TBH, having both is the best bet. Like Pepsi says, the diagrams in the Yamaha Service Manual are very useful. However, the service manual only really tells you what to do; it doesn't tell you how to do it. It's more geared up to people who have experience; it is after all designed for mechanics and dealer service. This is where they Haynes comes into its own as it gives you step by step instruction, and also some insight how to perform tasks without "special" tools, something you definitely don't get in the service manual.

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