OP, how do you know the heads are the cause of the low compression? Did you put some air in the low chambers with the valves closed and listen for leakage pas the valves, or... ???
When trying to start it it will not run, it backfires back thru the carburetor. I replaced the carb with a new one and same results. Figured timing or stuck valve/s, checked compression and 1-3-5-7 were all right around 120 dry, #2 was 30 and increased only slightly with oil in the cylinder. From what I have gathered and read it's most likely a valve issue, broke, bent, stuck.
