He said the replacement Mercury motor's serial number is 3129783. Can anyone tell me the year?
Hi there! Unless this engine is super old from say 1964 - 1972 for example then the serial number isn't the correct number. Typically some of the later model serial numbers may start with 0F, 0W, 0M, 0K,....But the older engines did start with just a number. Without knowing the engine H P, model, # of cylinders, it's just a guess but the serial number you posted can be found in the 1964 - 72 year range under the
120 GM 153 I/L4 1964-1972 engine model for example.
More info is needed and photos of the engine may help too.