I have a 68 Firebird with a 350 engine, 63,000 miles. When I accelerate hard I get a loud knocking from the engine, but at slow excel and slow rpms - nothing. Any idea what it could be? It sounds like a rod knock - not an octane knock, but wouldn’t a bad rod knock all the time?

It could be piston slap or maybe a main bearing. In any case the engine probably needs to be opened up.

Not necessarily. When you accelerate hard you put the rod bearings under a lot more force. Circuitsmith’s suggestions are also good possibilities.

It’s also possible that you have severe preignition and you’re hearing the shock wave of the wavefront banging against the upcoming piston. Causes for that could include carbon buildup (carbon retains heat and severe buildup can even increase compression), ignition timing too far advanced (have you checked?), or lean operation (it’s a carb…there’s no oxygen sensor to tweak the mix…you need to adjust it right to start).

Seeing as it’s a '68 Firebird, I have to ask: has this engien been worked on in the past? Is it modified?

The engine has never been modified. It’s 100% stock. I’ll try checking the timing.
I didn’t really want to open up the engine, but the car would be worth it - it’s a convertible.

2bl carb.

Oh, yeah, by all means, fix it!

If the timing (don’t forget the vacuum advance) doesn’t pan out, I’d look at the carb. I wouldn’t want to open it up without eliminating external possibilities first.