Recently I broke an exhaust valve spring on my 72. I was hoping that only the valve spring broke and no other damage occured. Yesterday a buddy and I were going to attempt and fix the broken spring. This is what we did....

bumped the motor until the problem cyclinder was finishing it's exhaust stroke. Removed both intake and exhaust rocker arms from that cyclinder so that the valves could close freely. Put an air hold fitting in the spark plug hole and attempted to pressurize the cyclinder. This is where we ran into a problem. The cylinder would not hold pressure. It would bleed off the pressure before we could try and compress the spring. We would pressurize at about 100-120 psi and this would bleed down in the matter of a minute or so. We tried the same technique on a good cyclinder and this one did hold pressure but we noticed that when we went to compress that spring we could force the valve down with our lever style spring compressor. This was at about 100-120 psi. (the max my wimpy little compressor could produce) But it did not bleed pressure like the problem cylinder.

So, before I head off to the pro's, my questions are...

Are we doing this correctly? Having the both rockers off I would assume the valves could seal and pressure should hold. Does it matter what part of the engine cycle the cylinder is no before you pressurize it?
Since the cyclinder will not hold pressure I must have some internal damage either to the piston or valves or other, correct?
Just for future reference, what pressure should be able to hold the valve up while compressing the spring?

I'm probably going to take off the head on that side and see what the internals look like. Hoping for the best but thinking the worst

 

I have changed many springs on the car and I just attach my shop air line to an adapter in the spark plug hole. The compressor MUST remain attached at all times. It will bleed off quickly if you do not stay connected. You should hear just a light hiss as air escaps.
I do use a rubber/plastic hammer to tap the retainer to free the keepers but yes the air should hold the valve up while you push down with your spring compressor but remember to give it a hit with the plastic hammer to jar the keepers loose.
Also stuff rags anywhere where you think a keeper could fall down into and become lost.

 

Yes, you probably have a bent valve ... heads need to go to machine shop.

It doesn't work that way ... the spring pressure has no direct correlation to cylinder pressure ... you need experienced help. It takes very little cylinder pressure to hold closed a GOOD valve & GOOD seat & GOOD seal.

Quit screwing around with it ... AND ... Take BOTH heads to machine shop along with your cam card & rocker specs ... have heads serviced with new springs that're clearanced to work with whatever VALVE lift your cam AND rockers produce.

 

Yeah, it would seem that cylinder has something wrong with it. Badly sealing valve, maybe?

Before you compress the spring you need to tap on the retainer, otherwise it sticks and the valve will open.

 

If your going to remove the head anyway you may want to try this. Before you remove the head, rotate the engine until the piston is about an inch and a half down in the cylinder. Now stuff some nylon rope into the spark plug hole. As much as you can get in there. Now rotate the crankshaft to force the piston and rope up into the valve heads.

Hopefully this will hold the valve up so you can change the spring. After you're done remove the rope, reinstall the plug and see how it runs.

It's worth a shot.

BigBlockk

Later.....

 

Pros? We dont need no stinkin pros! Whats the worst that happens, you have to buy a nice shinny 383.

 

As Norval said, keep the compressor connected and running while you remove the spring etc.

 

Yeah, if you've removed the rockers then the valves are definitely seated. And, if it won't hold air as well as a good cylinder then yep...bent valve. I would pull the head at this point and look everything over. If the valve got bent then it was for a reason...look for that cause and eliminate it...and make sure you apply the same fix to all the other valves.