Strange Intake Color?

[Forde]

Hello!

This is my second post ever and second for the evening. I'm really hitting the site hard on my first day!

I have a '78 XS400E, the one with the disk brakes front and rear.
I just had the frame powder coated, and I noticed something odd when disassembling the bike.


Looking down the intakes of the engine, the right cylinder intake appears to have this strange, rust coloration that the left cylinder intake does not. It rubs off with a little finger pressure and appears slightly powdery, almost like rust but I can't confirm what it is.

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Does anyone happen to know what this is? Is it a symptom of something with the engine that I am missing? The engine seems to run fairly evenly, and if anything, the right side is slightly stronger than the left.

 

[xschris]

Could be old gas varnish or a intake valve not sealing causing exhaust gasses to build up.

 

[Forde]

I sure hope it’s the former. I’ve never had to do lapping...

Also, could backfiring cause it? I know in the past before I had the jets sorted out the right would backfire quite a bit on deceleration. The left cylinder has only a slight discoloration, so that leads me to believe it’s something common to both cylinders. Not that it’s possible that valves aren’t seating on both sides, but the engine only has 7,000 total miles on it.

Thanks for the help Chris!