Compression problem on 84 k10

[30backs]

So I have been having a spark knock on my chevy and hunting the problem for months. I checked the compression in all cylinders today and all were 150 except for # 3 & 5 . They read barely above zero on the gauge any idea? I am personally leaning toward a possible blown head gasket but have no overheating issue or coolant in oil so I'm not positive.

 

[chengny]

Exhaust valves really burnt, valve stem binding causing valve to hang open (carbon build up on the stem or a bent valve stem), hole in piston crown (you'd notice excessive blow-by).

Pull the valve covers and check that the valves are free to stroke fully up and down. If they are not stuck, put air pressure on the cylinder (at TDC) and feel the end of the exhaust pipe. If your valves are toast, the air will blow right past them. Into the manifold, through the exhaust piping and out the tail pipe.

 

[highdesertrange]

although 350's are not known to blow head gaskets I seems logical since the cylinders are right next to each other. however it wouldn't hurt to do what chengny says, either way it appears that head has to come off. highdesertranger