I need a little help diagnosing a problem that just started on my 72. I took it out the other day when it was really hot out (95+ degrees with humidity). Car was driving fine, thermostat stayed at 190 where it always is, oil pressure was fine. I drove for about 45 minutes and was about 2 blocks from my house and slowing to stop at an interesection when I noticed that my exhaust suddenly sounded different. Was coming from the right side and it sounded like I had a hole in my muffler or something. I stopped at the intersection and my car almost died. I goosed the gas to keep her running and she stayed alive but was running very rough. I limped home the last block and pulled into my garage. As it sat in my garage she would idle but was very rough and almost stalling. I have a vacuum gauge mounted in my car which I looked at and the needle was very jumpy (not a constand reading). I shut her down and decided to let the car sit overnight to cool before I started checking stuff out.

Here's where I'm at today. I checked the plug wires, they look fine. No splits/tears or anything that I could see. I started the car and she turns over with no problems and runs but now I hear a loud ticking noise that increases as revs increase. Also when looking at the vacuum gauge it is very shakey. Where it used to give me a nice constant reading, now it shakes on whatever reading I'm getting.

So, what do you guys think so far. I'm guessing something with the valvetrain or maybe worse. I do have solid lifters which I haven't checked yet. Would this be a sympton of valve lash loosening up suddenly?

I'm open to suggestions at what to check.

 

Sounds like your're dropping a cylinder either due to electrical or mechanical reasons, the fact that you can hear a ticking sound may indicate a bad rocker,lifter, cam lobe, etc. If it were me after hearing that sound I'd pull the valve covers and look for something related to the rockers that doesn't look right including the push rods. Maybe a rocker loosened up or a stud pulled loose.
If everything still looks OK, start it up and pull one plug wire at a time until you find the one that's not making any difference, then you can concentrate on that one. I'm betting you'll see something right away if it's loose enough to make noise.

Good luck, hope it's simple.

 

This is right where I would start as well.

 

Take all of the plugs out and do a compression test. This will tell you if there is any mechanical damage. I'm thinking the fluctuating manifold vacuum could be a burnt valve. Also check the heat riser valve, it may have broken and slammed shut.

BigBlockk

Later.....

 

I'm guessing a broken valve spring, and probably an exhaust valve spring.
Don

 

Okay, here's where I'm at. I pulled the valve covers and took a look at the rockers. To my untrained eye everything looks okay. I checked to back sure the nuts holding the rockers were snug and they are. I snapped some pictures...





Anybody see something that I might not be recognizing as a problem?

Next I think I'm going to do as suggested and pull one plug wire at a time and try and isolate the problem cylinder. If the problem is not electrical will pulling the plugs one at a time still isolate the problem cylinder?
Also, since I'm a novice at these things and worse comes to worse and I can't figure this out, will it hurt the car to drive it over to a local mechanic in the condition it's currently in (about a mile away)?

 

If this is the cause what should I be looking for to identify it?

 

your looking for one that is loose. you will need to bump the motor over and check each one while the lifter is on the back side of the cam lobe. Watch for the valve to open, then close and then check for verticle movement in the pushrod with your fingers

 

And I wouldn't drive it until you locate the issue or at least eliminate things like pushrods about to come loose. You may have caught something just it time. If you do the pulling plug wire routine, when you get to the affected cylinder, you should notice that it doesn't have the effect on the engine performance that a good cylinder has even though it might be a mechanical issue.

 

ding, ding, ding, we have a winner! Found a broken exhaust valve spring with the help of a friend today. I'm following up here with how I go about fixing this...

http://forums.corvetteforum.com/show...post1556146217

 

It all depends on whether or not you feel like removing the head in question...

With the head still on
bump the engine around until the cylinder in question is at TDC
(pressurizing the cyl with 125 psi shop air will also keep the valve closed)
using your valve spring compressor, remove the spring retainer
remove the broken springs and replace with new
replace the reatiner and set the valve lash

Don

 

I have used the air compressor method for keeping up valves when changing springs in my LT1 and LS1. Works like a charm. Change the oil just in case. Have a pen magnet handy for the retainers. A good valve spring compressor is key.