I test drove a 91 Corvette 6-speed today that overall seemed decent for the money, but the spark knock sort of scared me. When I started it I could hear the clunk of spark knock for just a moment before it fired. When I let the clutch out in first, it would knock a little until it was going. once the car was going, it didn't knock any more. No knock under acceleration. When I accelerated, it did seem to have "flat spots" or "dead spots" in the rpm range, but that's probably easily solved by a tune-up and some studying of the auto x-ray. But what about the knock? Is it just a matter of replacing the knock sensor and checking the timing, or could this be a sign of something more serious? I've never experienced knock like this... only when under load, so I don't know what it means. Any help would be appreciated in helping me try to determine the problem. The car is 3 hours away, so before I make any decisions, I need to be relatively sure that I'm only dealing with minor tune-up issues.
Oh yeah... When I went back out with it a few minutes later to show the owner... it wouldn't do it unless I stalled the car letting out the clutch.

 

With your luck look at that as a sign.

Run

 

if it was knocking before it fired then it could not have been spark knock, it may be a rod knocking or maybe a lifter tapping but I don't think it was spark knock.

just my humble opinion, but lets here from some others first !!

 

This sounded 100% like spark knock. One clunk on startup, and the usual, tell-tale knock when it was on the brink of stalling from letting out the clutch with too-little gas.

If it was something like a wrist pin or a rod knocking, shouldn't that be present in more than just those two cases?