Kurgen

Gentlemen,

Occasionally, I get a popping noise that sounds like it is either coming from the headers or exhaust. It usually requires the car to warm up to happen. Interestingly, I drove the car for three hours on the highway and in the Woodward Cruise and it did not occur, restarting again the other day. I thought maybe it just didn't like Cleveland.

It does not occur at idle or when revving or when decelerating, but does occur during acceleration. Not in a fixed rhythm either, as if based on engine speed. I do have an old exhaust system and the mufflers have a couple holes in them.

Appreciate any thoughts.

Roco71

I had a afterfire problem that turned out to be a bad plug. It can also be caused by an exhaust leak I have heard that a vacuum leak will also cause it. I know that there are other causes that will contribute.

Kurgen

I may have an exhaust leak at the end of the header. I see exhaust coming up hear the firewall on the passenger side, but cant completely confirm it is from that spot. Headers appear solid, but will check. With an exhaust leak, I would have expected it to pop all the time, even at idle and more when letting off the gas that on it.

The whole intermittent nature of the issue is confusing me. Seems a vacuum leak would affect it all the time, hot or cold.

Kurgen

Thanks Roco71 - I saw more of your reply in email. Anyway, how would a vacuum leak effect the fuel burn? I can understand the plug issue and will get a new set asap and give it a go.

71coupe

I would also say exhaust leak. You might want to check your EGR valve too. How does a vacuum leak affect fuel burn? Because it's harder to light a lean air/fuel mixture when the fuel molecules are too far apart.

Try a new set of spark plugs first

Roco71

I had the problem about the same time I bought a new carburetor from Summit and the tech kept saying it was a vacuum leak. I googled it and that was one of the symptoms. It had nothing to do with my problem though. I am hoping that Lars will jump in on your post because he has some good comments on the causes of afterfire. Lars says that a lean carb. will cause the problem as well.

Kurgen

Thanks Guys. I will get a set of new plugs this weekend. I did check the header/exhaust pipe connection last eve and it appears to be separated a bit, but will confirm that as well. It was too hot when I checked it to get too close. The header connection to the block seems tight as I don't see any blow by exhaust from the joint but will check again. Based on what I see, there is an exhaust leak somewhere, just need to find it.

Kurgen

Gents,

Last eve, I check a couple of the plugs. The gap was off a bit (too tight) on some, but generally ok. Will check the others. Interestingly, and adding to the surprises I keep finding with this car, the PCV valve mounted in the valve cover is not connected to anything. I noticed in the book that the original was mounted to the intake but can't tell what it was connected to. My engine has a Victor Jr intake and Holley carb, so things are a bit different. Can you tell me where the PCV should be connected to? There is what appears to be a vacuum line connection at the bottom of the carb and that is connected to a coupling and several rubber lines heading toward the front of the car. Thanks again.

cv67

Should be a port on the back (base) of the carb may be a fitting in the rear of the intake for the power brakes also. Do not "T" and share those.

Kurgen

There is a port on the back of the carb that is split and at least part of it goes toward the front of the car. I did not follow it but will tonight.

Are you saying the PCV should be connected to this port?

Roco71

On mine its on the front of the carb. The back port is for the power brakes.

Kurgen

Seems I located at least part of the problem. I had a severely oil fouled plug in the rear most cylinder on the passenger side. Upon inspection of all the plugs, 2 others had oil residue on them although not fouled like that one. The popping seems to have stopped, indicating it was a detonation in the exhaust issue, which makes sense since I would suspect most of the gas from that cylinder was just pushing out the ex valve. As an aside, that cylinder also seems to have a noticeable slapping type noise that is rpm dependent. I am working to see if it is an exhaust leak at the header plate or a bad valve/lifter/push rod, etc.

If anyone has a quick trick to tell the difference, please let me know.