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Discussion Starter · #1 · (Edited)
So I have a misfire on cylinder number 7. The distributor was changed to a Delco Remy HEI. It ran fine when I bought it. I have changed wires, plugs, cap and rotor and still have a misfire on number 7 cylinder. I did a compression check on All the cylinders and it’s at 180- 185 psi across all cylinders. Number 7 cylinder was 183. I changed plugs that I know that were firing into number seven cylinder, and changed wires that I knew were firing and still when I pull the plug wire off of number 7 cylinder when it’s running there’s no difference. The engine still sounds the same with the same misfire. When I pull the plug in cylinder 7 it looks wet. What should the gap be on spark plugs on a 71 455? Any Help would be greatly appreciated
 

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So I have a misfire on cylinder number 7. The distributor was changed to a Delco Remy HEI. It ran fine when I bought it. I have changed wires, plugs, cap and rotor and still have a misfire on number 7 cylinder. I did a compression check on All the cylinders and it’s at 180- 185 psi across all cylinders. Number 7 cylinder was 183. I changed plugs that I know that were firing into number seven cylinder, and changed wires that I knew were firing and still when I pull the plug wire off of number 7 cylinder when it’s running there’s no difference. The engine still sounds the same with the same misfire. When I pull the plug in cylinder 7 it looks wet. What should the gap be on spark plugs on a 71 455? Any Help would be greatly appreciated
Wet plug means it is not firing. Set gap to .040" for the HEI even though it can go wider. Smaller gap requires less energy to fire.

Make sure your wires are not touching and the spark energy is jumping to another plug wire.

You may have a dead wire - wires do go bad. Maybe a new set of wires?

On the outside chance, you could have a valve out of adjustment and seeing the plug is wet, it would make sense the intake is working, but maybe the exhaust is not. Maybe a rocker arm has backed off, and gone sideways, so the exhaust valve is not working.
 

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Discussion Starter · #4 ·
Wet plug means it is not firing. Set gap to .040" for the HEI even though it can go wider. Smaller gap requires less energy to fire.

Make sure your wires are not touching and the spark energy is jumping to another plug wire.

You may have a dead wire - wires do go bad. Maybe a new set of wires?

On the outside chance, you could have a valve out of adjustment and seeing the plug is wet, it would make sense the intake is working, but maybe the exhaust is not. Maybe a rocker arm has backed off, and gone sideways, so the exhaust valve is not working.
Wet plug means it is not firing. Set gap to .040" for the HEI even though it can go wider. Smaller gap requires less energy to fire.

Make sure your wires are not touching and the spark energy is jumping to another plug wire.

You may have a dead wire - wires do go bad. Maybe a new set of wires?

On the outside chance, you could have a valve out of adjustment and seeing the plug is wet, it would make sense the intake is working, but maybe the exhaust is not. Maybe a rocker arm has backed off, and gone sideways, so the exhaust valve is not working.
Yes, today after posting I checked the temperature on the exhaust manifold for cylinder seven and it wasn’t as hot as the others. I was thinking I would pull the valve cover off and check valve adjustment as my next step. Thank You for the advice. I’ll let you know what I find out
 

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Discussion Starter · #6 · (Edited)
Wet plug means it is not firing. Set gap to .040" for the HEI even though it can go wider. Smaller gap requires less energy to fire.

Make sure your wires are not touching and the spark energy is jumping to another plug wire.

You may have a dead wire - wires do go bad. Maybe a new set of wires?

On the outside chance, you could have a valve out of adjustment and seeing the plug is wet, it would make sense the intake is working, but maybe the exhaust is not. Maybe a rocker arm has backed off, and gone sideways, so the exhaust valve is not working.
So today I took the valve cover off and turn the engine over and watch the valves open and close. Both intake and exhaust are opening and closing properly. Valve adjustment looks good. I got my Infrared thermometer out and took Exhaust readings. At low idle cylinder 7 exhaust temperature is very low compared to the others cylinders, but at high idle it starts the exhaust temp goes up on number cylinder 7.
Any other Ideas Why #7 cylinder is miss firing at lower RPM’s?
 

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So today I took the valve cover off and turn the engine over and watch the valves open and close. Both intake and exhaust are opening and closing properly. Valve adjustment looks good. I got my Infrared thermometer out and took Exhaust readings. At low idle cylinder 7 exhaust temperature is very low compared to the others cylinders, but at high idle it starts the exhaust temp goes up on number cylinder 7.
Any other Ideas Why #7 cylinder is miss firing at lower RPM’s?
Are you super-triple sure the distributor cap is seated properly? Get a spark plug wire tester or pull the plug wire and see if it will arc to the ground. Maybe pull the plug and keep it on the wire to the ground to inspect the spark.
 

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Guys, he said the misfire developed and then he changed all his secondary ignition components. So unless he took off a bad wire and put another bad wire right back on, odds are the issue lies elsewhere.
So.....valve train looks good from the outside....at least it's moving.
Next step?
I think I might do a leakdown test to see how well the valves are actually seating.
What are you running for an intake?
 

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Leakdown test is a good idea as well as a 'running compression test'. I have found flat exhaust lobes on camshafts doing a running compression test when the regular static compression test showed 170 psi in every hole. #7 was dead and it only showed up doing a running compression test. This was a '78 Chevy 305. Worn exhaust lobe on the cam.
 

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So today I took the valve cover off and turn the engine over and watch the valves open and close. Both intake and exhaust are opening and closing properly. Valve adjustment looks good. I got my Infrared thermometer out and took Exhaust readings. At low idle cylinder 7 exhaust temperature is very low compared to the others cylinders, but at high idle it starts the exhaust temp goes up on number cylinder 7.
Any other Ideas Why #7 cylinder is miss firing at lower RPM’s?

OK, let's recap what has already been done.

Delco Remy HEI. It ran fine when I bought it.

1.) Changed wires, plugs, cap and rotor

2.) Compression check on All the cylinders and it’s at 180- 185 psi across all cylinders. Number 7 cylinder was 183.

3.) Changed (swapped) plugs that I know that were firing into the number 7 cylinder.

4.) Changed (swapped) wires that I knew were firing to the number 7 cylinder

5.) Engine still sounds the same with the same misfire. When I pull the plug in cylinder 7 it looks wet.

6.) Took the valve cover off and turn the engine over and watch the valves open and close. Both intake and exhaust are opening and closing properly. Valve adjustment looks good.

Single cylinder won't fire, all others do.

If bad electronic module, it would be bad and all cylinders affected.
If bad coil, it would be bad and all cylinders affected.
If bad rotor, it would affect all cylinder.

Bad cap - cracked, poor quality where brass electrode is not extended far enough to make good contact with the rotor, drilled/positioned too far away from rotor to make contact.
--When you re-attached the coil to the top of the cap, did you use the same screws? Make sure none of the screws have gone through the top of the cap and are not sticking through causing a short.
---Check the center electrode in the cap - should have a curved surface that makes contact with the rotor button and should be able to move in/out against the spring - you did replace this with a new one, right?.

Since it was running well with the old parts, install the old cap and try that.

Rotor. May be bad. Make sure the contact that connects with the center electrode in the cap is making solid contact with it. I have had to lift up on that rotor contact because it was a cheap thin strip of metal and did not have a good enough bend in it to make solid contact with the cap electrode. Some of them can have the metal contact that signals your spark to the cap's electrode be too short and the gap can be too large for the spark energy to jump it and fire the plug.
---Check the distributor shaft that the rotor sits down onto. It could be bent or have a bad bushing causing a wobble and possibly creating a large air gap that the spark can't jump - so check the distributor shaft for wiggle/play.

The Reluctor wheel that triggers the plugs to fire could be bad or grounding out. There are 8 "teeth" and do the same thing as points do in a points distributor. One of them could be bad/bent/grounding or who knows what - I know nothing about factory HEI's.

You checked the rocker arms and the valve adjustment looked good. Looking good is not the same as the adjustment is good. The valve could be hanging open just a hair and you would not be able to see that. With the engine running, "Zero Lash" number 7 cylinder to make sure the valves are fully closing. Do a forum search and you will find how to "zero lash" your lifters. Watching the valve operation could also show a lifter not working correctly. Put a white mark on each pushrod where you can see it. When the engine fires up, the pushrod should be spinning and the white mark will be easier to see. It could take a little more than idle to get it to spin, but not much. If the pushrod is not spinning, this can mean the valve lifter is not spinning on the cam lobe - which is should do. No spinning could mean a bad cam lobe.

Just an odd problem to be sure since all other cylinders are firing. Do a few more checks/tests and give us the results. (y)
 

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Discussion Starter · #13 ·
if consistant ,,,, exhaust valve lobe or lifter went away
no rocker arm ticking ???
stock rockers ??? non adjustable
No ticking, stock rockers which are adjustable.
I pulled the valve cover off and rotated the engine and both intake and exhaust valves move up and down equally like the rest on that bank. I am going to do a leak down test sometime this week when I get time. Like I said it starts firing at higher RPMs because the exhaust temp goes up just like the rest at higher revs but not at idle
 

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OK, let's recap what has already been done.

Delco Remy HEI. It ran fine when I bought it.

1.) Changed wires, plugs, cap and rotor

2.) Compression check on All the cylinders and it’s at 180- 185 psi across all cylinders. Number 7 cylinder was 183.

3.) Changed (swapped) plugs that I know that were firing into the number 7 cylinder.

4.) Changed (swapped) wires that I knew were firing to the number 7 cylinder

5.) Engine still sounds the same with the same misfire. When I pull the plug in cylinder 7 it looks wet.

6.) Took the valve cover off and turn the engine over and watch the valves open and close. Both intake and exhaust are opening and closing properly. Valve adjustment looks good.

Single cylinder won't fire, all others do.

If bad electronic module, it would be bad and all cylinders affected.
If bad coil, it would be bad and all cylinders affected.
If bad rotor, it would affect all cylinder.

Bad cap - cracked, poor quality where brass electrode is not extended far enough to make good contact with the rotor, drilled/positioned too far away from rotor to make contact.
--When you re-attached the coil to the top of the cap, did you use the same screws? Make sure none of the screws have gone through the top of the cap and are not sticking through causing a short.
---Check the center electrode in the cap - should have a curved surface that makes contact with the rotor button and should be able to move in/out against the spring - you did replace this with a new one, right?.

Since it was running well with the old parts, install the old cap and try that.

Rotor. May be bad. Make sure the contact that connects with the center electrode in the cap is making solid contact with it. I have had to lift up on that rotor contact because it was a cheap thin strip of metal and did not have a good enough bend in it to make solid contact with the cap electrode. Some of them can have the metal contact that signals your spark to the cap's electrode be too short and the gap can be too large for the spark energy to jump it and fire the plug.
---Check the distributor shaft that the rotor sits down onto. It could be bent or have a bad bushing causing a wobble and possibly creating a large air gap that the spark can't jump - so check the distributor shaft for wiggle/play.

The Reluctor wheel that triggers the plugs to fire could be bad or grounding out. There are 8 "teeth" and do the same thing as points do in a points distributor. One of them could be bad/bent/grounding or who knows what - I know nothing about factory HEI's.

You checked the rocker arms and the valve adjustment looked good. Looking good is not the same as the adjustment is good. The valve could be hanging open just a hair and you would not be able to see that. With the engine running, "Zero Lash" number 7 cylinder to make sure the valves are fully closing. Do a forum search and you will find how to "zero lash" your lifters. Watching the valve operation could also show a lifter not working correctly. Put a white mark on each pushrod where you can see it. When the engine fires up, the pushrod should be spinning and the white mark will be easier to see. It could take a little more than idle to get it to spin, but not much. If the pushrod is not spinning, this can mean the valve lifter is not spinning on the cam lobe - which is should do. No spinning could mean a bad cam lobe.

Just an odd problem to be sure since all other cylinders are firing. Do a few more checks/tests and give us the results. (y)
Is the distributor 180 degrees out?
 

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Double check your firing order as well and the inside contacts on the plug wires. Put an ohm meter on #7 spark plug wire to make sure of resistance even new wires can be bad. Wires can crossfire with each other also. So separation is good. Also make sure that #7 is not routed to close to a vacumn hose or the power brake booster. You can switch plugs wires but they can arc to a short thru metal or the carbon black on a hose.

yes rotor and HEI’s coils can short in side only on one contact. Look inside the cap and under for any carbon tracking. HEI were notorious for tracking and arcing under there.

Also a misfire can be caused by a vacumn leak at the intake runner. It misses at high idle vacumn, then as throttle opens and vacumn drops that small vacumn leak is absorbed into the bigger rush of air from the throttle so it does not have as big an effect on the mixture in that cylinder. I would be carefully checking for a vacumn leak at that rear intake runner near #7.

If the Air fuel ratio (AFR) is too lean at idle the plug won’t fire. It needs hydrocarbons to fire under pressure and you have cylinder pressure, but maybe a bad AFR.

Smoke machine is the best way to detect this. Drive to a neighborhood garage that you like and ask them to hook up the smoke to the intake and watch all over, but particularly near that runner. They should not charge too much for that and before you throw more parts at it,mit may be worth a try. Bring a very bright flashlight, you need it to see the smoke.

Good luck and let us know how you do
 

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Discussion Starter · #17 ·
Yes, today after posting I checked the temperature on the exhaust manifold for cylinder seven and it wasn’t as hot as the others. I was thinking I would pull the valve cover off and check valve adjustment as my next step. Thank You for the advice. I’ll let you know what I find out
Wet plug means it is not firing. Set gap to .040" for the HEI even though it can go wider. Smaller gap requires less energy to fire.

Make sure your wires are not touching and the spark energy is jumping to another plug wire.

You may have a dead wire - wires do go bad. Maybe a new set of wires?

On the outside chance, you could have a valve out of adjustment and seeing the plug is wet, it would make sense the intake is working, but maybe the exhaust is not. Maybe a rocker arm has backed off, and gone sideways, so the exhaust valve is not working.
Hi Jim,
I finally got around to doing a leak down test on number 7 cylinder. I have 5% leakage on number 7 cylinder and I know for sure the intake valve is leaking as the air was coming out from the carb. I haven’t removed the head yet. Do you think this maybe a timing issue? Do you think Pinging caused the valve to go bad? I never heard any pinging while driving or under heavy throttle. What should the timing be?
Thank You
 

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Hi Jim,
I finally got around to doing a leak down test on number 7 cylinder. I have 5% leakage on number 7 cylinder and I know for sure the intake valve is leaking as the air was coming out from the carb. I haven’t removed the head yet. Do you think this maybe a timing issue? Do you think Pinging caused the valve to go bad? I never heard any pinging while driving or under heavy throttle. What should the timing be?
Thank You
Reading your post again, I have another question. What does "stock rockers which are adjustable" mean? Stock rockers use a nut which gets torqued down to 20-25 ft lbs, they are not adjustable. Please explain this - stock rockers as in factory stamped steel 1.5 ratio rocker arms, press-in or screw-in studs, factory rocker arm studs, poly locks instead of the factory nuts?

I would back the adjustment off on the intake valve. I pointed this out earlier - "zero lash" the lifters. You cannot look at the rocker arms and determine they are good because they all appear to operate the same. Not all rocker arms are the same exact ratio, a pushrod might be off, a valve may have been ground down/dressed on the top stem if a valve job was done, a valve can be sunken deeper than others during a valve job.

You may just have the 1 valve too tight. If you can hear air back flowing, back the rocker arm adjusting nut off so the rocker is loose. If you don't hear any air, then it is an adjustment. If you do hear air, then you have a valve/head problem. You could have worn valves/seats, a bent valve, a cracked valve, a piece missing if somehow something got sucked into the cylinder and bounced around getting caught under the valve, the valve hanging up on the valve guide, a weak/incorrectly installed valve spring height not applying enough pressure to seat the valve completely closed, OR a cracked head.

Pinging, or detonation, is not always heard. Often you will hear it very audible under load where the engine has to really grunt to get the car moving. But you can also have detonation at higher speeds/RPM that is masked by engine/exhaust sound. Timing too far advanced, low octane, high engine/cylinder temps can be causes you may not necessarily hear.

Detonation can sometimes show up on the nose insulator of the spark plug. If you use a magnifying glass to observe the insulator around the center electrode, you may see aluminum flecks/speckles. This is an indication that metal bits are being shed/melted from the pistons. Not good. So this can be a means to identify a problem.

Many factors can affect the timing on your engine - each engine is different and this has to take into consideration the driveline - trans & rear end gear ratio's. With iron heads, I always suggest Intial to be around 10-12 and 32-34 degrees total mechanical, vacuum advance disconnected, and all in around 3,200 RPM's. That is a base line as you can go up or down on the RPM's as well as the Total advance. Camshaft selection, Compression, Octane, and how the carb is jetted (rich or lean), and Exhaust choices can all be factors. Once you get the mechanical set where there is no pinging, engine temps are not too high, does not labor to turn over when starting, and has some pep, then you have to bring in the vacuum advance and dial that in by knowing how much additional timing it provides at high vacuum conditions.

So no easy 1 size fits all and this is why the factory used a generic and conservative tune on their engines so as to cover a number of bases rather than tune each engine/car individually. The region the car was going to be sold in also has bearing - higher/lower altitude, dry/damp climate, hot/cold weather types, etc., so a general tune setting covered a broad spectrum of adverse conditions. It was also for warranty purposes - the general tune was less likely to cause engine conditions that broke parts/pieces or took out an engine.

So if me, I would first do as I stated in backing off the rocker arm on the leaking valve and see if that does it. If it does, you could just choose to "zero lash" the valves and see if that cures the problem. If the valve stopped leaking, but "zero lash" did not cure the problem, then I might remove the spring on that valve (maintaining air pressure on the cylinder to keep the valve seated and not dropping down), secure the valve so it will not drop down once air pressure is removed/leaks down on the cylinder. You can re-pressurize the cylinder when you are ready to re-install the spring.

Take the spring down to the local machine shop and they should have the installed height specs for your year engine/head and they can test both the open installed height spring rate and closed spring rate. If it is not in spec, then you know you need new springs and they should be able to help you out in ordering what you need. Springs can go weak over time from excessive engine heat, over revving, or just a defective spring.

If the valve still leaks and you are 100% sure you can hear it through the carb with the rocker arm backed off, then I think I would pull the head and get to the root of the issue rather than take a chance that a problem I could solve rather easily becomes a problem that damages the engine - you don't want a valve snapping off the stem and dropping into the cylinder.

(y)
 

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Discussion Starter · #19 ·
Thanks Again Jim,
I will try the steps you explained before pulling engine. If I do pull engine I will rebuild it.
The motor has adjustable rockers(Allen rocker stud style adjustment with lock nut). Screw in rocker studs with oiling through push rods. Unfortunately I bought the car with no records of the engine build. It was a good buy considering it only had 22,000 original documented miles on the body. Original 389 cracked from sitting in a barn on a wood floor from 1966-99. I guess California cars didn’t get strong enough anti freeze in cars that would be in sub zero climate. Anyway… so last owner put a 1971 455 YC Engine in the he rebuilt. He sold it through a classic car dealership so I didn’t get any info on the engine other than pictures of it being built.
 

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Thanks Again Jim,
I will try the steps you explained before pulling engine. If I do pull engine I will rebuild it.
The motor has adjustable rockers(Allen rocker stud style adjustment with lock nut). Screw in rocker studs with oiling through push rods. Unfortunately I bought the car with no records of the engine build. It was a good buy considering it only had 22,000 original documented miles on the body. Original 389 cracked from sitting in a barn on a wood floor from 1966-99. I guess California cars didn’t get strong enough anti freeze in cars that would be in sub zero climate. Anyway… so last owner put a 1971 455 YC Engine in the he rebuilt. He sold it through a classic car dealership so I didn’t get any info on the engine other than pictures of it being built.
OK. Then my money would be on the poly lock being incorrectly adjusted just a little too tight and holding the valve open just enough so it does not seat tightly.

When you back the poly lock nut off on the #7 intake and test for an air leak,and that does it, I would "zero lash" all the lifters/rockers on that side and then do the other side to ensure whoever installed them knew what they were doing. This is done while the engine is running for the best results - in my opinion & experience. Others adjust the lifters differently than with the engine running.

You also want to make sure you have the correct length body on the poly lock - they can be long or short. Look at the allen screw. It should not be sunk into the poly lock, nor should it stick way up and out. You want the allen screw to be somewhat flush at the top or just a thread or 2 above.

Here are a couple posts to read through that will help.



 
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