[fred56]

I was out today and on the way back my SBC started clacking(rocker arms?) and as soon as I went to an idle it stopped clacking. What it seems to be doing is back firing/missing out the exhaust. I checked the timing/wiring/vacuum leaks and what I have come to the conclusion is that one or more of the exhaust valve springs are getting weak. At idle it purrs like a kitten but at 2000 rpm and up it pops/misses thru the exhaust not the carburetor. If the springs are weak can you compress(move them at all) them a little by hand? It seems to me that the springs are weak. They are very I mean very old. How can you measure spring pressure without taking them to a machine shop?
Thanks - Jerry

[F-BIRD'88]

Use a bathroom scale and a drill press. Check them at 1.75" and 1.70".

stock 85# Z28 #105-115 stock diameter race 125-135#

[dgcantrellsr]

If they are (very old) replace them,new one's are not that expensive. And while you got them off check for valve guide slop. Also may have lobes going on camshaft.

Originally Posted by fred56

I was out today and on the way back my SBC started clacking(rocker arms?) and as soon as I went to an idle it stopped clacking. What it seems to be doing is back firing/missing out the exhaust. I checked the timing/wiring/vacuum leaks and what I have come to the conclusion is that one or more of the exhaust valve springs are getting weak. At idle it purrs like a kitten but at 2000 rpm and up it pops/misses thru the exhaust not the carburetor. If the springs are weak can you compress(move them at all) them a little by hand? It seems to me that the springs are weak. They are very I mean very old. How can you measure spring pressure without taking them to a machine shop?
Thanks - Jerry

[GenYnNC]

What the gentleman above said... hope it's not cam lobes.

[EOD Guy]

Might ck your dizzy cap and rotor, mine was not secured properly, which led to a misfire condition, which led to plug fouling etc..... new plugs, cap and button ...... runs like it used to.

[Duntov]

Originally Posted by F-BIRD'88

Use a bathroom scale and a drill press. Check them at 1.75" and 1.70".

stock 85# Z28 #105-115 stock diameter race 125-135#

Back in the Seventies I bought my first adapter in order to pressurize a cylinder so I could replace the valve seals without removing the head.

I realized during the course of that job that it was the best diagnostic tool I had ever had. The first cylinder I pressurized was spewing pressure out of the ex valve/tail pipe, so fast I was worried if it would stay up.

Another cylinder had an intake valve nearly as bad and the carb belched up a puff fuel mixture from the intake. Another cylinder had really weak rings.

All the cylinders had a small leak you could hear but if it's bad there is no mistaking it. I keep a kit on the wall and use it once in a while.
Oh, If anyone does this who has never done it before, I recommend a swivel hose and not a quick coupling. Take no chance of it blowing off. keep fingers away from belts and pulleys

[fred56]

Thanks for the helpful information I am going to pull the heads over the winter and replace all the old stuff. I like the idea to use the bathroom scales to check for pressure, I will remember that. About the distributor I already checked it and while I have the motor apart I will check the cam lobes.

Thanks again for your help.
Jerry