Locked Up!, What might it be? Options

[74914LE]

Just finished getting a 72 1.7ltr. ready to start up after sitting for 29 years. Went over everything in the fuel, vacuum, and ignition systems. It turned over just fine without spark hooked up to it. Now for the moment of truth. Made sure I had fuel pressure and attempted to start it.

After about 3 tries, it fired up and started running!!!! After about 20 seconds, a rattling sound started, then got worse. I immediately shut it off. Then tried to turn it over again, and it would not turn over. I then got it on jack stands and blocked one wheel to put it in gear and see if I could spin the motor by hand with
transmission in 5th gear. The wheel will turn about 1/3 a turn in either direction and it locks.

Before I drop the motor and pull it apart to check for a dropped valve or worse, though I would see if anyone wanted to weigh in on what this might be....ideas?

Thanks in advance!

[eyesright]

I had my 93K mile '76 2.0 DJet for a year or so. It was driveable tho I knew it wouldn't be long before I tore it down. It rattled most of the time over 3k RPM. It wasn't a rod or main bearing and it just didn't seem like knocking. Then one day I went to start it and it turned over a single turn and it locked up. I could turn the wheel by hand and felt the piston hit something. It wasn't metalic but it was pretty solid.

On tear down, #3 and #4 head was CAKED with carbon with lots of carbon loose in the cylinders. The other head and cylinders were not. I assume that the carbon broke loose and fell into the cylinder and the piston crunched it up against the head.

(Anybody have some ideas how that side got so carboned up? )

Good luck