggestions. In addition, an easy check is to make sure that the #7 and the #8 plug wires are not next to one another, or parallel in the spark plug loom. Since they fire immediately in order, close proximity of those wires can cause a misfire.
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Lead Driver
Joined: Aug 2003
Posts: 7,592
Likes: 10
From: **** hole San Jose ca.
A wet cylinder will have the best compression since the oil adds to a better ring seal that gives it allusion that since it has the most cylinder compression that it's ok N0T.
May be a bad valve guide stem seal that is sucking oil.
Orich
May be a bad valve guide stem seal that is sucking oil.
Orich
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390 Missing Cyl 8 -Compression Normal and Exhaust Valve Intact
Ok did a compression test:
1 - 115
2 - 129
3 - 119
4 - 120
5 - 95
6 - 111
7 - 94
8 - 108
Cylinder 8 is the missing cylinder (only wet plug after truck is stopped and plugs pulled)
1 - 115
2 - 129
3 - 119
4 - 120
5 - 95
6 - 111
7 - 94
8 - 108
Cylinder 8 is the missing cylinder (only wet plug after truck is stopped and plugs pulled)
Fleet Mechanic
Joined: May 2001
Posts: 1,396
Likes: 4
From: Cupertino
Plug wire. Ohm out what you put in and retest. A couple of k-ohms is the right answer. Or, swap wires and see if the problem follows.
Compression is low but serviceable on a tired old engine. If it is just a driver, keep driving.
Compression is low but serviceable on a tired old engine. If it is just a driver, keep driving.
FTE Legend
Joined: Nov 2015
Posts: 30,912
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From: Angier, NC
The compression looks good to cly #8, I would be worried of # 5 & 7 but at a later date.
Ok wet plug I think it is not firing the fuel in the cly.
You said you replaced the plug & wire.
Have you checked or better just replace the cap & rotor.
When you had the valve cover off to check the valve spring you did not run the motor to see if both valves were moving did you?
I take it you are still running points? If so what shape are they in and what is the dwell set to?
Are you getting spark to that cly? Pull the wire and install a used plug, start the motor and see if it sparks.
If points and dwell is set right I would say a high point on the cam that the points open on is worn down and may not be opening to fire that cly.
Just my guess.
Dave ----
Ok wet plug I think it is not firing the fuel in the cly.
You said you replaced the plug & wire.
Have you checked or better just replace the cap & rotor.
When you had the valve cover off to check the valve spring you did not run the motor to see if both valves were moving did you?
I take it you are still running points? If so what shape are they in and what is the dwell set to?
Are you getting spark to that cly? Pull the wire and install a used plug, start the motor and see if it sparks.
If points and dwell is set right I would say a high point on the cam that the points open on is worn down and may not be opening to fire that cly.
Just my guess.
Dave ----
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390 Missing Cyl
Have you checked or better just replace the cap & rotor. Doing that Tuesday
When you had the valve cover off to check the valve spring you did not run the motor to see if both valves were moving did you? Checked and both move correctly
I take it you are still running points? Running HEI for the Ford FE390If so what shape are they in and what is the dwell set to?
Are you getting spark to that cly? Pull the wire and install a used plug, start the motor and see if it sparks. I haven't checked it hot yet but cold it does, when it duplicates missing while hot, I'll burn the fingers...
lol
If points and dwell is set right I would say a high point on the cam that the points open on is worn down and may not be opening to fire that cly.
When you had the valve cover off to check the valve spring you did not run the motor to see if both valves were moving did you? Checked and both move correctly
I take it you are still running points? Running HEI for the Ford FE390If so what shape are they in and what is the dwell set to?
Are you getting spark to that cly? Pull the wire and install a used plug, start the motor and see if it sparks. I haven't checked it hot yet but cold it does, when it duplicates missing while hot, I'll burn the fingers...
lol
If points and dwell is set right I would say a high point on the cam that the points open on is worn down and may not be opening to fire that cly.
Hotshot
Joined: Mar 2013
Posts: 14,255
Likes: 199
From: Phoenix, Az.
The miss prolly is a fouled plug. Is it wet from gas or oil?
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