I have an 86 z28 305 carb. the problem is that the car overheated and i was told i might need to replace the the heads(this is second time i need to replace them). How do i find out if the heads are actually bad

 

heads? more than likely you'd need head gaskets. You have water in the oil? blowing exhaust out of the radiator?

 

no water in oil... car just shakes alot when on

 

So it is missing? You need to determine why it is missing. Spark, fuel, air/compression. If you are missing, or have weak of one of those then you'll have a misfire. A bad head gasket, or bad valve's can cause a lack of compression to cause a misfire. Those are things that are why you'd need to have the heads removed. I'd start the basics. Check spark, perform full tuneup. Then go to fuel which is probably fine.

 

i was told that the spark plugs are good i watch the guy check them.. whats included in a full tune up?

 

tune up should be good

 

Have you performed a block test?

What was the original cause of the overheat, that caused head/head gasket damage?

If the original cause of overheat wasn't fixed, you'll replace that stuff over and over again it's finally corrected.

 

Magna flux and pressure test. ANY machine shop can perform that test for about $150 for both heads

 

the cause of the overheat was that fan did not turn on so know its made to stay on every time i turn on the car.. if machine shop says its one cylinder head thats bad should i replace that one or the whole thing?

 

the thing is you dont know for sure it is a cylinder head problem. A shop told you this or what? I'd take it to a known good repair facility and see what they can come up with.