Worn valve seals mentioned above cause oil to be sucked in the cylinders and black smoke out the tail pipe. Again, compression test done properly will show rings permitting blow by into crankcase and out the draft tube and oil input tube. Don't give up yet. Carbon build up around therings will case harden when an engine sits un-used for years. The rings have to flex as they travel up and down the tapered cylinder walls. Pull the pugs and put 2 oz of transmission fluid in each cylinder. Let sit for a week. After a week turn the engine over with no plugs and blow the excess transmission fluid out the spark pug holes. ( Put towels over the fenders). Re-test the compression. No improveent in readings= rebuild. Some improvement= do another week of soaking. All reading to spec= go to other problems!
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