Ohoh, doesn't sound good!

I would do the same as I did.... I bought a whole engine (including all sensors,... even the injection system was included) that was an overproduction of GM. Never used or installed, just tested.
If you want to go the same way, just give me a PM and I will help you getting the same. Price was $2450 I think.

 

oh, just saw it's a '91 one... so better look for a new one

 

I would suspect a head gasket leak between #3 & #5 cylinders. They are next to each other and when you get two cylinders next to each other with almost identical compression, that's where you look for the problem. Good luck, let us know how it goes.
david

 

Naw,

If you've got one cylinder at 70 lbs and the other at 80, don't think timing is your issue.

Allan

 

glad it worked out. Sounds like your rings are going but I would check again then put a squart of oil in the low cylinders if pressure increases it is your rings.