Re: chevy 350 compression
OK, lets make a few assumptions first.
It would be extremely rare to lose compression on 3 cylinders all at once, especially on different heads.
It's possible the cam went that fast, but it's more likely a 747 would land in your back seat. Cams usually go flat fast, but not all of a sudden, fast may be 50 miles or so, slowly getting worse. It takes time to wear through the case hardening.
I assume the numbers you gave are the right cylinders numbers, 1, 2, & 3, left front, right front, left second back.
If you had something that caused a misfire in those cylinders, then it would naturally cause the compression reading to be low. The misfire causes the raw fuel to wash the oil off the rings, dropping your compression readings. (I've seen lots of engines get rebuilt because of that.)
Your "blowdown test", how did you do it? I asked about percentage thinking you had done a Leak Down Test with the proper equipment. The right equipment will give a percentage of leakage indicating how bad, and helps determine what's wrong. Your ears do the rest to see where the leaking air is going.
I asked what the vacuum reading was, you didn't say, just steady, that actual reading would help. I'll have to assume it's low, maybe 12"? I think that's what the el cheapo vacuum gauges call "valve timing."
If the chain had slipped, causing "valve timing" problems, all cylinders would be affected, not just 3.
If 3 were misfiring, vacuum would be low anyway.
Let's analyze what we have so far.
The 350's firing order:
1 8 4 3 6 5 7 2
Not much "in common" between them, just 2 and 1 fire back to back, and all 3 are in the front of the engine.
I'd start with ignition, like crossfiring. Check the cap for tracking. If HEI check the bottom side of the rotor for any whitish marks indicating flash through, or rust.
Check that the plug wires as they run to the front are separated, and no whitish marks on them at all. Whitish marks are actually ash or solidified smoke, indicating leakage.
How'd those plugs look? Slightly oily and dirty I assume? -- likely ignition
Or very clean and white? -- indicating severe vacuum leak - like carb base gasket. (I doubt it)
What kind of plugs did you use? Hopefully no gimmick "trick" plugs "made to sell, not to use." American of course, Japs and Krouts can't make a Chevy run. Platinums don't belong in a Chevy, no matter what the pimply faced kid at the parts house said.
When you do the compression test again, take all the plugs out, block the carb open, then watch your gauge closely. A normal hole will rise to about 80% of the final reading on the first pulse, then the next 3 or 4 pulses keep pushing it up a little more.
On the low ones, add about a teaspoon full of oil, then crank it without the gauge about 4 or 5 revolutions to spread the oil inside, then retest. I'll bet it will come up to about the rest or more. Normally if those cyls had been running OK, that would indicate leaking rings. But if those cyls were already misfiring, the normal oil sealing the rings has washed away, so don't panic - yet.
Let us know what you find.
My guess - crossfiring, or wrong heat range plugs or "mickey mouse" plugs. No biggie!