Could backfiring cause this?
 

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What happened?

Long story short, engine was assembled, dyno'd, & tuned ran great. Installed in boat plug wires got crossed (don't ask) tried to start it was backfiring bad, got wires straightened out starts & runs but vibrates anything above idle. Checked timing, valves, carb, nothing wrong. pulled engine, pulled pan slight amount of I assumed assembly sludge, pulled #2 rod cap and #3 main to look & the pics are of what I found. Gen VI 502 .030 over aftermarket crank stock gm rods mild 515hp build. Approx 1 hour run time after dyno tuning.

A good backfire could cause a balancing issue.

randy,if you run your fingernail across the journal can you feel anything because from looking at the pick it looks ok.

Mike, I just pulled 1 of each but yes on the main & no on the rod journal. I can't remember what brand the crank is but the one that was replaced in this engine was a scat crank & it wiped the bearings with the static compression test that was done when it was originally built by Jasper Racing, that is why I had it gone through again so I wouldn't have these problems. The crank was replaced & short block built 5 yrs ago then stored until last yr then everything disassembled cleaned up & reassembled, dyno'd now this.

Can you see any ballancing weights on the crankshaft?? the viberation sounds like a ballancing issue..if they internally ballanced the engine then you will need a 396/427 harmonic ballancer and flywheel needs to ba machined off or 396/427 flywheel..

Probably not an issue since it dynoed fine.

When they run out of oil #3 main is the first to go. I don't know how hard you were running it for an hour...

If it ran smoothly on the dyno and now vibrates after installation, maybe it's a flywheel issue?