Any Idea What’s Causing This Gunk?

[luckydevil]

I’m trying to get this 1978 XS750 running and I noticed burnt oil or residue from incomplete combustion in the cylinder #2 intake port. Cylinders #1 and #3 look nice and clean. I’ve attached photos for reference.

I admittedly tried to run the bike with pretty nasty carbs before starting the rebuild, and it surely caused misfire/backfire.

Compression is consistent across all 3 cylinders, squeezing 135psi-141psi.

Thanks for the input!

 

[AUTOMAN]

Just one port doing this?

Valve guide dripping oil from rocker box.

Sloppy guides do this if really bad, but guide to head casting due to scored insertion interface on press fit is more likely.

New guides?

cliff

 

[luckydevil]

Yeah, just one port. I put new valve shims in, but didn’t touch guides. Who knows how long it’s been causing problems. Looks like I need to go shopping.

Thanks cliff!