What IS this stuff?!

[Dryad]

I'm currently starting a light rebuild of my 948cc engine, and when I removed the manifold I noticed that the inlet port has a thick deposit of black, brittle substance which can be broken off in small black shiny fragments. The corresponding cylinder head port has the same. However, the other inlet port is normal. What causes this?

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Manifold.jpg (762.06 KiB) Viewed 3026 times

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[Bill_qaz]

Looks like corrosion on picture from water ingress has it been sitting for a long time, was the intake covered?

[Dryad]

Looks like corrosion on picture from water ingress has it been sitting for a long time, was the intake covered?

Yes, the intake was covered, but it's the black shiny deposit I'm curious about. It's almost like tar that has dried up and gone brittle.

[Bill_qaz]

Oil residue from valve seals or worn valve guide?
What does the spark plug from that cylinder show.

[Dryad]

Oil residue from valve seals or worn valve guide?
What does the spark plug from that cylinder show.

I can't check the spark plug at the moment but I did do a compression test before I took the engine out, and I think one of those two cylinders showed a good 25psi below the other three, so you could well be right.

[oliver90owner]

Bill is not far off, I suspect. Likely a combination of oil leaking down the guide and poor valve seating, allowing that gunge to be forced back into the manifold by the combustion pressure.

[Dryad]

Yes, sounds right. I took the head off yesterday and it's clear there is a problem.

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[Bill_qaz]

Plug on right well oiled and not been firing properly. Which cylinder had low compression.
Take out the pistons and check bore and ring wear, all cylinders look to have carbon build up.