Leakdown can be done on a cold engine.

I'm not a fan of the "back side of the cam" approach for setting valve lash. But some favor it. It works, but is trickier and the spec is different (thinner feeler, as I recall, because the rocker multiplies the cam lift to get valve lift). Porsche said check lash at the valve head. You have to choose which way to go. Either way, if done right, will bring your lash close enough.

I measured the other side (#4) and it is the exact .038 inch measurement as #1. I can be sure of it being timed correctly then, yes?

So I tried this on two cylinders so far (1 and 6) Got about 50% leakdown on both, and heard air coming out of the crankcase, but absoulutely nothing from the intake (air filter out, throttle open) or exhaust (my muffler is off and I have race headers).

I thought leakdowns had to be done on a warm engine (so the rings are sealing) for accuracy...I assume getting a good bit of air past the rings is normal on a cold engine?

At this point I think I'll put it all back together and do a leakdown test with the engine warm. Maybe the slightly lower power is just environmental (summer temps) or in my head, but I'm still mystified where the occasional rattling noise I heard was coming from. I have it up on my lift when I'm listening.

I did a leak-down on my old motor that had been sitting for months on the engine stand. I was getting 6-8 %. A friend who bought the motor ran another test and was getting 4-6%.

So getting 50% past the rings does not sound reasonable to me. If your rings were that bad you would be smoking up a storm.

Still not sure why you're not doing the easiest thing, a quick compression check of each cylinder. Cold engine won't matter a bit for what you're looking for.

At least it is not timed so incorrectly as to cause the symptoms which have you worried. But those 50% leakdowns suggest at least two broken rings per bad cylinder or something equally unlikely. Typically we DIYers break rings during the installation phase, so the consequences, if not noticed when we break them or before we run the engine, are immediately obvious.

A 50% leakdown on two cylinders ought to mean a huge amount of blowby through the oil tank into the intake boot, and thus into the combustion chamber (as well as oil being sucked in), so lots of smoke all the time.

Cross checking with a compression checker seems in order. There is always a little hissing in the crank case, even with super ring sealing. I've not had the occasion to have one of my engines have a 50% leakdown via the rings, but I'd expect it to be quite a loud noise for an engine which was not running.

Not clear to me why a warm engine would seal better than a cold one. The rings are steel, the cylinder aluminum. It expands faster than the rings. The piston isn't supposed to be pushing the rings against the cylinder.

Still not getting to rattling/power loss, though those leakdowns could lead to the power loss.

I forget - what did you find in the oil? Just the usual mush on the magnets?

Does the rattle sound like maracas?
Could it be detonation or pinging?
Just throwing it out there