What method are you using for adjusting the valve lash? You can see you lifter with a flashlight and check if the plunger is collapsed.

Engine has been sitting for a while... Might want to set the valves on those cylinders at "0" lash.. Pull the valve covers and crank over the engine without any plugs to verify all the valves or operating properly. Then fire it up to build oil pressure, then shut er down and re-lash those valves.

The lifter may have collapsed sitting all this time and when you loosen the rocker arm the lifter isn't decompressing.

This might not solve the problem but just a thought....



This all in addition to what was mentioned above. That's what I'd start with.

Told ya so Sorry, had to get that out.

I just did a new set of lifters on my HP 500s and they were not pumped up at all. Luckily I had the manifold off because it was amazing how little difference I could feel on the pushrod when the lifter spring started to take up the slack. It would be really good to get oil pressure to those lifters to be sure, but you may have one collapsed and there is no way to adjust the lash. Try them at zero and worse comes to worse the manifold is not that hard to remove.

It looks like next step is going to be to pull the manifold and take a look at the lifter valley. Not really a big deal considering some of the alternatives.