Trouble Timing my 340

If the valve adjustment aren't the problem, I look in to cam timing, like its retarded, you want it 2 - 3 degrees advance. That means pulling the front timing cover and balancer off.

Heck I be swapping cams while in there just so you know what cam it is for sure. No need to pull the intake manifold, lifters can be pulled out with a magnetic rod through the cylinder heads.

Do a cranking compression test on all 8 cylinders and report back. Its really not that big of a deal ,lol and you can see how the plug look. If one or more valves are off the seats, the compression test should show it as being low on compression, then you know which cylinders to back the rocker screws off

On my iron rockers the screws are all about the same height. Look to see if any screws heads seem too far in / low. Hopefully its just an open valve and its not burnt just yet