So I've been driving my '76 914 2.0 pretty much daily now after getting it all smogged and registered but now I've started hearing some sounds from the exhaust. When giving around 80% throttle around 3k rpm I get a metallic crackling that goes away when i let off to about 40% throttle. I also get an occasional quiet backfire when shifting. Any ideas about what i should change?

Does the backfire occur when cold and gets better as the engine warms?

yeah it usually goes away as the engine warms up

sounds like the decel valve. have you tested it?

I'm pretty sure it works but i've messed with the adjusters and it could be set up wrong.

decel valves have adjusters?