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A 'leaded engine' runs fine on unleaded petrol although the lower octane level will require ignition retarded a couple of degrees but suffers valve seat erosion which of course will eventually cause a compression drop and running problems. It is possible this has happened and if yours doesn't have hardened valve seats then a compression check would be worthwhile. Cylinder heads with valve seats for unleaded were stamped with letters on the exhaust face close to spark-plug number 4. See pic below. Letters for 1.6 are M, MM, N, NN. 1.8 are S, SS. 2.0 are L, R, RR, P, PP.