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healthy diesels make small amounts of black smoke with some white on cold starts. Sick ones make blue or continuous white. Diesels are generally robust but require a strict schedule of oil changes. Bonus points for proof of maintenance. Again, bonus points for maintenance records and a spare parts kit. Common ailments of gas engines: wet or worn-out electric components, bad points and plugs. If the seller took the trouble to warm up the engine before showing you the boat, it may be because it is hard to start the engine when it is cold.
Steer clear of rare or very old engines unless you're certain there's an adequate supply of parts. Do the Smoke Test: Check for fuel leaks and a working bilge blower in gasoline engines. Before the seller cranks the engine, check to see if it is already warm.