Do I have a burnt/burned valve?

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jrlegacy23

jrlegacy23

68 Barracuda
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On my 70 318 there is a miss or a skip when at idle and applying the throttle. It has dual exhaust and it almost feels like the passengers side exhaust sucks in real quick in a consistant pattern while idling.
I am thinking that because of the year of the engine, and stock heads that unleaded fuel may have caused a burned/burnt valve. I pulled the plugs and cleaned them up and they all looked to be in good shape. Can bad/dirty points simulate this, or do you think the heads need replacing. I have a set of mid 80's heads laying around I could bolt on. I am chnging this motor out soon, but I want to make sure it is running good before I pull it.
Thanks for any info.
I could probably take a video and post what I am talking about with the exhaust.
 
If you are getting reversion into the exhaust and have eliminated an external cause of a misfire then my money if on a burned exhaust valve.
 
Start with a compression test dry. Then wet with a few squirts of oil in the cylinders. Then move onto a leakdown test.


If if is a problem due to "lack of lead" in the gas there is an easy check to see if your heads/valves have a problem. Remove the rocker covers and then remove the rocker arms. Look at the top of the valve stems and make sure that they are all the same height (a ruler/straight edge may be used).

On my 340 formula s the guy did not put hardened exhaust seats in the heads when he rebuilt the engine. In less than 10 k miles it started running rough. When I removed the rocler arms to repair the problem, the #7 exhaust valve stem was sticking up approx 1/4" higher than the other valves. The valve had beat up the seat so bad that the valve was sunk in approx 1/4" which shows up when you remove the rocker arms and let it seat on the "beat up seat". The rocker arms hold it open.

It is an easy check to remove the valve covers and rocker arms.
 
An old trick, take a dollar bill and put it up the exhaust with the engine at idle if it gets sucked up to the pipe its a burnt valve.
 
ogisme said:
An old trick, take a dollar bill and put it up the exhaust with the engine at idle if it gets sucked up to the pipe its a burnt valve.
Click to expand...

And if it does, add another 499.00 to the one dollar bill and do a valve job. Just kidding.
 
Throw a vacuum gauge on it, quick and easy way to diagnose engine condition.
A quick jumping vacuum reading is normally the sign of a burn / sticky valve.
 
I used a piece of paper to show how the exhaust flow is. It kind of has dead spots more than a suction. This is how it is running on a cold start up (40 degrees out), could it be a head gasket starting to go?

[ame]http://youtu.be/SYuzhB8fbNo[/ame]
 
I think on that warmed up video, it sounds just fine. Maybe if you had it idling in gear (with the parking brake on, of course) the unevenness would stand out more, if it exists.
 
cudavert said:
I think on that warmed up video, it sounds just fine. Maybe if you had it idling in gear (with the parking brake on, of course) the unevenness would stand out more, if it exists.
Click to expand...

Agreed. I didn't hear anything out of the way. And the old trick of testing for a bad valve by holding a piece of paper up to the exhaust is not accurate. Any time you have a misfire, no matter what the cause, the misfire will cause it to suck the paper back up to the exhaust.

And just looking at the plugs doesn't always tell the story either. I've seen plugs that looked ok yet they would misfire. Most plugs have a resistor built in and it can go bad and since it's inside the plug you can't see it.
 
cudavert said:
I think on that warmed up video, it sounds just fine. Maybe if you had it idling in gear (with the parking brake on, of course) the unevenness would stand out more, if it exists.
Click to expand...

Me too, I saw/heard no major issue.
 
Why change the points??? They won't cause a single cylinder to misfire. Just make sure the dwell is in spec and leave them alone. As for plugs, if they are old, replace them. Yes, a bad plug can cause a misfire but so can lots of things. How old are the wires?

A couple of thoughts from the video:

First, it actually sounds OK. I don't hear a major misfire, but it's tough to tell. I'm betting that your dual exhaust doesn't have an "H pipe" or crossover. This will cause the exhaust to "pulse" because of the uneven firing order. Remember that the engine fires left-right-right-left-right-left-left-right.

Second, the (warm) video does show a little about the health of your heads. Your valve guides are passing oil big time. When you rev, lift off and hit the gas again there is a pretty good puff of blue smoke. (there's also a puff of black smoke when you hit the gas, indicating that you are way rich, have too much pump shot, or both). I bet it blows a small cloud when you first fire it up in the morning. If these heads have some miles on them, that's typical, but loose guides will shorten valve seat life. If the heads have never been off the engine, then I wouldn't be surprised if they were getting tired. If the original timing chain and cam sprocket are in the engine they're on borrowed time and you will be doing a valve job to repair the damage.

As said before, you must DO A COMPRESSION TEST!!! There is no better way to diagnose the overall health of an engine. Just throwing parts at it and guessing is a waste of time and money. You can't tune an engine with uneven compression and internal vacuum leaks!

If this is the original engine, it is showing it's age. If you determine that you need to do a valve job, using a set of "302" casting as your cores makes a lot of sense. You can keep running what you have and them swap on the rebuilt heads. If you are planning to build another engine, why put any money into this one?
 
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