I have about had it with my "new" 406 SBC. After countless messing around, I still don't know what is wrong with it. It runs like complete crap. It was a completely new rebuild by a pro builder, It ran very strong at first, but had an issue with ex oil coming out of the seal on the front timing cover seal. After giving it 100 miles of break in and replacing the seal, I drove it on a 200 mile road trip and en-route, I broke a valve spring. It still ran good down the highway, but idled like crap when I pulled off the highway. I limped it home, and pulled the plugs at that time and they looked fine. Also, the dished pistons did not appear to hit the valve (luckily, it had an inner spring and I was not revving the engine). I replaced all the valve springs and readjusted the lifters twice (second time was running adjustment but I only went 1/4 turn past stop of ticking. After farting around with it some more and it running like a "turd-burger", I discovered a broken chrome distributor clamp that allowed the distributor to retard itself to the point where it almost wouldn't run. I replaced the clamp, moved the vacuum hose for the distributor to a full time manifold vacuum port and it seemed to run better. SO... I changed out the leaking timing cover...and road tested it. It is still running terrible and leaking oil from the timing cover seal yet again. I ran a compression test today and the readings are 200, 205, 205, a bunch of 210 and one 215. I also looked at the plugs and they look half oil fouled (on the side facing the valves) and half tan on the side facing the piston. (AC Delco R43TS). It almost looks like bad valve guides, but the heads were checked, reworked and vacuum tested. I pulled the rotor off and checked the mechanical advance on the HEI and it is clear and free. (I also rotated the cap clockwise and it snaps back easily too). I sucked on the vacuum advance hose and it operates fine. I checked the PCV valve and it sucks great at idle, but I don't know what happens at speed. It is connected to the port on my Quadrajet carb right under the fuel line on the very front of the carb, same place as always. The engine ran fine on day one, so the cam is set up right. I run M/T valve covers with no baffling in them, so I wonder if the PCV valve sucking in oil and fouling the plugs? Should I try different (stock) valve covers? Is my gas too low of octane? It revs in the driveway fine, but knocks when under load, when I accelerate. I had set the timing at 4 degrees BTDC (stock 400) and it pre-detonates like crazy. If I retard the timing, the knock goes away, but there is no power. When I set the timing (with the vacuum advance disconnected) the engine almost wanted to stall as I moved the distributor. As soon as I plugged the line back in, the RPM's jumped up. I have no idea what to do to fix it. I can really use some assistance...please!!! Ask questions, I'll answer as quick as possible and do whatever checks I can.