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fred56 09-07-2010 04:42 PM

SBC Valve spring pressure
 
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 09-07-2010 05:52 PM

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 09-07-2010 06:24 PM

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.


Quote:

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 09-08-2010 03:04 AM

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

EOD Guy 09-08-2010 03:47 AM

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 09-08-2010 04:55 AM

Possible leaky valve?
 
Quote:

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 09-08-2010 02:21 PM

SBC Valve spring pressure
 
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


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