Bought a '90 C-10 with the intent to switch over plastics (red) to my '86, but I'm wondering if fixing the engine would be a possible.  I know it's probably impossible to diagnose here, but this is the info I have.

Symptoms from PO are:
It backfired on a ride a few times.
Parked in garage, and went to start a few days later.
Loud pop when starting.  Assumed hydro-lock, and measured cylinders.  All OK with 1 & 4, 2 & 3.
Petcock was leaking.
Did compression test, and 1 & 2 were good, 3 & 4 would build pressure and then drop.
Put oil in cylinders 3 & 4 and hit starter.  Oil blew right out.
PO thinks it's valves??  If I could get some idea of what it might be, and attempt to fix it, I may try.  It would be a shame to break this bike up and part it out because it's in great shape cosmetically, with some nice farkles.  I'm no mechanic, but I have been able to wrench with the manuals and help here.  First valve adjust last week, carb rebuild, shock change etc.
Any help would be appreciated.  If it is too much to fix,  then I will part out.

You're on the right track. Could be as much a carb issue as anything. Worse case scenario is you drop your '86 engine in it and have a nice '90. Then part out your '86. First thing is check all the valve clearances to make sure nothing is too tight. If it is a valve issue then you can plop a rebuilt head on it or do the swap, but, I think your compression test would show an inability to build pressure to the 150+ area. Check with SISF on the carbs. JMHO

I would run the valves first. Recheck compression

Thanks,
Sounds like I'll check the valves.  Just the clearances??  Hopefully it's something fairly easy.  PO said bike ran well prior to this.  If valves were too tight, what could happen?

Thanks,
Sounds like I'll check the valves.  Just the clearances??  Hopefully it's something fairly easy.  PO said bike ran well prior to this.  If valves were too tight, what could happen?

The exact same symptoms you are having, for one.