As exhaust valve is 3/4 of the way from closing, the intake valve starts to open???
 

I replaced the head gasket on a generation IV 454. after i had it back together it wouldn't start. you could tell there was something wrong because the starter didn't have that usual sound to it like there was no compression. i noticed that when I cycled the engine around with a breaker bar that the intake valve would open, before the exhaust valve had closed all the way. Meaning that they were both open at the same time for a second. I didn't touch the cam, timing chain or gears when i replaced the head gasket. Is this a valve adjustment issue or is the cam miss aligned?

Re: As exhaust valve is 3/4 of the way from closing, the intake valve starts to open?
 

If you didn't mess with the timing chain, then nothing should be wrong with the cam/crank timing. I suspect that the valve lash is set too tight.

Re: As exhaust valve is 3/4 of the way from closing, the intake valve starts to open?
 

The intake and exhaust are open for a short time. Its overlap for whats called scavaging. It makes sure all the exhaust gets out of the cylinder and chamber. The valves may be out of adjustment and hanging the valves open. Re adjust per the book.
As Magic said if you didn't touch the cam or gears then your ok there. Just concentrate on what you did have your hands on. check the valves, plugs, wires

Re: As exhaust valve is 3/4 of the way from closing, the intake valve starts to open?
 

Make sure the distributor isn't misaligned.
Kim

Re: As exhaust valve is 3/4 of the way from closing, the intake valve starts to open?
 

Does the exhaust valve ever close? You might have valve guide that slipped out of the bore and hammered itself on to the valve, preventing it from closing. Either that or your lash is WAY too tight.

Re: As exhaust valve is 3/4 of the way from closing, the intake valve starts to open?
 

If you lost fluid from a lifter, you might have your lash set super tight. I pulled all the plugs and could turn mine over using the fan and I set the lash that way. Socket wrench and feeler gauge. Once it warmed up, did it again and it runs like a dream.

Re: As exhaust valve is 3/4 of the way from closing, the intake valve starts to open?
 

I prefer the EOIC method, it ensures the lifter is on the base of the cam and not the ramp. Pull all your plugs and rotate the engine until #1 exhaust valve just starts to open. At this point you adjust the intake valve until there is no vertical play (0 lash), it should not be tight, just adjust it until you can't lift the rocker off the push rod tip with light pressure, then tighten another 1/2 turn. Do this to all cylinders in the firing order. Then go back to #1 rotate the engine until the intake opens and starts to close. Adjust the exhaust valve the same way. Do all the ehaust valve in the firing order.