My friends nieces 80 Corvette LG4 carbed no ecm(yup a factory pos 305)has had a knock in it for months now.She drives it every day and to me it sounds like a rod or a cold start knock but it wouldnt have lasted so long if it was a rod.Timing is right on by factory specs 12 initial and about 28 total.This knock is constant at all speeds,rpms and temps.100,000 miles and nothing done to it but a tuneup.I dont see them much,because that part of my friends family think they are einstiens decendants and **** me off after a few min not to mention that they owe me a few grand,so i cant fully diagnose all at once.Sound loudest on pass side and you can slightly hear a air sound every time it pops.When you try to give it fuel it sounds like a old aircompressor(you know that unique sound they make while they fill the tank).

Im thinking wrist pin,piston slapping,the fitting that screws onto the AIR tubes(dont remem the name),manifold to head leak,valve that in the pass side exhaust.It'll be a couple weeks before i look at the car again so i thought i might get some ideas before i get there so i can try to go straight to it before they P me off.Thanks

 

Stop by Sears or wherever and pick up a mechanics' stethoscope so you can trace the source of the noise.

I've had an AIR pump cause some pretty scary sounding noise, hopefully that's the case here as well.

Very easy to probe around w/ the stethoscope and narrow down possible sources. Start at the accessories...alt, AIR, PS pump, A/C clutch, then move to the valve covers and listen over each rocker, then the header primaries if it has them, etc.