Exhaust advice: Discoloured downpipes

I’ve bought a used 2006 Yamaha XJR1300 with 3000 miles on clock which runs very well. However the front downpipes are discoloured and have gone slightly purple.

What do you think is the cause?

Should the fuel mixture be checked? I’ve been told that leaving the bike running when it’s stationary could cause this.

Matthew Ritchie, email

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This is happening more and more on bikes with stainless exhausts, because manufacturers are setting bikes up to run leaner to pass emissions testing, so the bike runs hotter, and stainless steel systems do discolour as a result.

It’s not something to worry about ­ some good metal polish and a lot of elbow grease will remove the worst of the discolouring, though it won’t completely eradicate the problem and you’ll have to re-polish your pipes again when the discolouring returns.