I'm still waiting to hear back from him. I need to hear what tests he did and when the vibration started. I'll definitely pursue the balancer issue. He did say he used an octane booster. The 250hp engine was rated at 10.5:1 CR.


Verne

Wrong flywheel/flex plate? Seen it happen way to often.

Well, I just got off the phone with him. The car had no vibration at all before the engine rebuild. It was run without all the pulleys, trans and flex plate disconnected and the vibration was still there. He even pulled the balancer and stuffed a cardboard tube on the crank to keep the oil from running out and it still vibrated.
Supposedly, the crank was retested for balance and passed. I suggested he find a good shop to check the crank again, making sure the rod journals were ground at the same distance from the crank centerline, and weigh the rods and pistons again.

He's sending me the carb to go through and fix the lean issues.
I told him to skip the octane booster and lead substitute in the future.

Verne

Verne to me if that engine had no vibration before the rebuild but developed one after the rebuild, something was not done right then. Sounds like the engine is completely apart now. Going to have to go thru everything again with a fine tooth comb and see what was done and was it done right. Have the rods checked for out of round and straightness and have them magged for cracks. Sounds like he had the crank double checked and its fine. So now take a hard look at the rods like i mentioned and then the pistons. Check pistons for cracks and piston pins too. Check push rods, lifters, rocker arms, etc. There is definitely something wrong inside. Even get the engine balanced while he is at it. It doesnt cost a bunch and you know everything is good. What kind of carb is this engine using? A Q-JET?

As mentioned -Check the weight of the rods-
I know this is a 327 but I seen it before on a 396 where someone used a single non dimple rod in the rebuilding of a 396 L78 that was 6 grams off and destroyed the engine in under 2500 miles

Did the shop who machined it put in a exchange crankshaft? It’s either cracked or the bob weight figure is way off. Was it balanced? I had a shop screw up the bob weight calculation 1 time by figuring in 2 small end connecting rod weights. I would also make sure the flywheel & balancer is a non counter weighted style.

Agree i mentioned balance too. This engine just seems like a real mess. Maybe a
wrong rod, crank is bad, on and on. I mentioned do a complete visual inspection on the bottom end. And also mentioned a complete balance job on the crank, rods, etc. Vibration means something is totally wrong with the rotating mass.

Agree. I was thinking detonation due to an extreme lean condition, but the plugs show otherwise.

The carb is the correct Rochester 4GC. I believe the crank and rods are the originals. Reportedly they did some "shaving" on the rods, so they are suspect. The pistons are new, but they are being cleaned and will be weighed. The valves appear to be new but I don't know about the other valve train pieces. I doubt any of those parts could cause a vibration. As I said before, the trans, flex plate, pulleys and balancer were removed and it was test run and still had the vibration.
When I talk to him again, I'll suggest the balancing. He should take those parts to another shop for that.
He said he adjusted the mixture screws using a vacuum gauge but I forgot to ask him what the readings were. There might be a leak which would cause the lean condition. I'll ask him if he has the correct PCV system hooked up. I want to know those things before making any changes to the carb.

Verne

Verne, shaving on the rods? interesting. Maybe he meant polishing the rods and removing the flash and possible shot peen. But shaving could of meant lighting them up somehow. But if material is removed from a rod, you need a balance job on the crank and rods. His best bet would be to buy a good set of news rods on the market and lighter. Agree valve train should have nothing to do with this vibration problem. I always been big having an engine balanced anytime you have it apart for a rebuild or building an engine. You will be amazed how smooth they run. Racing engines are always balanced. IMO i highly doubt its a carb problem. You are right see what happens to his changes first before a carb rebuild. Keep me posted.